Principles & Applications Electronics Principles & Applications Sixth Edition Charles A. Schuler Chapter 14 Electronic Control Devices and Circuits ©2003 Glencoe/McGraw-Hill

INTRODUCTION The Silicon Controlled Rectifier Full-Wave Devices Feedback in Control Circuitry Troubleshooting

This two-transistor circuit is stable in either of two states. Q1 Q2 On state Off state

A four-layer structure replaces the two-transistor circuit. Q1 } { Q2 p Q1 n p n Q2

The Silicon Controlled Rectifier (SCR) Anode Anode Anode p Q1 n p Gate Gate Gate n Q2 Cathode Cathode Cathode

The SCR can be turned on at its gate terminal. With a dc source, the SCR stays on after it is gated. Load Anode Cathode Gate current Load Gate pulse occurs here Time

With an ac source, the SCR turns off at the zero-crossing. Load Anode Cathode Gate on off current Load Gate pulse occurs here Turns off here Time

The gate can be pulsed for each positive alternation. Load Anode Cathode Gate current Load Time

The average load current can be decreased by gating the SCR later. Anode Cathode Gate current Load Time

…. and later. Load Anode Cathode Gate current Load Time

…. or, not at all. Load Anode Cathode Gate current Load No gate pulses: ILoad = 0 Time

A full-wave rectifier allows use of both alternations. Load Bridge rectifier Load current Gate

Two SCRs can provide full-wave control. Load Two SCRs can provide full-wave control. Gated early for full power Gated later for low power

The TRIAC provides control of both ac alternations. Main terminal 2 Load Gate Main terminal 1 Gated early Gated late

Solid state relays provide optical isolation.

DIAC Volt-ampere Characteristic Curves VP+ -V +V VP- -I

DIAC Volt-ampere Characteristic Curves Load Decreasing R will gate the TRIAC earlier.

Illustration courtesy Powerex, Inc.

Thyristor Quiz The three terminals of an SCR are anode, cathode and __________. gate SCR turnoff in ac circuits occurs at the __________ crossing. zero In ac control, load power is increased by gating the SCR __________ in the cycle. earlier The device equivalent to two SCRs for full-wave control is the __________. triac The diode often used to trigger triacs is the __________. diac

Servomechanisms use negative feedback to control velocity and/or position. VREF Negative feedback Tachometer Error Amplifier Motor Velocity set If the mechanical load changes, causing the velocity to change, the error amplifier will respond by adjusting the motor drive to reduce the change.

In this servo, gears drive a variable resistor to provide VREF Error Amplifier Motor Position set Gearbox VREF In this servo, gears drive a variable resistor to provide position feedback. Feedback

Suppose, at time t1, a servo is commanded to a new position. This is the ideal response Position Time t1

This is the critically damped response Position Time t1

This is the overdamped response Position Time t1

This is the underdamped response Position Time t1

Which response is the best? Tuning a servomechanism involves adjusting the loop gain and the phase to achieve the best response. Position Time Which response is the best?

Troubleshooting SCRs can fail by opening or shorting. A shorted SCR means full load power. An open SCR means no load power. A defective gate circuit can produce either full power or no power. TRIAC troubleshooting is much the same.

Servo Troubleshooting Mechanical problems can be confused with electrical faults. Slippage and excessive mechanical play can cause various symptoms and should be eliminated before tuning or troubleshooting. The underdamped response is often caused by excessive gain. The overdamped response is often caused by insufficient gain.

Servo Quiz Servos use negative __________ to control position or speed. feedback When servo response is sluggish, the response is __________. overdamped When servo response is oscillatory, the response is __________. underdamped The overdamped response can be caused by __________ gain. insufficient The underdamped response can be caused by __________ gain. excessive

REVIEW The Silicon Controlled Rectifier Full-Wave Devices Feedback in Control Circuitry Troubleshooting