SR036/SR037
Applications Information, continued
VN2460N8
Unregulated Voltage
Fuse
1N4001
220µF
120VAC
or
240VAC
12V Coil
Relay
Gate
Source
3.3V
Logic
Control
Circuit
VOUT
HVIN
VN2110K1
SR036
1µF
GND
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.
VN2460N8
Unregulated Voltage
Fuse
1N4001
220µF
120VAC
or
240VAC
5V Coil
Relay
Gate
Source
VOUT
1KΩ
Logic
Control
Circuit
5.0V
2N3904
HVIN
SR037
GND
1µF
100Ω
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:
40mA
β
5.0V-
1KΩ-Vbe
R =
, where Vbe = 0.6V and β =100
40mA
=100Ω
6
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