6

SR036/SR037

Applications Information, continued

12V Coil

Relay

220

µF

1

µF

SR036

VN2460N8

120VAC

or

240VAC

Fuse

Logic

Control

Circuit

Unregulated Voltage

3.3V

VN2110K1

1N4001

HV

IN

GND

Gate

Source

V

OUT

Figure 4: Driving 12V Relay Coils

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

regulated 3.3V for the logic control circuitry while the unregu-

lated 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.

220

µF

1

µF

SR037

VN2460N8

120VAC

or

240VAC

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

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

regulated 5.0V for the logic control circuitry while the unregu-

lated 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 calcu-

lated as:

R =

5.0V -

40mA

1K - V

40mA

where V

= 0.6V and

= 100

= 100

be

be

β

β

Ω

Ω

,