AC16FSM datasheet(135/223 Pages) TECCOR

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Application Notes

AN1003

AN1003 - 3

http://www.teccor.com

Thyristor Product Catalog

+1 972-580-7777

Control Characteristics

A relaxation oscillator is the simplest and most common control

circuit for phase control. Figure AN1003.7 illustrates this circuit

as it would be used with a thyristor. Turn-on of the thyristor

occurs when the capacitor is charged through the resistor from a

voltage or current source until the breakover voltage of the

switching device is reached. Then, the switching device changes

to its on state, and the capacitor is discharged through the thyris-

tor gate. Trigger devices used are neon bulbs, unijunction tran-

sistors, and three-, four-, or five-layer semiconductor trigger

devices. Phase control of the output waveform is obtained by

varying the RC time constant of the charging circuit so the trigger

device breakdown occurs at different phase angles within the

controlled half or full cycle.

Figure AN1003.7

Relaxation Oscillator Thyristor Trigger Circuit

Figure AN1003.8 shows the capacitor voltage-time characteristic

if the relaxation oscillator is to be operated from a pure DC

source.

Figure AN1003.8

Capacitor Charging from DC Source

Usually, the design starting point is the selection of a capacitance

value which will reliably trigger the thyristor when the capacitance

is discharged. Trigger devices and thyristor gate triggering char-

acteristics play a part in the selection. All the device characteris-

tics are not always completely specified in applications, so

experimental determination is sometimes needed.

Upon final selection of the capacitor, the curve shown in Figure

AN1003.8 can be used in determining the charging resistance

needed to obtain the desired control characteristics.

Many circuits begin each half-cycle with the capacitor voltage at

or near zero. However, most circuits leave a relatively large

residual voltage on the capacitor after discharge. Therefore, the

charging resistor must be determined on the basis of additional

charge necessary to raise the capacitor to trigger potential.

For example, assume that we want to trigger an S2010L SCR

with a 32 V trigger diac. A 0.1 µF capacitor will supply the neces-

sary SCR gate current with the trigger diac. Assume a 50 V dc

power supply, 30° minimum conduction angle, and 150° maxi-

mum conduction angle with a 60 Hz input power source. At

approximately 32 V, the diac triggers leaving 0.66 VBO of diac

voltage on the capacitor. In order for diac to trigger, 22 V must be

added to the capacitor potential, and 40 V additional (50-10) are

available. The capacitor must be charged to 22/40 or 0.55 of the

available charging voltage in the desired time. Looking at Figure

AN1003.8, 0.55 of charging voltage represents 0.8 time constant.

The 30° conduction angle required that the firing pulse be

delayed 150° or 6.92 ms. (The period of 1/2 cycle at 60 Hz is

8.33 ms.) To obtain this time delay:

6.92 ms = 0.8 RC

RC = 8.68 ms

if C = 0.10 µF

then,

To obtain the minimum R (150° conduction angle), the delay is

30° or

(30/180) x 8.33 = 1.39 ms

1.39 ms = 0.8 RC

RC = 1.74 ms

Using practical values, a 100 k potentiometer with up to 17 k min-

imum (residual) resistance should be used. Similar calculations

using conduction angles between the maximum and minimum

values will give control resistance versus power characteristic of

this circuit.

Triac Phase Control

The basic full-wave triac phase control circuit shown in

Figure AN1003.9 requires only four components. Adjustable

resistor R1 and C1 are a single-element phase-shift network.

When the voltage across C1 reaches breakover voltage (VBO) of

the diac, C1 is partially discharged by the diac into the triac gate.

The triac is then triggered into the conduction mode for the

remainder of that half-cycle. In this circuit, triggering is in Quad-

rants I and III. The unique simplicity of this circuit makes it suit-

able for applications with small control range.

Switching

Device

Voltage

Current

Source

Triac

SCR

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Time Constants

8.68

–

×10

0.1

–

×10

--------------------------

86,000 Ω

1.74

–

×10

0.1

–

×10

---------------------------

17,400 Ω

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