AC08FSM datasheet(175/223 Pages) TECCOR

175 / 223 page

Application Notes

AN1007

AN1007 - 3

http://www.teccor.com

Thyristor Product Catalog

+1 972-580-7777

Figure AN1007.5

Normally Closed Switch Circuit

Optocoupled Driver Circuits

Random Turn-on, Normally Open

Many applications use optocouplers to drive thyristors. The com-

bination of a good optocoupler and a triac or alternistor makes an

excellent, inexpensive solid state relay. Application information

provided by the optocoupler manufacturers is not always best for

application of the power thyristor. Figure AN1007.6 shows a stan-

dard circuit for a resistive load.

Figure AN1007.6

Optocoupled Circuit for Resistive Loads (Triac or

Alternistor)

A common mistake in this circuit is to make the series gate resis-

tor too large in value. A value of 180

Ω is shown in a typical appli-

cation circuit by optocoupler manufacturers. The 180

Ω is based

on limiting the current to 1 A peak at the peak of a 120 V line

input. This is good for protection of the optocoupler output triac,

as well as the gate of the power triac on a 120 V line; however, it

must be lowered if a 24 V line is being controlled, or if the RL

(resistive load) is 200 W or less. This resistor limits current for

worst case turn-on at the peak line voltage, but it also sets turn-

on point (conduction angle) in the sine wave, since triac gate cur-

rent is determined by this resistor and produced from the sine

wave voltage as illustrated in Figure AN1007.2. The load resis-

tance is also important, since it can also limit the amount of avail-

able triac gate current. A 100

Ω gate resistor would be a better

choice in most 120 V applications with loads greater than 200 W

and optocouplers from Quality Technologies or Vishay with opto-

coupler output triacs that can handle 1.7 APK (ITSM rating) for a

few microseconds at the peak of the line. For loads less than

200 W, the resistor can be dropped to 22

Ω. Remember that if the

gate resistor is too large in value, the triac will not turn on at all or

not turn on fully, which can cause excessive power dissipation in

the gate resistor, causing it to burn out. Also, the voltage and dv/

dt rating of the optocoupler's output device must be equal to or

greater than the voltage and dv/dt rating of the triac or alternistor

it is driving.

Figure AN1007.7 illustrates a circuit with a dv/dt snubber network

included. This is a typical circuit presented by optocoupler manu-

facturers.

Figure AN1007.7

Optocoupler Circuit for Inductive Loads (Triac or

Alternistor)

This “T” circuit hinges around one capacitor to increase dv/dt

capability to either the optocoupler output triac or the power triac.

The sum of the two resistors then forms the triac gate resistor.

Both resistors should then be standardized and lowered to

100

Ω. Again, this sum resistance needs to be low, allowing as

much gate current as possible without exceeding the instanta-

neous current rating of the opto output triac or triac gate junction.

By having 100

Ω for current limit in either direction from the

capacitor, the optocoupler output triac and power triac can be

protected against di/dt produced by the capacitor. Of course, it is

most important that the capacitor be connected between proper

terminals of triac. For example, if the capacitor and series resis-

tor are accidentally connected between the gate and MT2, the

triac will turn on from current produced by the capacitor, resulting

in loss of control.

For low current (mA) and/or highly inductive loads, it may be nec-

essary to have a latching network (3.3 k

Ω + 0.047 µF) connected

directly across the power triac. The circuit shown in Figure

AN1007.8 illustrates the additional latching network.

Load

Triac

51 k

0.02 µF

(4) IN4004

PS2502

120 V ac

Q2008L4

BS08D

Rin

VCC

180

120 V

60 Hz

MT2

MT1

Hot

Neutral

Load Could Be

in Either Leg

Rin

VCC

100

Neutral

100

120 V

60 Hz

MT2

MT1

Hot

0.1 µF

AC08FSM Scheda tecnica(PDF) 175 Page - Teccor Electronics

AC08FSM Scheda tecnica(HTML) 175 Page - Teccor Electronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Scheda tecnica Scarica

Link URL