Electronic Troubleshooting

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

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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

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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

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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

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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

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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 gt 0V
• IG1 gt 0V and Q1 starts to turn on

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Power Control Devices

• SCRs
• Basic DC operation
• With E applied and Vin gt 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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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 DIACs
  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

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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