1. Electronic Troubleshooting
Chapter 13
Power Control Devices

2. Power Control Devices Characteristics
Bipolar and MOSFETs can be used to control large loads and motors
However they only can control DC Loads and Motors
Most large Loads and Motors are AC
Types of devices used to control them are:
SCRs (Silicon Controlled Rectifiers)
TRIACs (TRIODE for AC - ??))
DIACs (Diode for AC - ??)
Ancillary Devices
Reed Switches
Opto-coupliers

3. Power Control Devices Topics covered SCRs TRIACs Phase Control
Ancillary Devices
Reed Switches
Opto-coupliers
Phase Control
Problems with TRIAC and SCR circuits
Characteristics
Used to control current for AC Loads
Sometimes for DC loads

4. Power Control Devices SCRs Characteristics Type of Thyristors
Acts like a switch not as a variable resistance
Key ratings
Maximum Voltage rating – regardless of polarity
30 to 3000V ratings are normal
Maximum voltage without damage or false activation
Maximum Current
3000A
Construction
Uses alternating layers of P and N semiconductor materials like in bipolar transistors

5. Power Control Devices SCRs Characteristics Construction
Uses 4 layers and three connections
Gate (G); ANODE (A); Cathode (K)
Functions as two transistors in the circuit shown
Typical packages

6. Power Control Devices SCRs Characteristics Construction
Uses 4 layers and three connections
Gate (G); ANODE (A); Cathode (K)
Functions as two transistors in the circuit shown
Typical packages

7. Power Control Devices SCRs Basic DC operation
A simple SCR DC circuit is shown – top right and the equivalent transistor circuit that will be analyzed – bottom right
With E applied and Vin = 0V
IG1 = 0V and Q1 is off
With Q1 off Q2 lacks base current and is off
With both transistors off the SCR appear like a reverse biased diode
Almost no current between
A and K or to the load
With E applied and Vin > 0V
IG1 > 0V and Q1 starts to turn on

8. Power Control Devices SCRs Basic DC operation
With E applied and Vin > 0V
IC1 starts to flow and Q2 starts to conduct
IC2 starts to flow into the base of Q1 and Q1 turns on harder
More IC1 flows, and Q2 turns on harder
The snowballing continues until both transistor are in saturation
Once the turn-on process starts the input voltage that started the process can be removed
The SCR will stay on until the
cathode voltage = anode voltage

9. Power Control Devices SCRs Basic DC operation
Sample Circuit: an Intrusion Alarm
With light (probably IR) striking the photoresistor it has a low value
The voltage divider formed by it and R1 yields a gate voltage too low to activate the SCR
Too low to make the sonic alarm output sound
When an intruder breaks the light
beam the photoresistor has a much
higher resistance and the
SCR turns on
The alarm will stay on
until S1 is opened
regardless s of the light beam

10. Power Control Devices SCRs Basic AC operation Two modes of operation
Zero Voltage Switching
SCR is turned on when the AC voltage crosses a little above zero volts (instantaneous voltage not rms)
Phase Control (Covered after TRIACs)
The timing of the trigger that turns on the SCR is delayed from the zero crossing of the AC voltage
Characteristics
Current only flows during ½ of the AC voltage cycle
Sample circuit operation
See figure 13-5 on page 378 or on the next slide

11. Power Control Devices SCRs Basic AC operation Sample circuit operation
With S1 open
All the line voltage drops across the SCR
Lamp is off
With S1 Closed
All the line voltage drops across the SCR only on the negative part of the cycle
During the positive part of the cycle the SCR is on and almost all the voltage is dropped across the lamp

12. Power Control Devices SCRs TRIACs More Efficient AC operation
Provides more power to
the device under control
Use a rectifier between the
AC source and the SCR
Will feed the SCR the full-wave rectified AC signal and the motor all the available AC power from the line – not ½
TRIACs
Conducts AC in both directions
Acts like two SCRs in parallel, but facing in opposite directions

13. Power Control Devices TRIACs The symbol reflects the parallel SCR
description
Still has gate connection along with T1 and
T2 connections (some time MK1 and MK2)
The gate triggers operation when
With T2 positive with respect to T1 – a positive gate with respect to T1 triggers operation
With T2 negative with respect to T1 – a negative gate with respect to T1 triggers operation
Voltage and Current ranges available
Usually significantly less than for SCR
Reasonable values
50-600V and A

14. Power Control Devices TRIACs
Ancillary Devices used to control the zero crossing mode with DC signals
Types covered: Reed Switches; Opto-coupliers
Reed Switches
Range of packaging
As shown
In a DIP for insertion on a PCB
Operation
When a current flows through the wire
The spring tensioned ferrous contacts are activated completing a circuit

15. Power Control Devices TRIACs
Ancillary Devices used to control the zero crossing mode with DC signals
Opto-coupliers
Use either Light Activated SCRs (LASCR) or OptoTRIACs and a LED
Gates are either not shown
or shown not connected on circuits
Ancillary Device packaging
Can be obtained as discrete components and assembled
Or both types come as part of a Solid State Relay package

16. Power Control Devices TRIACs Sample Circuit Operation
Vin could be coming from:
logic circuit
microcontroller
microprocessor, etc
With Vin =0V
The TRIAC is off and all the voltage is dropped across it
With Vin = a logic one or higher voltage
The micro switch is activated
When the instantaneous AC voltage is high enough the TRIAC is activated

17. Power Control Devices TRIACs Sample Circuit Operation
With Vin = a logic one
The TRIAC will continue to be activated on each positive and negative transition while the micro switch is activated
Sample w/Optocoupler
See Figure on page 382 and on the next slide
The Q-NOT flip flop output goes low and the LED inside the optocoupler turns on
Activates internal Opto TRIAC

18. Power Control Devices TRIACs Sample w/Optocoupler
That activates the Power TRIAC
This repeats every 1/2cycle while the digital input is a Logic 0
For low current applications the internal TRIAC may be sufficient

19. Power Control Devices Phase Control Characteristics
Provides smooth control of amount of power delivered to a load instead of switching the power on and off using SCRs or TRIACs
Commonly used in lamp dimmers and motor speed controls
Ancillary Device – A DIAC
Two terminal device that act like two diodes in parallel facing opposite directions
Or a TRIAC without a gate
Acts like a reversed polarity diode until a breakdown voltage is reached
Then it has a very small resistance
Not dependent on polarity

20. Power Control Devices Phase Control Ancillary Device – A DIAC
Acts like a reversed polarity diode - continued
Breakdown voltage of 30 V is common but others such as 8 volts are available
Used to provide a triggering spike to the TRIAC to turn it on
Without the DIAC a slowly rising voltage would slowly turn the TRIAC on
Sample Circuit Operation
As the switch closes the
TRIAC is off and for simplicity
the AC is at zero crossing
The voltage on C1 slowly rises
due to the time constant; from
R1, R2 and C1

21. Power Control Devices Phase Control Sample Circuit Operation
Switch closed - continued
After the breakdown voltage of the DIAC is reached on C1 – the DIAC fires
The TRIAC conducts for the remainder of the ½ cycle
By adjusting the POT you can vary the delay before the DIAC fires
Thus effecting the power delivered to a motor or lamp
Varies the motor speed
Varies the lamp intensity

22. Problems with TRIAC and SCR circuits
Slow Turn-On
SCRs and DIACs need a rapid rise in gate voltages
A slow rise in gate voltages result in slow activations of the SCR or TRIAC
DIACs provide a voltage spike for SCRs and TRIACs
After the voltage on the Cap reaches The DIAC’s breakdown voltage it provides a low impedance path for the Cap to discharge into the SCR/TRIAC gate
Inductive Loads
Sometimes SCRs and TRIACs
remain on past the point
when VAK or VT1-T2 =0V
CEMF is the prime cause

23. Problems with TRIAC and SCR circuits
Inductive Loads
When a switch in series with an inductive load is opened
A CEMF instantaneously develops across the load to cause the current to continue flowing
For physical switches arcing can occur and sometimes damage switches
Some protection is needed - RC discharge path
Large rapid voltage swings across SCRs and TRIACs can cause them to turn on
Discharge path as shown
Resistor helps prevent a Tank circuit from consisting of the inductor and Cap