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 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

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 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

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 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