Pages 22 to 41

                                                      Pages:    1 to 21

                                                      Pages:    42 to 62
                                                      Pages:    63 to 85


The pages can be printed and collated into a book for easy reference.

More circuits and projects can be found on TALKING ELECTRONICS website:

Colin Mitchell

Tel: 0417 329 788




AC Trigger   
Arc Welder
Audio Alarm  
Audio Mixer
BC 547    NPN transistor
BC557     PNP transistor
BD 139   BD140 transistor
Bipolar Transistors  
CA 3130 OP-amp
Capacitor Data
Car Voltage Converter 
Chip Pinouts
Circuit Symbols 
Common Base/Emitter/
Collector Circuits 
Crystal Set   
Darlington Transistor  
DC Millivoltmeter  
Difference Amplifier
Dual Power Supply
Electronic Dice   
FET Voltmeter  
Field Strength Meter MkII
Flashing LED  
FM Transmitter    FM Bug
Germanium Diode 
Hearing Aid
Infinity Bug
Infrared LED
Infrared Light Beam 
Kitt Scanner
Lamp Dimmer   
Lamp Flasher 
Light Beam Relay  
Light Switch  
LED Chaser
LED Dice
LED Flasher
LM 340  
LM 386
Logic Gates
Logic Probe  
Metal Detector 

20, 21


Morse Code Generator
MPF 102  FET
OA 91  diode 
OP Amp  
Peak Reading VU Meter
Photo Diode  
Photo Electric Relay 
Power Diode  
Power Supply
Relay Driver
RF Monitor Meter  
RF Prescaler 
SCR   C122D
Silicon Controlled Rectifier 
Regulator 78xx  79xx
Signal Diode   
Semiconductor Devices  
Solar Charger
Square Wave Oscillator
Steam Simulator
Surface Mount Resistors
Time Delay  
Transistor Amplifier
Transistor Pinouts 
Touch Switch   
Ultrasonic Transmitter
Unijunction Transistor 
UJT Time Delay 
Wein Bridge Oscillator  
Zener Diode   
1N4001   Power diode
1N4148  Signal diode
27MHz Links
2N2646  UJT
2N 3055   transistor
4017   Decade counter
555 Light Switch   
555 Timer  
7-Segment Displays
74c14 Hex Schmitt 
741   OP Amp
7555   CMOS 555
7805   +5v  Regulator
7905   -5v  Regulator

22, 60
8, 10


Page 22

2N3055 Power Transistor
The 2N3055 is a medium speed NPN Silicon Power Transistor designed for general purpose switching and amplifier applications.


   DC current Gain (hFE) = 20-70 @ lc = 4.0A
  Collector-Emitter Saturation Voltage = 1.0V @ lc = 4.0A

Text Box: Absolute Maximum Ratings
Collector-Emitter Voltage (VCEO)      60v
Collector-Base Voltage (VCBO)        100v
Emitter-Base Voltage (VEB)              7.0v
Collector Current Continuous (lc)     15A
Base Current Continuous (IB)            7A
Total Device Dissipation (Ptot)        115W
Collector- Emitter Leakage Current 
(VCE = 30V, lB = 0)                                               0.7mA
DC Current Gain (HFE) 
lc = 4.0A     VCE = 4.0V                                             20-70
lc = 10.0A    VCE = 4.0V                                  5 (Minimum)
CoIlector-Emitter Saturation Voltage 
IC = 4.0A    IB = 0.4A                                          1.1v
lc = 10.0A   IB = 0.4A                                          8.0V
Ft @ IC = 3.3A                                                    0.8MHz






Low Ripple Regulated Power Supply
The excellent characteristics of the 2N3055 at high currents (high hFE and low collector-emitter saturation voltage) makes it ideal as a series regulator transistor in regulated power supplies. The power supply circuit shown below can be used when high current with low ripple is required. Q, and Q2 form a high power Darlington. ZD1 and R1 provide a reference voltage at the base of Q1 The voltage output will be:-       VOUT  =  Zener Voltage - 1.2v

Page 23
Car Voltage Converter for radios and cassettes
This circuit is suitable for dropping a 12v car battery to the correct voltage to run portable cassette players/radios etc. Using a 2N3055 might seem like a bit of an overkill but they are cheap. The output voltage will be 0.7v lower than the zener voltage, due to the voltage drop across the base-emitter junction of the 2N3055. The 10 ohm series resistor stops excessive current being drawn in the case of a short. The diode (1N4001) protects the transistor in case of reverse voltage being applied.


The output will drive transistor radios, cassette players etc. If the current drain is over 500mA, it is a good idea to put a heat sink on Q1. Mounting the converter in a metal box with Q1 on the lid (but insulated from it with a mica washer) will act as a good heatsink.


2N2646 Unijunction transistor
The 2N2646 is intended for general and industrial triggering and oscillator circuits where circuit economy is of primary importance. lt is a high speed switching device with a low saturation voltage.

Absolute maximum ratings  

Power Dissipation                  300mW
RMS Emitter Current                50mA
Peak Emitter Current
(Capacitor discharge <10F)        2A
Emitter Reverse Voltage              30V
Interbase Voltage                         35V


Intrinsic Standoff Ratio (VBB = 10v)                            ή               0.69
Interbase Resistance (VBB = 3v, Ie = 0)                      RBB0           6.7 
Emitter Saturation Voltage (VBB = 10v, IE = 50mA)    VE(sat)   
Emitter Reverse Current (VB2E = 30V IB1 = 0)             IE0              .001
Peak Point Emitter Current (VBB = 25v)                      IP                0.8
Valley Point Current (VBB = 20v RB2 = 100R)              IV                  5
Base-One Peak Pulse Voltage                                   V0B1             8.5

Basic Theory
The unijunction transistor (UJT) has 3 terminals: Emitter (E). Base-one (B1 and Base-two (B2). Between B, and B2 the UJT has a resistance of from 4.7k to 9.1k.
In operation the UJT emitter voltage VE is lower than the emitter peak voltage VI. The emitter will be reverse biased and only a small leakage current will flow. When VE equals VI the emitter current will increase enormously. At the same time the emitter-B1 resistance will fail to a very low level.

 Page 24

 Basic UJT Pulse Trigger Circuit

This is a basic relaxation oscillator. C charges through R, until the emitter reaches VP at which time the UJT turns on and discharges C1 via RB1. When the emitter has dropped to approximately 2v, the emitter stops conducting and the cycle starts again.

The design of the UJT trigger is very broad.
RB1 is limited to values below 100 ohms for most applications. R1 should be a value between 3k and 3M. Supply voltage can be from 10 to 35v. If the circuit is being used to trigger an SCR, RB1 must be low enough to prevent DC voltage at the gate from exceeding the maximum voltage that will not trigger the SCR. In practice, keep RB, below 50 ohms.
The 2N2646 is specifically designed for SCR trigger circuits. RB2 is typically 100 ohms.

Page 25
UJT/SCR Time Delay Relay

This circuit provides an efficient, high power and accurate time delay circuit. The SCR should be selected to suit the application. R5 and the zener diode maintain a stable supply for the UJT.


Initially the SCR is off. The timing sequence is started by shorting out C1. C1 then charges through R1 and R2 until the UJT triggers, developing a pulse across R4 which turns on the SCR. Holding current for the SCR is supplied by current through R5 and D2. When the SCR triggers, it pulls the voltage across the UJT to <2 volts. This discharges C1.
The load this circuit will drive depends on the SCR used. A suitable type would be a C106Y. This has a maximum current rating of 4A. This would be enough to drive a relay (even one with a low coil resistance), globes or an electric bell.

This is the simplest metronome circuit. It produces a 'click' similar to that of the traditional mechanical device. The rate is variable from 40 to 220 beats per minute. R1 sets the high rate limit and R2 the low rate limit. Virtually any speaker is suitable. Supply voltage is from 12 to 18v. While an 8 ohm speaker is suitable in this circuit, more volume and higher efficiency can be obtained with a high impedance speaker, such as a 40 ohm unit.

Page 26

MPF102, 5, 6 Field Effect Transistors

The MPF102-6 series are N-channel Junction-type field effect transistors.
The FET is a three terminal semiconductor device. Input voltage is applied to a GATE terminal and controls the current flowing from SOURCE to DRAIN terminals.
An important feature of the FET is its very high input impedance. Since the FET makes use of a small input voltage to control a large output current, its gain is specified in terms of TRANSCONDUCTANCE. Transconductance (gfs) is equal to the change in drain current (dI0) divided by the change in gate voltage (dVG) and the formula is usually written as follows:-

gfs = 100Q(dlD/dVG) where:
gfs is the transconductance in micromhos
ID is the drain current in DC mA
VG is the gate/source voltage in DC volts.

Definitions of specifications

VGS (Gate/Source Voltage)

This is the maximum voltage which may appear between gate and source. IDSS (Drain current at zero gale voltage)
This is the current which will flow in the drain/source circuit when VGS = 0. It is given for specific drain/source voltages.
BVGSS (Gate/Source breakdown voltage).
The voltage at which the gate junction of a JFET will enter avalanche. Vp (Gate/Source pinchoff voltage).
This is the gate-to-source voltage at which the field just closes the conduction channel. This is given for a specified value of VDS. The value of the drain current is specified (usually 1A).


FET Type





Ptot (mW)


25v @ IG 1A

0.5-8.0V @ VDS 15v

2-20mA @ Vus 15v










25v @ IG 1A

0.5-8.0V @ VDS 15v

4-16 mA @ Vus 15v










25v @ IG 1A

0.5V-4V @ VDS 15v

4-10mA @ Vus 15v



All types are mounted in T092 plastic cases with pin connections as shown above.


Operation and Applications

The basic mode of operation of the FET amplifier is shown below. This is referred to as the common source amplifier. The gate to source circuit is the input and the drain to source circuit is the output.

When a moderate reverse or negative voltage is applied between gate and source, the gate junction becomes 'reverse biased' i.e. the voltage on the gate reduced the current flowing between the source and the drain. At a higher gate-source voltage, the drain-source current is cut to practically zero. This is referred to as the gate-source pinchoff voltage and is listed in the specifications as VP at a drain-source current of either 1 or 10uA. In practical circuits, the DC bias is developed across R2, due to the current being through it. This then puts the source at a positive potential relative to ground. The gate is at ground potential and therefore is at a negative potential relative to the source, R, sets the input impedance of the circuit since the gate of the FET draws virtually no current at all and so is seen by the load as a very high impedance.


Page 27


All the circuits and applications in these pages assumes the use of 'N-channel' Junction FETs, i.e. FETs in which the drain-source material is made of N-type silicon. However, these JFETs may be replaced in the circuits with P-channel JFETs if the polarity of the power supply is reversed.

Typical Design for a Common-Source Circuit

When used as an amplifier, the FET is biased to a certain part of its response curve for lowest distortion and maximum available voltage swing. Assume that the FET has the following operating parameters

  VDs = 8V (where VDs is the voltage between drain and source)
   ID = 0.5mA (where ID is the drain current)
VGS = -2V (where VGS is the gate-drain voltage or bias)
The power supply voltage is 22.5v

FET Applications Source Follower Circuit:

The source follower circuit is suitable where a high input impedance and low output impedance is required, but no voltage gain is needed. The figure below shows a typical source follower stage. Input impedance is set by the gate resistor RG. Output impedance is very low.

Page 28
RF Preselector

The uses for the FET are not limited to audio applications. The circuit below is for an RF preselector (a tuned amplifier) for the broadcast bands. The FET is a very good device to use in this application, due to its low cross modulation characteristics. Most cheaper receivers use ordinary bipolar transistors to keep costs down. The FET RF amplifier can also take higher signal levels without distortion. The preselector has a Volume Control style gain control between the FET and the emitter follower output stage. This means that only the FET has to handle high signal levels.

The tuning capacitor does not have to be exactly the same value as shown in the circuit, any capacitor covering a similar range is suitable. The aerial coil is wound on a 200mm length of ferrite rod. The main winding consists of 42 turns of 22B&S enamelled wire. The second winding consists of a further 6 turns. The preselector gives a marked improvement on the reception of weak signals and aids in the attenuation of adjacent channel interference and noise.


LDR Applications Light Beam Relay

In this circuit the LDR is held at a low resistance by light from a small globe. The circuit is actuated when the beam is broken. The resistance of the LDR then goes high. The circuit is set up so that with the light shining on the LDR the input voltages at the two input terminals of the 741 op amp hold its output 'low'. When the LDR goes to high resistance the op amp's output goes 'high'. This turns the transistor 'on' and pulls in the relay.

Simple timer

The very high impedance of the FET makes it suitable for a wide variety of timer circuits. The circuit below gives one such example. With C1 given a value of 1F, it will give timing periods of 40 sec, and with a value of 100F it gives a period of 35 minutes. The FET is wired as a source follower and has its gate taken to the junction of a time constant network R1-C1 When the supply is first connected, C1 is discharged, so Q1 gate is at ground potential, and the source is a volt or two higher. The base of Q1 is connected to the source of Q1 via R3, so Q2 is turned on and 12v is across R5, When the supply is connected, C1 starts to charge via R1, so the voltages on the gate of Q1 (and on the source) rise exponentially towards the 12v supply. When the voltage reaches approximately 10.5v the bias on Q1 falls to zero and Q2 switches off, the voltage across R5 falls to zero.

 Page 29

FET Voltmeter

The very high input impedance of the FET makes it the ideal basis of a voltmeter. The circuit below has a basic sensitivity of 22M ohms per volt. Maximum full scale sensitivity is 0.5V, and input sensitivity is a constant 11.1 M ohms on all ranges. R7,R8 R9  form a potential divider across the 12v supply. R8 is adjusted for a zero meter deflection. Any potential across the gate circuit of Q1 causes the circuit to 'unbalance'. To avoid drift, the power supply should be stabilized if possible.

555 Light Switch

The use of the 555 timer 1C with an LDR provides a high performance light switch.
An LDR is a Light Dependent Resistor and is a very low cost way of detecting light.
The 555 is used with its trigger and thresholds tied together to provide a Schmitt trigger with a very low input current but which can drive a relay taking up to 200mA of current. The trigger is activated when the light level on the LDR falls below a predetermined level. The relay energizes when the voltage on pins 2 and 6 is greater than 2/3Vcc. It de-energizes when the voltage falls below 1/3VCC. This gives a hysteresis of 1/3VCC.


The 555 can supply current up to 200mA, so the relay type is not critical. Any with a coil resistance from 100-280 ohms would be suitable.

Page 30
Light Sensitive Switch

This circuit makes use of the wide change of resistance of the LDR. Between positive and negative supply there is a voltage divider. The bottom section is a variable resistor RV1. The top half is formed by the LDR and a 4.7K ohm resistor in series. In low light conditions when the resistance of the LDR is very high, the bias to the Darlington pair formed by TR1 and TR2 is very low, and they do not conduct. When the light level rises, the resistance of the LDR falls. This turns the transistors 'on' and pulls in the relay.

 The LDR should be an ORP12 or similar. The relay should have a pull in voltage of 9V or lower and a coil resistance of 280 ohms or higher.

Photo Electric Relay

This circuit is basically a bistable multivibrator. When the light level is low and the resistance of the ORP12 is high, transistor Q1 conducts and Q2 is off. As the level of illumination increases the resistance drops until Q1 cuts off and Q2 turns on, energizing the relay coil.

 The relay should have a coil resistance of 180 ohms or higher and a pull in voltage of 9V or lower

Page 31

Text Box:



   Low power consumption

   IC compatible

   Long life


Absolute Maximum Ratings







Reverse Voltage






Av forward Current






Peak Forward Current






Power Dissipation







Page 32 

LED Basics
Text Box: Specifications
Forward Voltage (IF = 20mA)          
Red                      1.7v Typ.      2.0v Max 
Green                   2.2v Typ.     2.8v Max
Yellow                  2.1v Typ.     2.8v Max
Amber                  2.1v Typ.     2.8v Max
Orange                 2.0v Typ.      2.8v Max

Peak Emission Wavelength
Red                       697nm
Green                    565nm
Yellow                   585nm
Amber                   600nm
Orange                   635nm
Note: Luminous intensity is measured with a light sensor and filter combination that approximates the CIE eye response curve. 








LEDs are used in the 'forward biased' mode. i.e. positive on the anode and negative on the cathode. This voltage drop is stated in the specifications (eg 1.7V for a red LED), If the LED is used on a higher voltage than this, a current limiting resistor must be used.

The following formula can be used:-

R = (E -1.7) x 1000/I

R is the resistance in ohms. E is the DC supply voltage. I is the LED current in milliamps.    
A common LED current is 20mA.
Some calculated values are:-

For   6v use 220 ohm.
For   9v use 390 ohm.
For 12v use 560 ohm.
For 24v use 1.2k ohm.

If a LED is reverse biased, it will break down, in a similar way to a zener diode. This occurs at 3-5V. It usually damages the diode if a high current flows.


Operating LEDs from the mains

This circuit uses a capacitor as a voltage dropping element. A 1N4148 diode is placed across the LED for rectification. As the voltage across the LED is negligible compared with the supply, capacitor current is almost exactly equal to mains voltage divided by the capacitor reactance. At 50Hz, a 0.47F will result in a LED current of about 16mA. Resistor Rs limits current on transients. A value of 270 ohms is adequate.

Page 33

The Flashing LED
The Flashing LED has a chip inside the device to produce the flash-rate. Simply connect the LED to a supply voltage (4v to 10v) and the LED will flash at a rate of approx 2Hz. No external resistor is needed up to 10v. For voltages higher than 10v, the resistor should be 100 ohms for each volt above 10v.
This is the only LED that does not need a resistor when connected to a supply as it has an internal resistor. All other LEDs MUST have a resistor in series to limit the current and prevent damage.



LED Flasher
This circuit for a LED flasher is very simple and cheap to make and will work on any voltage between 3v and 12v. As the voltage is raised the value of R1 must be increased - The speed can be changed by altering the value of C1 and C2 and/or R2 or R3. Raising the value of C1 and C2 slows the rate down. Raising the value of R2 and R3 also slows it down.



LED Chaser
This circuit acts as a LED chaser. The 4017 is driven by a 555 working as a free-running multivibrator. The speed can be changed by altering C1 or R1.

Page 34

CQY89 Light Emitting Diode -  Infrared LED
The CQY89 is an infrared LED, similar in performance to conventional LEDs, but emitting light in the infrared region. This is visible to the human eye. Unlike conventional LEDs, infrared LEDs are usually pulsed rather than fed with continuous DC. They find wide use in alarms and in remote control equipment.



Maximum Forward Current        130mA
Maximum Reverse Voltage         5V
Maximum Power Dissipation      215mW
Maximum Forward Current        130mA
Beamwidth between half intensity directions (IF = 100mA]       40 typ.
Wavelength at peak emission (lr = 100mA) (λpk)                   930nm typ.

BPW34 photosensitive diode
This device is mainly used in combination with a light source for go/no go detection as in card readers and industrial safety devices.


VR Forward voltage                           32V
Total power dissipation                     150mW
Spectral sensitivity (VR =5V)               70nA/lx
Dark Reverse Current (VR=10; Ee=0)    2nA
Light Reverse Current
(VR = 5; Ee= 1mW/cm2;λ = 930nm)       10A


BPW34/CQY89 Infrared light-beam relay

The light is picked up by the photodiode a BPW34. It is wired so that a current is generated that is proportional to the light falling on it. The FET acts as a source follower and impedance matches to the next stage. The amplifier after this acts as a bandpass filter. Its output is coupled to a CMOS Schmitt trigger, followed by a rectifying circuit and a pulse stretcher. This drives a transistor and a buzzer and LED.

Page 35



A CMOS oscillator drives an output stage consisting of a BC547 transistor and two CQY89 infrared LEDs. Current drive is limited by the 680 ohm resistor. If greater range is required, this resistor may be reduced to a minimum of 150 ohms with a consequent increase in current consumption.


 7 Segment LED Displays
The 7 segment display is found in many displays such as microwaves, lifts, ovens etc. It consists of 7 LEDs that have been combined into one case to make a convenient device for displaying numbers and some letters. There are basically two different size displays.  0.3 and 0.5. The two sizes are shown below:

                0.3" and 0.5"displays

Displays come is a range of colours and brightness levels.
Most come in super-bright and these are preferred so the display can be seen during the day. They are not much more expensive but give a much better illumination.
All displays also come in COMMON CATHODE and COMMON ANODE. 
The COMMON CATHODE display has all the cathodes of the LEDs tied together and connects to the pin that goes to the 0v rail. This is the most common type of display.

Page 36

The Common Cathode and Common Anode displays are wired as shown below:

The project above from JJM turns on each segment of the display to show how each letter and number is produced.  The second photo is a white 7-segment display.

Electronic Die

This circuit consists of three sections: an oscillator, a counter, and the display. The oscillator uses three sections of a 4069 hex inverter. The 4029 is a four bit counter with the capacity to count from zero to 15. The 4511 driver/decoder takes binary output and decodes it to drive a seven segment display. The current to the 7-segmenl display is limited by seven 560 ohm resistors. The display is a common cathode type, and any 7-segment display can be used.


Page 37


This circuit uses a 7-segment display as the output of a basic counter circuit. The 7490 counts decimal pulses and converts them to a BCD code. Its output is fed to a 7475 latch. This stores the outputs from the decade counter. The four binary outputs are taken from the 7475 to a 7447 LCD to the 7 segment LED decoder, which drives the display. 




Page 38

The SC151 D is a medium power plastic package TRIAC designed for economical mains power and lighting control. Unlike SCRs, the SCI 51 D is a bidirectional thyristor - when triggered, it conducts in both directions and can be triggered by a positive or negative gate signal. TRIAC (Triode AC Semiconductor). The diagram below shows the V/l characteristics of the Triac. A gate current of the specified level of either polarity will trigger the triac into conduction in either quadrant, provided the applied voltage is less than VB0.

Triggering may be from DC, rectified AC or pulse sources such as unijunctions, neon lamps or breakdown devices such as the ST4.


Voltage Rating                                                                             400V
Current Rating                                                                             15A RMS
ITSM Maximum peak one cycle non rep. surge current                 110A
IDRM Blocking Current at 25C                                                      0.1mA max
dv/dt Off State, Tc = 100C, Rated VDRM, gate O/C                      250V/S (typ.)


IGT Max DC Gate Trigger current VD=12v, 25C                           50mA
VGT Max Gate Trigger voltage VD= 12v  25C                              2.5V

Triac as a switch

This gives improved performance over a conventional switch, as there can be no arcing or contact bounce. This circuit shows a simple three position power control. In position one there is no gate connection, so power is off. In position two there is gate current during one half cycle only and load power is half wave. In position three the gate is triggered on both half cycles and the power is full on. For a simple on-off switch, just delete the diode.

Because the contacts only carry current for the few microseconds needed to trigger the triac, the actual switch can be almost any small device: reed relays, thermostats, pressure switches or program/timer switches.


Page 39
Lamp dimmer/Heater controller

R1 and C1 are a phase shift network - they produce a variable delay in the waveform applied to the ST4 and hence the triac. When the voltage across C1 reaches the breakdown voltage for the ST4, C1 partially discharges into the triac gate through the ST4. This pulse triggers the triac into conduction for the remainder of the half cycle.

This easy-to-build controller is ideal for dimming lights, and controlling the output of electric heating type appliances. The light or heater element etc is placed where the 'LOAD' is marked on the circuit.

ST4 Asymmetrical AC Trigger Switch

The ST4 is an integrated triac trigger circuit that provides wide range hysteresis-free control of voltage.
It behaves like a zener diode in series with a silicon bilateral switch (a symmetrical device).
The zener provides asymmetry since the switching voltage is increased in one direction by the zener breakdown voltage.

Switching voltage:
VS1                          14v-18v
VS2                                            7-9v

Switching current
I S1   I S2                                80A

 On-state voltages

VF1   (I = 100mA)    7-10v
VF2   (I = 100mA)    1.6v max
Peak pulse voltage
V0                                3.5v min

Page 40

C122D/C122E Silicon Control Rectifier        

 The C122Dand 122E are medium power plastic package SCRs designed chiefly for mains power and motor control. The SCR is a unidirectional device, (current flows through it in one direction, from anode to cathode).

The SCR is a three terminal semiconductor device. The three terminals are the anode (A), cathode (K). and the gate (G). With no voltage applied to the gate terminal, if a voltage is applied to the SCR anode and cathode terminals, (anode positive with respect to cathode) current flow is prohibited. If the supply is reversed the flow is likewise prohibited. Thus with no signal applied, the SCR appears as an open circuit as long as its diode junctions do not break down. The SCR is brought into conduction by applying a current into the gate terminal. This will cause it to conduct in the forward direction (i.e. with the anode positive and the cathode negative). The gate voltages required vary from approximately 1.5- 6.0v. Once the SCR is turned on the gate no longer controls the circuit and the SCR only drops out of conduction when the anode-cathode voltage falls to near zero. At this instant, the current through the device falls to zero.


(Repetitive off state voltage. Max between anode and cathode)          500V
(RMS Current through SCR)                                                                     8 Amps
IGT (Peak Positive gate current) (Tc = 25C)                                                 25mA
VGT (Peak Positive gate voltage) (Tc = 25C)                                                1.5V
(AV) (Max Gate power)                                                                             0.5W
IH Holding Current (Current below which the SCR will drop out of conduction) (Tc = 25C)   30mA
dv/dt Rate of change of on-state voltage (Max. rate of change of
anode-cathode voltage which will not turn SCR on)                                   50V/sec(typ.)  
The C122D differs only in that its VDRM is only 400v as against 500v for the C122E.

The SCRs listed above are medium power SCRs (Silicon Controlled Rectifiers) designed primarily for economical mains power and motor control. They are three terminal devices (see above). The electrodes are anode, cathode and control gate. They are unidirectional devices i.e when triggered 'on' they only conduct in one direction. The SCR is a 'regenerative' device. It is triggered 'on' by injecting a signal into the gate. As noted earlier, once the gate has triggered the SCR 'on' it no longer controls the gate. The only way to cause the SCR to stop conducting from cathode to anode is to drop the anode cathode voltage to a level where the current flowing from anode to cathode is below the 'holding level'. This is indicated in the figure above. In practice, this is not a problem, since SCRs are normally used to control fluctuating voltages such as the AC mains. The 'drop out' of the SCR occurs as the mains voltage goes through zero.

Page 41


SCRs are current rather than voltage triggered devices. This means that they must be fed from a relatively low impedance source i.e. one in which the voltage won't drop down under load from the gate. In a way analogous to a relay or a solenoid, the SCR requires certain minimum anode current if it is to remain in the 'closed' or conducting state. If the anode current drops below the minimum level, the SCR reverts to the forward blocking or 'open' state. The following circuit shows a basic R-C-Diode trigger circuit giving full half wave control. On positive half cycles the capacitor C will charge to the trigger point, at a speed determined by the time constant of R and C. On the negative half cycle, the capacitor is reset by CR2, resetting it for tire next charging cycle, Thus the triggering current is supplied by the line voltage.


C122D, C122E, C106D SCRs   Phase Control Circuit

Improved phase control circuit

The following diagram shows a circuit using a neon lamp as a breakdown device. This gives smoother control and improved performance. The neon triggers when the voltage across the two 0.1 capacitors reaches the breakdown voltage of the lamp (60-90V). Control extends from 95% to full off. 

The neon lamp phase controlled circuit shown below combines the low cost of the simple RC circuit shown before but gives improved performance. The circuit below gives half wave control from 95% on to full off. Full power can be easily obtained by putting a switch across the SCR. The circuit uses a neon. This gives the following improvements:

 A higher impedance circuit can be used for control.

 As a result, the control element (which is a 100k pot in the circuit below) can be replaced by a high impedance device such as a thermistor or light dependent resistor, for heating or light control applications.





This completes Pages 22 to 41 of Data Book 1

Sept 2008    Nothing is copyright. You can copy anything     Colin Mitchell

                                       to pages:    42 to 62