AC08FSM datasheet(174/223 Pages) TECCOR

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AN1007

Application Notes

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AN1007 - 2

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Thyristor Product Catalog

Figure AN1007.3

Waveform Across Static Switch

A typical example would be in the application of this type circuit

for the control of 5 A resistive load with 120 V rms input voltage.

Choosing a value of 100

Ω for R1 and assuming a typical value of

1 V for the gate to MT1 (VGT) voltage, we can solve for VP by the

following:

VP = IGT (RL + R1) + VGT

Note: RC is not included since it is negligible.

VP = 0.025 (24 + 100) + 1.0 = 4.1 V

Additionally the turn-on angle is

[

θ = 1.4°]

The power lost by the turn-on angle is essentially zero. The

power dissipation in the gate resistor is very minute. A 100

Ω,

0.25 W rated resistor may safely be used. The small turn-on

angle also ensures that no appreciable RFI is generated.

The relay circuit shown in Figure AN1007.1 and Figure AN1007.2

has several advantages in that it eliminates contact bounce,

noise, and additional power consumption by an energizing coil

and can carry an in-rush current of many times its steady state

rating.

The control device S1 indicated can be either electrical or

mechanical in nature. Light-dependent resistors and light- acti-

vated semiconductors, optocoupler, magnetic cores, and mag-

netic reed switches are all suitable control elements. Regardless

of the switch type chosen, it must have a voltage rating equal to

or greater than the peak line voltage applied. In particular, the

use of hermetically sealed reed switches as control elements in

combination with triacs offers many advantages. The reed switch

can be actuated by passing DC current through a small coiled

wire or by the proximity of a small magnet. In either case, com-

plete electrical isolation exists between the control signal input,

which may be derived from many sources, and the switched

power output. Long life of the triac/reed switch combination is

ensured by the minimal volt-ampere switching load placed on the

reed switch by the triac triggering requirements. The thyristor rat-

ings determine the amount of load power that can be switched.

Normally Closed Circuit

With a few additional components, the thyristor can provide a

normally closed static switch function. The critical design portion

of this static switch is a clamping device to turn off/eliminate gate

drive and maintain very low power dissipation through the clamp-

ing component plus have low by-pass leakage around the power

thyristor device. In selecting the power thyristor for load require-

ments, gate sensitivity becomes critical to maintain low power

requirements. Either sensitive SCRs or sensitive logic triacs must

be considered, which limits the load in current capacity and type.

However, this can be broader if an extra stage of circuitry for gat-

ing is permitted.

Figure AN1007.4 illustrates an application using a normally

closed circuit driving a sensitive SCR for a simple but precise

temperature controller. The same basic principle could be applied

to a water level controller for a motor or solenoid. Of course, SCR

and diode selection would be changed depending on load current

requirements.

Figure AN1007.4

Normally Closed Temperature Controller

A mercury-in-glass thermostat is an extremely sensitive measur-

ing instrument, capable of sensing changes in temperature as

small as 0.1 °C. Its major limitation lies in its very low current-

handling capability for reliability and long life, and contact current

should be held below 1 mA. In the circuit of Figure AN1007.4, the

S2010LS2 SCR serves as both current amplifier for the Hg ther-

mostat and as the main load switching element.

With the thermostat open, the SCR will trigger each half cycle

and deliver power to the heater load. When the thermostat

closes, the SCR can no longer trigger and the heater shuts off.

Maximum current through the thermostat in the closed position is

less than 250 µA rms.

Figure AN1007.5 shows an all solid state, optocoupled, normally

closed switch circuit. By using a low voltage SBS triggering

device, this circuit can turn on with only a small delay in each half

cycle and also keep gating power low. When the optocoupled

transistor is turned on, the gate drive is removed with only a few

milliamps of bypass current around the triac power device. Also,

by use of the BS08D and 0.1 µF, less sensitive triacs and alter-

nistors can be used to control various types of high current loads.

120 V rms (170 V peak)

VP-

≅1 V rms or 1.6 V peak MAX

VP+

VT+

VT-

Sin

–

4.1

170VPK

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

1000 W Heater Load

120 V ac

60 CPS

D2015L

CR1—CR4

CR4

CR3

CR1

CR2

S2010LS2

0.1 µF

510 k

SCR1

Twist Leads to Minimize

Pickup

Hg in Glass Thermostat

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