Electronics Principles & Applications Fifth Edition Chapter 14 Electronic Control Devices and Circuits ©1999 Glencoe/McGraw-Hill Charles A. Schuler

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

This two-transistor circuit is stable in either of two states. On state Off state Q1Q1 Q2Q2

Q1Q1 Q2Q2 p p n n A four-layer structure replaces the two-transistor circuit. Q1Q1 } { Q2Q2

The silicon controlled rectifier (SCR) Q1Q1 Q2Q2 Anode Gate Cathode p p n n Anode Gate Cathode Anode Gate Cathode

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

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

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

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

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

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

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

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

DIAC volt-ampere characteristic curves +I -I-I +V -V-V VP+VP+ VP-VP-

Load A popular diac-triac control circuit Decreasing R will gate the TRIAC earlier.

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. Motor Tachometer Error Amplifier Negative feedback Velocity set V REF If the mechanical load changes, causing the velocity to change, the error amplifier will respond by adjusting the motor drive to reduce the change.

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

Position Time t1t1 Suppose, at time t 1, a servo is commanded to a new position. This is the ideal response

Position Time t1t1 This is the critically damped response

Position Time t1t1 This is the overdamped response

Position Time t1t1 This is the underdamped response

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