Question & Answers About Electrical Machines-II (part-4)

31. Why is the MMF method of estimating the voltage regulation considered as the optimistic method?

Compared to the EMF method, MMF method, involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes care of saturation effect. As this method require more effort, the final result is very close to the actual value. Hence this method is called optimistic method.
32. State the condition to be satisfied before connecting two alternators in parallel
The following are the three conditions to be satisfied by synchronizing the additional Alternator with the existing one or the common bus-bars.

•    The terminal voltage magnitude of the incoming Alternator must be made equal to the existing Alternator or the bus-bar voltage magnitude.
•    The phase sequence of the incoming Alternator voltage must be similar to the bus-bar voltage.
•    The frequency of the incoming Alternator voltage must be the same as the bus-bar voltage.

33. How do the synchronizing lamps indicate the correctness of phase sequence between existing and incoming Alternators?

The correctness of the phase sequence can be checked by looking at the three sets of lamps connected across the 3-pole of the synchronizing switch. If the lamps grow bright and dark in unison it is an indication of the correctness of the phase sequence. If on the other hand, they become bright and dark one after the other, connections to any two machine terminals have to be interchanged after shutting down the machine.

34. What are the advantages and disadvantages of three dark lamps method of synchronizing?


Advantages:


•    The synchronous switch using lamps is inexpensive

•    Checking for correctness of the phase sequence can be obtained in a simple manner which is essential especially when the Alternator is connected for the first time or for fresh operation after disconnection
Disadvantages:
•    The rate of flickering of the lamps only indicates the frequency difference between the bus-bar and the incoming Alternator. The frequency of the incoming Alternator in relation to the bus-bar frequency is not available.

35. How synchronoscope is used for synchronizing Alternators?
Synchronoscope can be used for permanently connected Alternators where the correctness of phase sequence is already checked by other means. Synchronoscope is capable of rotating in both directions. The rate of rotation of the pointer indicates the amount of frequency difference between the Alternators. The direction of rotation indicates whether incoming Alternator frequency is higher or lower than the existing Alternator. The TPST switch is closed to synchronise the incoming Alternator when the pointer faces the top thick line marking.

36. Why synchronous generators are to be constructed with more synchronous reactance and negligible resistance?

The presence of more resistance in the Synchronous generators will resist or oppose their synchronous operation. More reactance in the generators can cause good reaction between the two and help the generators to remain in
synchronism in spite of any disturbance occurring in any one of the generators.

37. List the factors that affect the load sharing in parallel operating generators?
The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors
•    Prime-mover characteristic/input
•    Excitation level and
•    Percentage synchronous impedance and its R/X ratio

38. How does the change in prime mover input affect the load sharing?

An increase in prime-mover input to a particular generator causes the active- power shared by it to increase and a corresponding decrease in active-power shared by other generators. The change in reactive power sharing is less appreciable. The frequency of the bus-bar voltage will also subjected to slight increase in value.

39. How does change in excitation affects the load sharing?

The decrease in excitation in one generator causes the reactive power shared by it to decrease and a corresponding increase in reactive-power shared by

other generators. The change in active-power sharing is less appreciable. There will be a slight decrease in terminal voltage magnitude also.

40. What steps are to be taken before disconnecting one Alternator from parallel operation?

The following steps are to be taken before disconnecting one Alternator from parallel operation

•    The prime-mover input of the outgoing generator has to be decreased and that of other generators has to be increased and by this the entire active-power delivered by the outgoing generator is transferred to other generators.

•    The excitation of the outgoing generator has to be decreased and that of other generators have to be increased and by this the entire reactive-power delivered by the outgoing
generator is transferred to other generators.

•    After ensuring the current delivered by the outgoing generator is zero, it has to be disconnected from parallel operation.
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Question & Answers About Electrical Machines-II (part-3)

21. Upon what factors does the load angle depend?

The magnitude of load angle   increases with increase in load. Further the load angle is positive during generator operation and negative during motor operation.

22. An Alternator is found to have its terminal voltage on load condition more than that on no load. What is the nature of the load connected?

The nature of the load is of leading power factor , load consisting of resistance and capacitive reactance.

23. Define the term voltage regulation of Alternator.

The voltage regulation of an Alternator is defined as the change in terminal voltage from no-load to load condition expressed as a fraction or percentage of terminal voltage at load condition ; the speed and excitation conditions remaining same.

Voltage regulation in percentage , URP = [(|E|-|U|)/|U| ]x i00

24. What is the necessity for predetermination of voltage regulation?

Most of the Alternators are manufactured with large power rating , hundreds of kW or MW, and also with large voltage rating upto 33kV. For Alternators of such power and voltage ratings conducting load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.

25. Name the various methods for predetermining the voltage regulation of 3-phase
Alternator.

The following are the three methods which are used to predetermine the voltage regulation of smooth cylindrical type Alternators
•    Synchronous impedance / EMF method
•    Ampere-turn / MMF method
•    Potier / ZPF method

26. How synchronous impedance is calculated from OCC and SCC?

Synchronous impedance is calculated from OCC and SCC as
|Zs| = E0/Isc(for same If)
A compromised value of Zs is normally estimated by taking the ratio of (E0/Isc)
at normal field current Ifn. A normal field current Ifn is one which gives rated voltage
Ur on open circuit.
|Zs| = Ur/Iscn

27. What are the advantages and disadvantages of estimating the voltage regulation of
an Alternator by EMF method?


Advantages:


•    Simple no load tests (for obtaining OCC and SCC) are to be conducted
•    Calculation procedure is much simpler

Disadvantages:

•    The value of voltage regulation obtained by this method is always higher than the actual value

28. Why is the synchronous impedance method of estimating voltage regulation considered as pessimistic method?

Compared to other methods, the value of voltage regulation obtained by the synchronous impedance method is always higher than the actual value and therefore this method is called the pessimistic method.

29. In what way does the ampere-turn method differ from synchronous impedance method?

The ampere-turn /MMF method is the converse of the EMF method in the sense that instead of having the phasor addition of various voltage drops/EMFs, here the phasor addition of MMF required for the voltage drops are carried out. Further the effect of saturation is also taken care of.

30. What are the test data required for predetermining the voltage regulation of an
Alternator by MMF method?
Data required for MMF method are :
•    Effective resistance per phase of the 3-phase winding R
•    Open circuit characteristic (OCC) at rated speed/frequency
•    Short circuit characteristic (SCC) at rated speed/frequency
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Question & Answers About Electrical Machines-II (part-2)


11. What is distributed winding?

When coil-sides belonging to each phase are housed or distributed in more than one slot under each pole region then the winding is called distributed winding
A full pitch coil has width of coil otherwise called coil-span as 180ยบ
Where   - angle between adjacent slots in electrical degree and x=
1,2,3…

12. Why is short pitch winding preferred over full-pitch winding ?

 Advantages


•    Waveform of the emf can be approximately made to a sine wave and distorting harmonics can be reduced or totally eliminated.
•    Conductor material , copper , is saved in the back and front end connections due to less coil-span.
•    Fractional slot winding with fractional number of slots/phase can be used which in turn reduces the tooth ripples.
•    Mechanical strength of the coil is increased.

13. Write down the formula for distribution factor.
Kd = sin(m b /2) / m sin(b /2) or Kdn = sin(mn  b /2) / m sin(n   b/2)

m= number of slots/ pole/ phase
  b- angle between adjacent slots in electrical degree
n = order of harmonic
 
14. Define winding factor.
The winding factor Kd  is defined as the ratio of phasor addition of emf induced in all the coils belonging to each phase winding to their arithmetic addition.

15. Why are Alternators rated in kVA and not in kW?
 
The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class.

Apart from the constant loss incurred in Alternators is the copper loss, occurring
in the 3 –phase winding which depends on I2 R, the square of the current delivered by the generator. As the current is directly related to apparent – power delivered
by the generator , the Alternators have only their apparent power in
VA/kVA/MVA as their power rating.

16. What are the causes of changes in voltage in Alternators when loaded?
Variations in terminal voltage in Alternators on load condition are due to the following three causes:
•    Voltage variation due to the resistance of the winding, R

•    Voltage variation due to the leakage reactance of the winding, Xt
•    Voltage variation due to the armature reaction effect, Xa 

17. What is meant by armature reaction in Alternators?
The interaction between flux set up by the current carrying armature
conductors     a and the main field flux     m is defined as the armature reaction. 

18. What do you mean by synchronous reactance?
 
Synchronous reactance X s= (Xl + Xa)
The value of leakage reactance Xl is constant for a machine based on its
construction. Xa depends on saturating condition of the machine. It is the
addition of Xa , which represent the armature reaction effect between two
synchronously acting magnetic fields that makes the total reactance Xa to be called syncheornous reactance. 
19. What is meant by synchronous impedance of an Alternator?
The complex addition of resistance, R and synchronous reactance , jXs can be represented together by a single complex impedance Zs called synchronous impedance.
In complex form     Zs = (R + jXs )
In polar form     Zs = | Zs | <
Where     | Zs | =  ¥(52  + X2  )
And           tan-i  (Xs /R)
 
20. What is meant by load angle of an Alternator?
The phase angle introduced between the induced emf phasor, E and terminal voltage phasor , U during the load condition of an Alternator is called load angle.
 
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Question & Answers About Electrical Machines-II (part-1)

1.   Why almost all large size Synchronous machines are constructed with rotating field system type?

The following are the principal advantages of the rotating field system type
construction of Synchronous machines:
•    The relatively small amount of power, about 2%, required for field system via slip-rings and brushes.
•    For the same air gap dimensions, which is normally decided by the kVA rating, more space is available in the stator part of the machine for providing more insulation to the system of conductors, especially for machines rated for 11kV or above.
•    Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.
•    Stationary system of conductors can easily be braced to prevent deformation.
•    It is easy to provide cooling arrangement for a stationary system of conductors.
•    Firm stationary connection between external circuit and system of conductors enable he machine to handle large amount of volt-ampere as high as 500MVA.
 

2.   Write down the equation for frequency of emf induced in an Altenator.
 
Frequency of emf induced in an Alternator,f ,expressed in cycles per second or
Hz, is given by the following equation
F = (PN)/120 Hz,
Where P- Number of poles
N-Speed in rpm

3.   How are alternators classified?
 
According to type of field system
•    Stationary field system type
•    Rotating field system type

According to shape of field system
•    Salient pole type
•    Smooth cylindrical type

4.   Name the types of Alternator based on their rotor construction.
Alternators can be classified into the following two types according to its rotor construction
•    Smooth cylindrical type alternator
•    Salient pole alternator

5.   Why do cylindrical Alternators operate with steam turbines?

Steam turbines are found to operate at fairly good efficiency only at high speeds. The high speed operation of  rotors tends to increase mechanical losses and so the rotors should have a smooth external surface. Hence, smooth cylindrical type rotors with less diameter and large axial length are used for Synchronous generators driven by steam turbines with either 2 or 4 poles.

6.   Which type of Synchronous generators are used in Hydro-electric plants and why?

As the speed of operation is low for hydro turbines use din Hydro-electric plants, salient pole type Synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.

7.   What are the advantages of salient pole type construction used for Synchronous machines?

Advantages of salient-pole type construction are :
•    They allow better ventilation
•    The pole faces are so shaped that the radial air gap length increases from the pole center to the pole tips so that the flux distribution in the air-gap is sinusoidal in shape which will help the machine to generate sinusoidal emf
•    Due to the variable reluctance the machine develops additional reluctance power which is independent of excitation

8.   Why is the stator core of Alternator laminated?

The stator core of Alternator is laminated to reduce eddy current loss.

9.   How does electrical degree differ from mechanical degree?

Mechanical degree is the unit for accounting the angle between two points based on their mechanical or physical placement.

Electrical degree is used to account the angle between two points in rotating electrical machines. Since all electrical machines operate with the help of magnetic fields, the electrical degree is accounted with reference to the magnetic field. 180 electrical degree is accounted as the angle between adjacent North and South poles.
 
10. What is the relation between electrical degree and mechanical degree?

Electrical degree   e and mechanical degree   m are related to one another by the number of poles P, the electrical machine has, as given by the following equation
e   (P/2)   m 
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Build an electronic workbench tools PART-2

D. PART LIST
        Here are the part list for each tools :


  1. Power Supply
    • Resistor :
      • R1, R2, R7, R8 = 220 Ohm, 1/4W, 10% ...... 4 pcs
      • R3, R4 = 680 Ohm, 1/4W, 10% .................. 2 pcs
      • R5, R6 = 1k, 1/4W,10% .............................. 2 pcs
      • R9 = 47 Ohm, 20W, 10% ........................... 1 pcs
      • R10 = 33 Ohm, 10W, 10% .......................... 1 pcs
      • Variable VR1, VR2 = 5k .............................. 2 pcs
    • Capacitor :
      • C1, C2 = 1000uF/16V ................................. 2 pcs
      • C3, C4 = 1000uF/25V ................................. 2 pcs
      • C5, C6 = 2200uF/50V ................................. 2 pcs
      • C7, C8 = 10uF/16V ..................................... 2 pcs
      • C9, C10 = 10uF/25V .................................... 2 pcs
      • C11, C12 = 10uF/50V .................................. 2 pcs
    • Semiconductor :
      • Diode BD1~BD3 = 1N4007 (4 pcs) .......... 12 pcs
      • LED 5mm, big size, red ............................... 6 pcs
      • Voltage Regulator IC1 = 7805 ..................... 1 pcs
      • Voltage Regulator IC2 = 7905 ..................... 1 pcs
      • Voltage Regulator IC3 = 7812 ..................... 1 pcs
      • Voltage Regulator IC4 = 7912 ..................... 1 pcs
      • Voltage Regulator IC5 = 317 ....................... 1 pcs
      • Voltage Regulator IC6 = 337 ....................... 1 pcs
    • Others :
      • Transformer T1 = P220V/S30V, 1A ............ 1 pcs
      • Fuse & Fuse Holder F1 = 1A, 5mm ............. 1 pcs
      • Switch SW1 = on-off, 220V-AC ................. 1 pcs
      • PCB 1 layer about 10cm x 10cm ................. 1 pcs
  2. Mini Amplifier Design 1
    • Resistor :
      • R1 = 33 Ohm, 1/4W, 10% .......................... 1 pcs
      • Variable VR1 = 10k .................................... 1 pcs
    • Capacitor :
      • C1 = 10uF/10V ........................................... 1 pcs
      • C2, C6 = 100nF .......................................... 2 pcs
      • C3 = 100uF/10V ......................................... 1 pcs
      • C4 = 560pF ................................................ 1 pcs
      • C5 = 47uF/10V ........................................... 1 pcs
    • Semiconductor :
      • Amplifier IC1 = TBA820M (8-pin) .............. 1 pcs
    • Others :
      • Speaker SP1 = 2-1/2", 8 Ohm, 1/4W .......... 1 pcs
      • PCB 1 layer about 10cm x 10cm ................. 1 pcs
  3. Mini Amplifier Design 2 (Alternative)
    • Resistor :
      • R1 = 10 Ohm, 1/4W, 10% .......................... 1 pcs
      • Variable VR1 = 10k .................................... 1 pcs
    • Capacitor :
      • C1, C2 = 10uF/10V ..................................... 2 pcs
      • C3 = 100nF ................................................. 1 pcs
      • C4 = 47nF ................................................... 1 pcs
      • C5 = 220uF/10V.......................................... 1 pcs
    • Semiconductor :
      • Amplifier IC1 = LM386 (8-pin) ................... 1 pcs
    • Others :
      • Speaker SP1 = 2-1/2", 8 Ohm, 1/4W .......... 1 pcs
      • PCB 1 layer about 10cm x 10cm ................. 1 pcs
  4. Signal Generator
    • Resistor :
      • R13 = 1k, 1/4W, 10% ................................. 1 pcs
      • Variable VR1 = 900k (500k) ....................... 1 pcs
    • Capacitor :
      • C22 = 10nF ................................................. 1 pcs
    • Semiconductor :
      • Timer IC1 = 555 (8-pin) .............................. 1 pcs
    • Others :
      • PCB 1 layer aobut 5cm x 5cm ..................... 1 pcs
  5. Logic Probe
    • Resistor :
      • R1 = 220 Ohm, 1/4W, 10% ........................ 1 pcs
      • R2 = 22k, 1/4W, 10% ................................. 1 pcs
      • R3 = 22 Ohm, 1/4W, 10% .......................... 1 pcs
      • R4 = 100 Ohm, 1/4W, 10% ........................ 1 pcs
    • Capacitor :
      • C1 = 1uF/10V ............................................. 1 pcs
    • Semiconductor :
      • Timer TTL IC1 = 74121 .............................. 1 pcs
      • Inverter TTL IC2 = 74LS04 ........................ 1 pcs
      • LED 7-segmen common anode .................... 1 pcs
    • Others :
      • PCB 1 layer about 5cm x 5cm ..................... 1 pcs
  6. 8-Bit Indicator
    • Resistor :
      • R20~R27 = 220 Ohm, 1/4W, 10% ............... 8 pcs
      • R30~R37 = 1k, 1/4W, 10% ......................... 8 pcs
    • Semiconductor :
      • LED 5mm with multi coloured...................... 8 pcs
      • Transistor Q1~Q8 = BC548B ..................... 8 pcs
    • Others :
      • PCB 1 layer about 5cm x 10cm .................. 1 pcs
  7. Completeness
    • Others :
      • Bread Board (GL-No.12) .......................... 3 pcs
      • Hi-Quality IC socket (14 pin) ..................... 1 pcs
      • Hi-Quality IC socket (20 pin) ..................... 4 pcs
      • PCB 1 layer about 3cm x 15cm .................. 1 pcs
      • Mini jack + socket 3mm  ............................ 4 sets
      • Tester terminal check .................................. 1 set
      • Slide swich 2 position (toggle) ..................... 2 pcs
      • Dial button for Variable Reg. ....................... 2 pcs
      • Dial button for Sig-Gen. & Vol. .................. 2 pcs
      • DB-25-F connector ................................... 1 pcs
      • DB-9-F connector ..................................... 1 pcs
      • Mini crocodile clip (black) .......................... 1 pcs
      • Selector switch 3 position (optional) ........... 1 pcs
      • Plastic grid for speaker 8cm x 8cm. ............ 1 pcs
      • Heatsink for voltage regulator ..................... 6 pcs
      • Box size ± 25cm x 35cm x 5 cm ................. 1 pcs 



         


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Transistor Circuits


Types of transistor


There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. This page is mostly about NPN transistors and if you are new to electronics it is best to start by learning how to use these first. The leads are labelled base (B), collector (C) and emitter (E).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!
A Darlington pair is two transistors connected together to give a very high current gain.
In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

                                                              Transistor circuit symbols





Transistor currents

transistor currents The diagram shows the two current paths through a transistor. You can build this circuit with two standard 5mm red LEDs and any general purpose low power NPN transistor (BC108, BC182 or BC548 for example). The small base current controls the larger collector current.
When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.
When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off.
A transistor amplifies current and can be used as a switch.
This arrangement where the emitter (E) is in the controlling circuit (base current) and in the controlled circuit (collector current) is called common emitter mode. It is the most widely used arrangement for transistors so it is the one to learn first


Functional model of an NPN transistor

Functional model of NPN transistor The operation of a transistor is difficult to explain and understand in terms of its internal structure. It is more helpful to use this functional model:
  • The base-emitter junction behaves like a diode.
  • A base current IB flows only when the voltage VBE across the base-emitter junction is 0.7V or more.
  • The small base current IB controls the large collector current Ic.
  • Ic = hFE × IB   (unless the transistor is full on and saturated)
    hFE is the current gain (strictly the DC current gain), a typical value for hFE is 100 (it has no units because it is a ratio)
  • The collector-emitter resistance RCE is controlled by the base current IB:
    • IB = 0   RCE = infinity   transistor off
    • IB small   RCE reduced   transistor partly on
    • IB increased   RCE = 0   transistor full on ('saturated')
Additional notes:
  • A resistor is often needed in series with the base connection to limit the base current IB and prevent the transistor being damaged.
  • Transistors have a maximum collector current Ic rating.
  • The current gain hFE can vary widely, even for transistors of the same type!
  • A transistor that is full on (with RCE = 0) is said to be 'saturated'.
  • When a transistor is saturated the collector-emitter voltage VCE is reduced to almost 0V.
  • When a transistor is saturated the collector current Ic is determined by the supply voltage and the external resistance in the collector circuit, not by the transistor's current gain. As a result the ratio Ic/IB for a saturated transistor is less than the current gain hFE.
  • The emitter current IE = Ic + IB, but Ic is much larger than IB, so roughly IE = Ic.
There is a table showing technical data for some popular transistors on the transistors page.

Darlington pair
touch switch circuit
Touch switch circuit

Darlington pair

This is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together: Darlington pair current gain, hFE = hFE1 × hFE2
(hFE1 and hFE2 are the gains of the individual transistors)
This gives the Darlington pair a very high current gain, such as 10000, so that only a tiny base current is required to make the pair switch on.
A Darlington pair behaves like a single transistor with a very high current gain. It has three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. To turn on there must be 0.7V across both the base-emitter junctions which are connected in series inside the Darlington pair, therefore it requires 1.4V to turn on.
Darlington pairs are available as complete packages but you can make up your own from two transistors; TR1 can be a low power type, but normally TR2 will need to be high power. The maximum collector current Ic(max) for the pair is the same as Ic(max) for TR2.
A Darlington pair is sufficiently sensitive to respond to the small current passed by your skin and it can be used to make a touch-switch as shown in the diagram. For this circuit which just lights an LED the two transistors can be any general purpose low power transistors. The 100kohm resistor protects the transistors if the contacts are linked with a piece of wire.


Using a transistor as a switch

transistor and load When a transistor is used as a switch it must be either OFF or fully ON. In the fully ON state the voltage VCE across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current Ic. The output device switched by the transistor is usually called the 'load'. The power developed in a switching transistor is very small:
  • In the OFF state: power = Ic × VCE, but Ic = 0, so the power is zero.
  • In the full ON state: power = Ic × VCE, but VCE = 0 (almost), so the power is very small.
This means that the transistor should not become hot in use and you do not need to consider its maximum power rating. The important ratings in switching circuits are the maximum collector current Ic(max) and the minimum current gain hFE(min). The transistor's voltage ratings may be ignored unless you are using a supply voltage of more than about 15V. There is a table showing technical data for some popular transistors on the transistors page. For information about the operation of a transistor please see the functional model above.

Protection diode

If the load is a motor, relay or solenoid (or any other device with a coil) a diode must be connected across the load to protect the transistor from the brief high voltage produced when the load is switched off. The diagram shows how a protection diode is connected 'backwards' across the load, in this case a relay coil. Current flowing through a coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

 
Protection diode for a relay


 

 

 

 

 

 When to use a relay

Relay, photograph ยฉ Rapid Electronics
Relay, photograph ยฉ Rapid Electronics
Relays  Photographs © Rapid Electronics 
Transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! Advantages of relays:
  • Relays can switch AC and DC, transistors can only switch DC.
  • Relays can switch high voltages, transistors cannot.
  • Relays are a better choice for switching large currents (> 5A).
  • Relays can switch many contacts at once.
Disadvantages of relays:
  • Relays are bulkier than transistors for switching small currents.
  • Relays cannot switch rapidly, transistors can switch many times per second.
  • Relays use more power due to the current flowing through their coil.
  • Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil. 
    •  

Connecting a transistor to the output from an IC

Most ICs cannot supply large output currents so it may be necessary to use a transistor to switch the larger current required for output devices such as lamps, motors and relays. The 555 timer IC is unusual because it can supply a relatively large current of up to 200mA which is sufficient for some output devices such as low current lamps, buzzers and many relay coils without needing to use a transistor. A transistor can also be used to enable an IC connected to a low voltage supply (such as 5V) to switch the current for an output device with a separate higher voltage supply (such as 12V). The two power supplies must be linked, normally this is done by linking their 0V connections. In this case you should use an NPN transistor.
A resistor RB is required to limit the current flowing into the base of the transistor and prevent it being damaged. However, RB must be sufficiently low to ensure that the transistor is thoroughly saturated to prevent it overheating, this is particularly important if the transistor is switching a large current (> 100mA). A safe rule is to make the base current IB about five times larger than the value which should just saturate the transistor.

Choosing a suitable NPN transistor

The circuit diagram shows how to connect an NPN transistor, this will switch on the load when the IC output is high. If you need the opposite action, with the load switched on when the IC output is low (0V) please see the circuit for a PNP transistor below. The procedure below explains how to choose a suitable switching transistor.
NPN transistor switch
NPN transistor switch
(load is on when IC output is high)
Using units in calculations
Remember to use V, A and ohm or
V, mA and kohm. For more details
please see the Ohm's Law page.
  1. The transistor's maximum collector current Ic(max) must be greater than the load current Ic.
    load current Ic =   supply voltage Vs
    load resistance RL
  2. The transistor's minimum current gain hFE(min) must be at least five times the load current Ic divided by the maximum output current from the IC.
    hFE(min)  >   5 ×     load current Ic  
    max. IC current
  3. Choose a transistor which meets these requirements and make a note of its properties: Ic(max) and hFE(min).
    There is a table showing technical data for some popular transistors on the transistors page.
  4. Calculate an approximate value for the base resistor:
    RB =   Vc × hFE    where Vc = IC supply voltage
      (in a simple circuit with one supply this is Vs)
    5 × Ic
    For a simple circuit where the IC and the load share the same power supply (Vc = Vs) you may prefer to use: RB = 0.2 × RL × hFE
    Then choose the nearest standard value for the base resistor.
  5. Finally, remember that if the load is a motor or relay coil a protection diode is required.
Example
The output from a 4000 series CMOS IC is required to operate a relay with a 100ohm coil.
The supply voltage is 6V for both the IC and load. The IC can supply a maximum current of 5mA.
  1. Load current = Vs/RL = 6/100 = 0.06A = 60mA, so transistor must have Ic(max) > 60mA.
  2. The maximum current from the IC is 5mA, so transistor must have hFE(min) > 60 (5 × 60mA/5mA).
  3. Choose general purpose low power transistor BC182 with Ic(max) = 100mA and hFE(min) = 100.
  4. RB = 0.2 × RL × hFE = 0.2 × 100 × 100 = 2000ohm. so choose RB = 1k8 or 2k2.
  5. The relay coil requires a protection diode.

PNP transistor switch
PNP transistor switch
(load is on when IC output is low)

Choosing a suitable PNP transistor

The circuit diagram shows how to connect a PNP transistor, this will switch on the load when the IC output is low (0V). If you need the opposite action, with the load switched on when the IC output is high please see the circuit for an NPN transistor above. The procedure for choosing a suitable PNP transistor is exactly the same as that for an NPN transistor described above.

Using a transistor switch with sensors

transistor and LDR circuit 1
LED lights when the LDR is dark
transistor and LDR circuit 2
LED lights when the LDR is bright
 
The top circuit diagram shows an LDR (light sensor) connected so that the LED lights when the LDR is in darkness. The variable resistor adjusts the brightness at which the transistor switches on and off. Any general purpose low power transistor can be used in this circuit. The 10kohm fixed resistor protects the transistor from excessive base current (which will destroy it) when the variable resistor is reduced to zero. To make this circuit switch at a suitable brightness you may need to experiment with different values for the fixed resistor, but it must not be less than 1kohm.
If the transistor is switching a load with a coil, such as a motor or relay, remember to add a protection diode across the load.
The switching action can be inverted, so the LED lights when the LDR is brightly lit, by swapping the LDR and variable resistor. In this case the fixed resistor can be omitted because the LDR resistance cannot be reduced to zero.
Note that the switching action of this circuit is not particularly good because there will be an intermediate brightness when the transistor will be partly on (not saturated). In this state the transistor is in danger of overheating unless it is switching a small current. There is no problem with the small LED current, but the larger current for a lamp, motor or relay is likely to cause overheating.
Other sensors, such as a thermistor, can be used with this circuit, but they may require a different variable resistor. You can calculate an approximate value for the variable resistor (Rv) by using a multimeter to find the minimum and maximum values of the sensor's resistance (Rmin and Rmax):
Variable resistor, Rv = square root of (Rmin × Rmax)
For example an LDR: Rmin = 100ohm, Rmax = 1Mohm, so Rv = square root of (100 × 1M) = 10kohm.
You can make a much better switching circuit with sensors connected to a suitable IC (chip). The switching action will be much sharper with no partly on state.

A transistor inverter (NOT gate)

transistor inverter circuit Inverters (NOT gates) are available on logic ICs but if you only require one inverter it is usually better to use this circuit. The output signal (voltage) is the inverse of the input signal:
  • When the input is high (+Vs) the output is low (0V).
  • When the input is low (0V) the output is high (+Vs).
Any general purpose low power NPN transistor can be used. For general use RB = 10kohm and RC = 1kohm, then the inverter output can be connected to a device with an input impedance (resistance) of at least 10kohm such as a logic IC or a 555 timer (trigger and reset inputs). If you are connecting the inverter to a CMOS logic IC input (very high impedance) you can increase RB to 100kohm and RC to 10kohm, this will reduce the current used by the inverter.
     
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    Build an electronic workbench tools part-1

    ELECTRONIC WORKBENCH


    A. PREFACE 

            One of the most important device for the hobbyist is a bread board. Bread board make it easy to wiring  circuit or to break it up again without the need to solder the components. This topic will cover about how to make some tools to attached to bread board, and this device will make our works better. For this purpose we will call this device as electronic workbench (ewb). In my opinion, the less tools on the working desk is the very good things, because the more tools on the desk will make our mind become dizzy and not concentrate to the circuit that we want to make.
            The tools that we want to accompany to ewb are the simple test devices and power supply. The test devices will be build also not a sophisticated ones, and maybe already there somewhere in your junk box. I never try to make the sophisticated tool with using hard to find parts at the market. So I put the circuits that I already made some times ago and usually already throw to the junk box. It is better to used it again, for recycle and the important purpose. If you don't have circuits like these, you may make a new one. The tools ie :
    1. Power Supply : this device is absolutely necessary for making some experiment later on. Here I choose 3 pairs of power supply ususally used, ie; +/-5V (for TTL circuits), +/-12V (for CMOS circuits) and the variable one (adjustable) up to +/-30V (this is for analog circuits). 

    2. Mini Amplifier : a small mini amplifier, and using popular IC, preserved for 2 type of  designs and both already tried, quiet the same. This amplifier also can used for locating the existing audio signal (frequency) range. 

    3. Signal Generator : a simple signal generator, using popular IC 555 Timer, configured as multivibrator, and the parts was chosen for the audio frequency range (20Hz ~ 20kHz). 

    4. Logic Probe : a simple logic tester, using 7-segmen display. This tool can be used to show 3 state of logic, ie: "L" for low logic, "H" for high logic and blinking "8" for clock logic. 

    5. 8-Bits Indicator : this circuit show the logic up to 8-bit. So we could indicate the state logic from signals, eg : data signals, address signals, etc. 

    6. Connector DB-25 and DB-9 : the popular type connectors for using with interface circuits to or from computer. There both are standard for LPT paralel port and serial COM port connectors. Besides that we can use it for our experiment circuits with microcontroller later on.
            That's the tools which I think most needed untill now, and not necessary need to much fund, but quiet good enough for experiments and for amateur hobbyists.

    B. SCHEMATIC 

            Over all the circuits for this ewb is like this :





    Usually, positive power supply often to be used, but for the negative seldom, so for you who don't need it, can be omitted, but infact that it would be need one day. For mini amplifier, you can use ex radio amplifier or from ex active computer speaker not used anymore. The pupose is depend on the needs. It can be used for tracing the signal. Signal generator needs in experiment to produce some pulses or as oscillator. Logic probe needs to detected logic state from digital circuits. 8-bit indicator usually used for detected visual logic state from many signals, if you want to compare the logic states.


            In the implemented to the ewb device, these tools connected to terminals which I use the very simple way,  using good quality IC sockets, so it would be compatible with bread board working. Pin descriptions used like this :



    Power Supply terminals pun on the left side and for the other tools pun on the right side. More detail can be seen at construction section. It is more wisely to print this picture and put it on the back cover ewb box, so if we forgot the terminal purpose, we can refer to this picture.


    C. LAYOUT 

            Here I already design PCBs for each tools and the last choice depends on your decision to make it with this pcb layout or use some thing else, like veroboard, or direct connection (rat nest). Infact that for my prototype, because for some tools already made before, so the picture prototype not the same with this design. Some PCBs layout not tried yet, please check it by yourself again if you want to use it. Here are these PCBs layout :  






    PCBs  Layout Design

     




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    Low cost 8031 microcontroller development board PART-2

    F. FIRMWARE

     

     

            8 kB ROM for the first moving will contain with monitor program, where this monitor program I take from Paul Stoffregen (using assembly) after modified to fit with 8031 type. The next experiment, this will contain with BASIC interpreter. So the user program can use BASIC language, more easy to learn. Of course the ROM can be make 2 pcs, so the firmware can be changed easily. One using assembly and the other using BASIC language. It is your choice. EPROM can be programmed using commercial EPROM programmer or another diy programmer like my 2764-programmer or my universal eprommer. We also can use EEPROM device like 2864 type to experiment with. EEPROM can be programmed using  Willeprom ver 3.1s like mine.

            Here are the firmware programs :
    1. Modified from Paul design (PaulMon) 

    2. BASIC Interpreter with 8031 basic system (Basic31 V1.2a) 

    3. BASIC Interpreter with 8052 basic system (Basic52 V1.1)
    These are the displays for paulmon monitor program on hyper terminal : 

















         Remember that, paulmon monitor program has autobaudrate detect, so when connected the development board to PC and after run hyper termninal, push the key once until the welcome message appear on the screen. Hyper terminal set to 9600 bps connection.
            Doing more experiment to another monitoring program, you can try these sample :
    1. Minimum Monitor (MinMon) from the book written by Sencer Yeralan 

    2. Mon51 from the book writen by Scott MacKenzie 

            After I searh the net at several old sites (link could be followed below), I got a few resources about 8051 microcontroller. Very much monitor program could be used was made by another people, and all of it is free. Here a few of them (but still in original version, not implemented to my circuit) some simple while others more complex :
    1. Steve 8051 Bootloader Ver 3.0 

    2. Goodhue (dari Signetics Co.) Bootstrap 8051 

    3. Scott Mon51 Ver 12 from the author 

    4. Steve Kemplin MonPlus 8031 Ver 3.1 

    5. Ibrahim AVCI MonXXX 8XC52 

    6. Rigel Corp. rMinMon 8051 from the author 

    7. Ron Stubbers RTC31 Monitor 8051

    8. UltraMon 8051 

    9. Debug51 Monitor Debugger
            If I could have enough time, I will do experiment with those programs monitor later.
            Here are the displays on hyper terminal :













    G. PROTOTYPE

     

            As told above , my prototype size about 8cm x 15 cm and work nicely. To test the board works or not, you can use the sample program like this that showing the big text to monitor display.

    Fig 1. Development Board 8031-ah














    Beside using 8031 microcontroller, I also tried another types like : 8051(note : pin-31 tied to ground, so the internal ROM not used), 8952 (note : this type has flash ROM/PEROM), monitor program can be written or erased many times, EPROM not used). All could work successfully. Besides that, we could use flash ROM/EEROM like the type 2864 (8 kBytes), which could written or erased many time also, so we did not bother by reprogramming the EPROM.
    Here is the complete design.
            Between the three board that I design, there is a little difference for the I/O terminals. The difference between each boards I/O could be seen here.

    H. ADD-ON CARDS

            As the time goes by, I made one by one the expansion board (module) for any project as I needed it. Here are the collections of the add-on cards, and I will update it as I make the new one :

    I. APPLICATION

            First application that I have built was running the 24 leds from PPI-8255 port. This program also can be used for testing the PPI-8255 port as that is already works properly or not. Here are the applications for this running leds project.





        Experiment later with this board :
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