Electronic Troubleshooting

Chapter 13Power 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 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

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 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

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 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

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