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More  AC-DC Power Supplies

Low Power AC to DC Power Supply 2

Last week I showed how to use a small common mode choke as a fully isolated transformer and a 130v sidac to generate about 100mw of DC power from the AC line.  This second circuit still uses the same inexpensive tiny off-the-shelf 5mH common mode filter coil with a split bobbin as a high frequency transformer but uses two SCRs instead of a sidac to discharge a capacitor.  This method increases the available power to about 500mw. 

The circuit routes short 300v pulses to the primary winding of the coil every half cycle of the 60Hz line frequency.  It uses two sensitive gate silicon controlled rectifiers (SCR) to discharge a 0.1uF high voltage capacitor.  By winding about 6 turns of wire on the secondary of the common mode coil, enough voltage is produced to charge up a 2200uF 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 100ma of current from the regulated supply. Unlike the circuit posted last week, this circuit only works with 120vac power lines.  The SIDAC circuit posted last week will work fine for 240vac power lines.

Three resistors connected to the two SCRs form a voltage divider.  The resistor values are selected so the SCRs will not be triggered on until the peak AC voltage reaches about 150 volts.  This is close to the 170v peak of a 120vac line.  Once turned on, the series capacitor will discharge into the coil but at twice the peak voltage or about 300 volts, due to the charge pump action of the SCRs and diodes.  This higher voltage puts more energy into the coil.

An SCR has a large negative resistance characteristic.  Once the device’s gate terminal has sufficient current, 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 point 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 300v stores 4.5 millijoules of energy.  Since the capacitor is discharged 120 times per second, the maximum power available from this type of circuit is 540mw.
5mH Common Mode Coil  EC103D1 SCR
5mH Common Mode Coil  EC103D1 SCR

Click on Drawing Below to view PDF version of Schematic

Low Power AC to DC Power Supply 2 circuit designed by Dave Johnson



More  AC-DC Power Supplies

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