Electronic Troubleshooting
Chapter 13
Power Control Devices
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
Power Control Devices
• Topics covered
• SCRs
• TRIACs
• Ancillary Devices
• Reed Switches
• Opto-coupliers
• Phase Control
• Problems with TRIAC and SCR circuits
• SCRs
• Characteristics
• Used to control current for AC Loads
• Sometimes for DC loads
Power Control Devices
• SCRs
• Characteristics
• Type of Thyristos
• 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
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
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
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
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
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
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
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 open
» 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 an and
almost all the voltage is
dropped across the lamp
Power Control Devices
• SCRs
• 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
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 0.8-25 A
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
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
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
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 13-11 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
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
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
• Characteristics
• 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
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
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
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 it’s breakdown voltage the
DIAC 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
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