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Old 21st April 2005, 12:16 PM   #1
bowdown is offline bowdown  Australia
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Question Help on power supply

Hi Guys just wandering if someone can help me with this situation.

I have a 2KVA toroidal that produces 71 V AC without a load which is about +/- 100V DC but i can only get 100V caps so i wanted to try and drop the voltage to +/-90V DC or maybe even lower. How can i go about this and does someone have a schematic to do it?? PLease please please this thing cost me a fortune, I asked for 65VAC but they said its more because they have almost 10% regulation on it.

Please help

Cheers

Bowdown
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Old 21st April 2005, 12:54 PM   #2
sek is offline sek  Germany
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Hi,

do you know how a (toroidal) transformer works and how it is constructed?

It is possible to achieve your goal without wasting power (heat) or paying complicated electronics. Unwinding some rounds of the secondaries, that is. But you'd have to be sure that you know what you are doing.

Of course, you could also regulate the voltage down a bit, but at 2kVA this won't work without a switching regulator. And then you could have bought an expesnive switching supply in the first place...

So, the forum can surely explain what to do, but I first want to be sure you know what you're doing. Because doing it wrong would deliver some hundres volts at some hundred amperes (peak).

Cheers,
Sebastian.
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Old 21st April 2005, 01:01 PM   #3
dhaen is offline dhaen  Europe
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Put a small load on it and see what the DC drops to. Often a few extra no-load volts can be due to garbage on the mains charging up the caps to a potential that's higher than the actual peak of the mains sinewave.
As far as cap ratings go, it really is best to keep within ratings. Measure your mains at different times of the day to determine the maximum value and decide whether your caps are acceptably rated.
If the caps are NG, consider using 63v caps in series.
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Old 21st April 2005, 01:05 PM   #4
SY is offline SY  United States
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If all else fails, use a bucking transformer on the primary. Sorry, I'm not near a scanner so can't post a schematic, but I bet someone else will.
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Old 21st April 2005, 01:11 PM   #5
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Default Reducing voltage.

Bowdown, I have had this situation before, the easiest way to lower the voltage is to wind several turns of wire in buck (reverse to existing windings) This seems a little wastefull but it is easy to do and does not add any appreciable load to the transformer.
This also saves you from having to unwrap and unwind the item. Regards, Steve
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Old 21st April 2005, 04:18 PM   #6
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Default Re: Reducing voltage.

Quote:
Originally posted by gearheaddruid
Bowdown, I have had this situation before, the easiest way to lower the voltage is to wind several turns of wire in buck (reverse to existing windings)
And have that bucking wire, wired in series with the primaries?
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Old 21st April 2005, 04:20 PM   #7
Eva is offline Eva  Spain
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Adding turns to the secondaries in reverse direction reduces output voltage while adding them to the primary in reverse increases voltage. Adding them in the same direction produces the opposite effect, be careful
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Old 21st April 2005, 04:55 PM   #8
dhaen is offline dhaen  Europe
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It's safer to do it with the secondaries, you also need less turns - although the wire'll be thicker.
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Old 21st April 2005, 08:41 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
I second turns removal.
If you're really clever you could try to intercept the 65Vac turn and cut it then tap in to the 2 loose ends, forming a 4 turn secondary consisting of 2 * 65v and 2 * 5V to power the regulated driver stage.
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Old 22nd April 2005, 01:22 AM   #10
bowdown is offline bowdown  Australia
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Hey thanx guys for all the replies so quick.

Ok i dont know how to unwind the toroidal and its core is full of resin for a mounting screw, so i dont think i can add or remove any windings. I think i might just have to try some sort of regulated power supply, not sure how to design one though, i have had heaps of experience with raw power supplies but thats really all.

cheers

Bowdown
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