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Old 11th November 2008, 11:04 AM   #1
00940 is online now 00940  Belgium
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Default Question about voltage multipliers

I know how to use basic voltage doublers, triplers and the like. But is there a way to multiply an ac voltage by 1.5 ? I need about 24-25VDC regulated out of a 18VAC xformer for a small load (about 50ma).

Thanks in advance.
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Old 11th November 2008, 11:29 AM   #2
Mooly is offline Mooly  United Kingdom
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Best bet is a doubler and regulator for so small a current.
78L12 + 12 volt zener for example.
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Old 11th November 2008, 11:35 AM   #3
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Under such a light load, the transformer may put out somewhat more than 25 volts. If you're lucky, you may need nothing more than a bridge rectifier and a 7824 regulator. Measure the transformer secondary voltage with no load. Otherwise, I agree with Mooly.
Voltage doubler and a regulator.
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Old 11th November 2008, 11:51 AM   #4
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Thanks. It seems like I have no choice...

- actually the transformer is also used for other stuff at 200ma/17VDC, so I don't expect it to put it a lot more voltage than what it's rated for. A quick check shows me about 25VDC after rectification and filtering.

- with a doubler, I can expect about 44VDC. That means about 20*0.05= 1W of dissipation for the regulator. That's why I asked about this.
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Old 11th November 2008, 12:05 PM   #5
Mooly is offline Mooly  United Kingdom
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Could you perhaps play around with the value of the caps in the doubler --- using the reactance as a wattless dropper and so on --- and still have reasonable DC voltage for the regulator to work off without the ripple component being too high.
Would have to think about that.
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Old 11th November 2008, 04:41 PM   #6
Elvee is offline Elvee  Belgium
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Hi,

Fractional voltage multiplier do exist, but they're probably too complicated for your application.
I would use a regular voltage doubler, but with a small input capacitor serving also as ballast, and a shunt regulator at the output.
A capacitive supply in fact.
The power that will be wasted is tolerable, and the complexity is minimal.
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Old 11th November 2008, 04:58 PM   #7
lineup is offline lineup  Sweden
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One small 2x12 VAC,
<= 5 VA
transformer is not a big cost.
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Old 11th November 2008, 10:29 PM   #8
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Quote:
Originally posted by lineup
One small 2x12 VAC,
<= 5 VA
transformer is not a big cost.
No, but space is an issue. The aim of this is to provide a higher regulated voltage for an (headphones) amplifier front end, using the existing power rails.

I think I will go for a voltage doubler but I'll stick a pair of 33R/2W resistor in serie before the voltage doubler.
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Old 12th November 2008, 10:04 AM   #9
lineup is offline lineup  Sweden
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i see.
can you please post your circuit used later
and tell us how well it works

thanks
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