Re: Need Voltage Doubler Circuit

Dave.H wrote:
I have a regen radio powered by two 23 volt wallwarts in series, but
would like to use a 24 VAC transformer with a voltage doubler to
provide the 45v B+, I've found circuits via Google, but they don't
give the values for 24 VAC. Also don't say what diode to use. I have
about 50 1N4004's lying around I could use. Transformer I'm planning
on using is M6672 @ www.dse.com.au

The diodes should work fine as long as the DC current load is less than about a half amp. A simple doubles connects two capacitors (negative side of one and positive side of the other) to one side of the 24 supply and two diodes (cathode or banded end of one and anode of the other) to the other side of the 24 volt supply. Then connect the free anode to the free negative capacitor end and the free cathode to the free positive capacitor end. The output is taken across those last two connections, so that the two capacitors are acting as a pair of stacked half wave supplies. Under no load, this will produce at least 24*sqrt(2) *2 minus two diode drops or about 66 volts. Keep in mind that the 24 volts AC output may rise quite a bit under no load, since it is rated for full load.

Any load will sag some the voltage, and add twice line frequency ripple to the output. You may want to add an RC low pass filter stage with the series R selected to lower the voltage to closer to your desired 45 volts, and the capacitor from that output to the other side of the supply as an additional ripple filter. Or you may just lower the voltage under load by adding some resistance in series with the 24 volt AC output. This will not reduce the ripple as much, but will make the 24 volt transformer run a little cooler. I just looked up the transformer and it has several taps, so you can try different combinations to get very close to the desired output voltage.

For the two doubler capacitors, I would use something like 5,000 to 10,000 microfarads per amp of output load and rated for at least 16 volts DC. Your diodes and transformer will allow about 300 to 400 mA load current. If your load current is at or below 400 mA, that implies something around 2,200 to 4,700 uF 16 or 25 volt capacitors for the doubler.

Of course, if you use the full 30 volt winding, you can get pretty close to 45 volts output with a bridge rectifier made with 4 of your diodes and a single capacitor, say, 2,200 to 4,700 uF, rated for 50 volts. This will give you a voltage around 40 to 45 volts, depending on load current that can be up to about 0.7 amps.

--
Regards,

John Popelish
.

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