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Old 24th May 2006, 08:57 PM   #1
preiter is offline preiter  United States
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Default Can you drive two rails from one transformer output?

I came across this power adapter...

http://www.goldmine-elec-products.co...?number=G15566

14 VAC @ 1220 ma for $2.

I was wondering if you could hook the single output to 2 recitifers to get + and - rails at 610mA. Will that work?

If so, it's a pretty interesting option for a power supply for op-amp circuits.
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Old 24th May 2006, 09:21 PM   #2
sss is offline sss  Israel
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u mean voltage doubler?
yes ,ii will work .
but the output voltage will be half wave rectified ,soits not that "clean"
to drive an op amp (if its amplifiying audio signals) u'll need to use some heavy filtering ...
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Old 24th May 2006, 09:29 PM   #3
preiter is offline preiter  United States
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I wasn't really thinking about half wave. I was thinking of two full-wave rectified signals with each getting half of the current.

I guess there's one way to find out...
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Old 25th May 2006, 05:21 AM   #4
Pierre is offline Pierre  France
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There is a way, half wave using 2 diodes. If current is not high, you won't get problems in getting a clean output with reasonable capacitance.
- Use one of the outputs as GND
- Connect the other output to the anode of one diode (its cathode is the positive output) and to the cathode of the other diode (its anode is the negative output).
- Connect at least 1000uF capacitor in parallel with each output, to GND.

This worked for me in a switching power supply, at 80KHz but the principle is just the same, but you need more filtering.
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Old 25th May 2006, 08:11 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi Pierre,
that sounds like two half wave rectifiers in parallel.

Am I reading your response correctly?

Dual polarity supplies from one secondary winding! Seems the only downside is high ripple.
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Old 25th May 2006, 10:17 AM   #6
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What DC rails are you after?

Rectification of 14VAC (whichever way you do it) gives you 19.75V DC minus the volt drop of the recitifier. Rectifiers typically drop 1.1V to 1.5V. You want complimentary rails so I'd sugest a virtual earth circuit (google it for more detail).

A good virtual earth is basically a resistor divider pair in series across the supply (giving you half of your DC supply at the mid point) feeding a non-inverting unity gain opamp (to handle current through the virtual earth).


Sam.
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Old 25th May 2006, 01:08 PM   #7
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Quote:
Originally posted by samborambo
What DC rails are you after?

A good virtual earth is basically a resistor divider pair in series across the supply (giving you half of your DC supply at the mid point) feeding a non-inverting unity gain opamp (to handle current through the virtual earth).


Sam.
Good in theory, but only works when the supply loads are reasonably balanced; any difference greater than the op-amp Iout(max) causing the rails to shift.

But I know you knew that
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Old 26th May 2006, 03:46 PM   #8
CBS240 is offline CBS240  United States
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Quote:
Originally posted by sss
u mean voltage doubler?
yes ,ii will work .
but the output voltage will be half wave rectified ,soits not that "clean"
to drive an op amp (if its amplifiying audio signals) u'll need to use some heavy filtering ...

good rejection linear regulator/amplifier to drop each side to 12V. Add a series current source/limiters, and you have a great balanced PS for Op-amps.
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