10

SR036/SR037

B092005

Applications Information, continued

12V Coil

Relay

220µF

1.0µF

SR036

120VAC

or

230VAC

Fuse

Logic

Control

Circuit

Unregulated Voltage

3.3V

VN2110K1

1N4001

HV

IN

GND

Gate

Source

V

OUT

Figure 4: Driving 12V Relay Coils

GN2470

1.0

The circuit shown in Figure 4 uses the SR036 to supply a

regulated 3.3V for the logic control circuitry while the unregulated

voltage is used to drive a 12V relay coil. The operating voltage

for a 12V relay coil is typically very wide and can therefore

operate directly from the unregulated line.

220µF

1.0µF

SR037

120VAC

or

230VAC

Fuse

Logic

Control

Circuit

Unregulated Voltage

5.0V

100

Ω

1K

Ω

2N3904

1N4001

5V Coil

Relay

HV

IN

GND

Gate

Source

V

OUT

Figure 5: Driving 5V Relay Coils

GN2470

1.0

The circuit shown in Figure 5 uses the SR037 to supply a

regulated 5.0V for the logic control circuitry while the unregulated

voltage is used to drive a 5.0V coil relay. To overcome the voltage

variation of the unregulated line, a bipolar transistor is used to

drive the coil with a constant current. The resistor value from the

emitter to ground sets the desired coil current. For an arbitrary

coil current of 40mA, the resistor value can be calculated as: