AN3168

Triggering circuits

Doc ID 17193 Rev 1

9/20

2.5

SCR triggering circuit with negative power supply

In the previous section, it has been demonstrated that choosing a negative power supply is

the best solution to control Triac or ACS / ACST. But in some applications, an electronic

circuit may have to control both Triac and SCR. And SCR can only be triggered by sourcing

a current to the gate.

Such applications are encountered when a switch has to control a DC load on the mains.

This occurs for example for some pumps used in coffee machines, which feature an internal

diode (see Figure 9). Also some magnetic door locks can be controlled only during one half

cycle. Then an SCR can be a cheaper solution than a Triac.

The circuit has then to be modified to allow the SCR to be triggered from a negative power

supply. The schematic in Figure 9 shows the addition of a PNP transistor (Q1), a low-voltage

diode (D1), a resistor (R2) and a capacitor (C1). This schematic is similar to the schematic

used to trigger Triacs with a positive power supply [see Reference 2].

The circuit operation is the following:

1.

Q1 is OFF, C1 capacitor is charged thanks to D1 and R3.

2.

Q1 is switched on by the “CTRL” signal, C1 capacitor is discharged through R2, the

SCR gate and Q1. A positive gate current pulse is then applied and the SCR switches

on.

C2 capacitor is used to increase the dV/dt immunity of X1 SCR, which is a very sensitive

The gate pulse width has to be set by the values of R2 and C1. The point is to keep the gate

temperature.

remains above 2.4 mA during the whole “CTRL” pulse which lasts 400 µs (see Figure 10).

This allows correct operation even for very low ambient temperature operation.

Note:

Figure 9.

Drive schematic to trigger an SCR with a negative power supply

R1

R2

1k

R3

20k

C1

470n

C2

10n

D1

D1N4448

Q1

PN2907

X1

X00602MA

CTRL

R4

1k

R5

1k

D2

Vac

5V

L1

LOAD

V T

I T

V D

V CTRL

I T