1

Final Control

• Introduction• Thyristors

–SCR–TRIAC–DIAC

• Stepping Motors• Summary

2

•Thyristor SCR-Silicon Controlled Rectifier Triac Diac etc

A thyristor is a four-layer semiconductor device, consisting of alternating P type and N type materials (PNPN). A thyristor usually has three electrodes: an anode, a cathode, and a gate (control electrode). The most common type of thyristor is the silicon-controlled rectifier (SCR).

Thyristors are used in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators.

Hockey Puck SCRhttp://www.larkinpower.com/Thyristors.htm

SCR Symbol

3SCR Origins

A commercially practical solid state rectifier has been operated by engineers at General Electric's Clyde, New York Rectifier Engineering Laboratory. This silicon-controlled rectifier operates in the same manner as a thyratron, and is capable of switching 1000 watts. A power rating of this magnitude is sufficient for most military and commercial applications. The device is expected to be first used in missiles.

The size of the silicon controlled rectifier is approximately twice that of a signal type transistor and 1/100 the size of a thyratron. The predicted life of the device is over 300,000 hours as compared to the 1000 hour life of a thyratron.

(Electronic Design, Jan. 8, 1958, p. 7) The first SCRs were compared to thyratrons because

that's what vacuum-tube era engineers understood. For today's engineer, we'd probably have to reverse the analogy--a thyratron, a gas-filled grid-controlled rectifier, was like an SCR.--Steve Scrupski

Thyratron Power Supply at Fermilab

Technical Papers

4

Operation of an SCR

http://www.tpub.com/neets/book7/26c.htm

5

SCR Characteristicshttp://www.nteinc.com/Web_pgs/SCR.html

Igate

(turn-on)

Imax

(Main)

Gate Controlled

turn-on time

0.8-25 mA .8-7 Amps

<2 usec

6

SCR V-I curvehttp://www.tpub.com/neets/book7/26d.htm

V

I

VAK= 1-1.5 volts

Very HighReverse voltageto break it down

Very High

7

SCR Circuit

Characteristics of 2N1595

Peak Reverse Blocking Voltage = 50 voltsRMS Forward Current (max) = 1.6 ampsTypical Gate Trigger Current= 2 mATypical Gate Trigger Voltage= .7 voltsTurn-on time = .8 usec Turn-off time=10 usec

8SCR Output

9

SCR for Overvoltage Protection

Vin

Computer

10

Phase Control with an SCR

http://www.fairchildsemi.com/pf/1N/1N4148.html

Characteristics of 2N1595

11SCR Phase Control

12

Diacs andTriacs

• The Diac and Triac are bilateral thyristors, meaning that they conduct current in both directions

• The Diac is designed to conduct when breakdown occurs in both directions The triac is like two parallel SCR’s, with one in each direction.

• Triac’s have less current carrying ability than SCR’s

Diac Triac

13A Triac Full-Wave Circuit

Τau = RC = .12 msec

14

A Triac Full-Wave Circuit

Τau = RC = 3.8 msec

15Diac-Triac Phase Control

Power Control with Thyristors and Triacs

16

Diac-Triac Light Dimmer

http://www.geocities.com/tjacodesign/dimmer/dimmer.html

17

Stepper Motors

A stepper motor system is an electro-mechanical rotary actuator that converts electrical pulses into unique shaft rotations. This rotation is directly related to the number of pulses. The speed is synchronous to the rate of pulsing.

Stepper motors feature bi-directional control, built-in braking, variable torque, power control, precision accuracy, high resolution, open-loop control, and direct interface to digital systems. Stepper Motors - General Description

A step motor converts electrical energy into discrete motions or steps. The motor consists of multiple electrical windings wrapped in pairs (phases)

around the outer stationary portion of the motor (stator). The inner portion (rotor) consists of iron or magnetic disks mounted on a shaft

and suspended on bearings. The rotor has projecting teeth which align with the magnetic fields of the

windings. When the coils are energized in sequence by direct current, the teeth follow the sequence and rotate a discrete distance necessary to re-align with

the magnetic field.The number of coil combinations (phases) and the number of teeth determine the

number of steps (resolution) of the motor. For example, a 200 step per rev (spr) motor has 50 rotor teeth times 4 coil combinations to equal 200 spr.

There are no brushes between the rotor and stator assembly; a stepper motor is a multipole (polyphase) brushless DC motor.

These multiple coil pairs can be connected either positive or negative resulting in four unique full steps. When the coils are sequenced correctly, the motor rotates for- ward. When the sequence is reversed, the motor rotates in reverse.

http://www.cs.uiowa.edu/~jones/step/#introduction http://www.doc.ic.ac.uk/~ih/doc/stepper/

18A Stepper Motor

In the KP4M4-001 stepper motor, the permanent magnet lies North - South along the shaft. It is encased in two "stacks" each with 25 teeth round the rim. The teeth on the South stack are out of phase with the teeth on the North stack by half the gap between teeth as can be seen in the photo of the shaft shown above.

http://www.doc.ic.ac.uk/~ih/doc/stepper/kp4m4/

19

SimpleStepper Operation

http://www.st.com/stonline/books/ascii/docs/1679.htm

Permanent Magnet or can also be switched

20

Summary

• Introduction• Thyristors

– SCR– TRIAC– DIAC

• Stepping Motors• Summary