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Circuits designed
by David Johnson, P.E.
Last Updated on:
Wednesday, June 02, 2021 01:45 PM
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Low Power AC to DC Power Supply |
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you need some DC power fully isolated from the AC power line, this circuit may be just
the ticket. The circuit uses an inexpensive tiny off-the-shelf 5mH common mode
filter coil from with a split bobbin as a high frequency transformer. The
circuit routes short 130v pulses to the primary winding of the coil every half cycle
of the 60Hz line frequency. It uses a silicon bilateral voltage triggered switch
called a “sidac” to discharge a 0.1uF high voltage capacitor. |
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By winding about 10 turns of wire on the secondary of the common mode coil,
enough voltage is produced to charge up a 470uF capacitor through a bridge rectifier to
activate a 5v regulator. The amount of power produced by discharging the 0.1uF
capacitor is enough to provide about 20ma of current from the regulated supply. For
many applications, this is sufficient power. If more power is needed, the value of
the 0.1uF capacitor could be increased. For 240vac, 50Hz power lines, the value of
the 0.1uF capacitor should be decreased to 0.047uF with a 600v rating and a 280v sidac
should be used. |
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A “SIDAC” operates much like a two terminal TRIAC. The device has a
large negative resistance characteristic. Once the trigger voltage is exceeded, the
device switches on and forms a low impedance current path. The current continues to
flow until it drops below the device’s holding current, at which the device switches off
again. The switch can handle about 20 Amps of peak current, which is perfect for quickly
discharging a capacitor. The energy stored in a capacitor is 0.5CV^2, where C is the
capacitance in farads and V is the voltage. A 0.1uF capacitor charged up to 130v
stores 845 microjoules of energy. Since the capacitor is discharged 120 times per
second, the maximum power available from this type of circuit is 100mw. For 240v
systems, I would recommend a 280v sidac and a 0.047uF capacitor, rated at 600v. With those
changes, the power available will increase to about 200mw. |
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6.8mH Common Mode Coil |
Sidac |
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Click
on Drawing Below to view PDF version of Schematic |
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eMail David A.
Johnson, P.E. about this circuit
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