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Old 14th July 2001, 10:31 AM   #1
hifi is offline hifi  Sweden
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hello..

I got a +-22V transformer (secondaries) but i need +-18. can u just unwind a bit of the wire...to get 18v..well i know i can but if i do do i need to rewind the orginal winding to cower all of the toroid core? or can i just leave about 20% naked regarding the secondairy windings??


/micke
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Old 14th July 2001, 03:52 PM   #2
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Default well

Do you have to have just 18? And then comes, how much are current are you talking about? (guessing that at 18v, this can't be really driving anything, so a resistor might be better than screwing with the transformer).
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Old 14th July 2001, 04:24 PM   #3
hifi is offline hifi  Sweden
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+-18V after rectifying about +-25V its for the aleph 3 im tired running it from my humming variable mains transformer

/micke
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Old 14th July 2001, 09:46 PM   #4
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Default unwinding

You can unwind, there is no problem there. If the windings are "imperfect", you will get more leakage etc, so you might want to try several angular positions when you mount the transformer (if inside case).

You could also try to use inductor loading to reduce the voltage (transformer, bridge, series inductor, capacitor bank). Search for PSUD2 and you will have a nice tool for simulating the power supply.
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Old 14th July 2001, 10:52 PM   #5
grataku is offline grataku  United States
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I did it once with NO PROBLEMS WHATSOEVER. Just be gentle and try not to disturb the windings you are not going to remove and do a tight re-tape.
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Old 15th July 2001, 01:40 PM   #6
hifi is offline hifi  Sweden
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Its done now..by the way... i find a nice way to mount the transformer if you have a thight case...fill the "inside" the hole in the dounut of the torid with epoxy or that kind of glue you melt with a heatgun... then just screew it in without the upper mounting bracket...


works nice i think it did reduce som hum aswell..


/micke
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Old 24th July 2001, 11:59 PM   #7
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Quote:
Originally posted by hifi
Its done now..by the way... i find a nice way to mount the transformer if you have a thight case...fill the "inside" the hole in the dounut of the torid with epoxy or that kind of glue you melt with a heatgun... then just screew it in without the upper mounting bracket...


works nice i think it did reduce som hum aswell..


/micke
The epoxy fill for the center hole is one of the standard mounting methods for toroids. The epoxy is normally put there at the factory.

The epoxy should hold the windings tightly. If the cure is slow enough, it might get through the plastic wrap on the transformer, creating an even tighter hold on the windings. This is probably the cause for the reduced hum.

The hot-melt glue is probably a bad idea. Under heavy load, the transformer could get hot enough to melt the glue (that's not very hot). This could result in some nasty damage near the transformer (as it falls ).
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Old 25th July 2001, 06:20 PM   #8
hifi is offline hifi  Sweden
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nope... the glue has withstand about 70c (celsius) witout melting it works fine....

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Old 6th September 2001, 12:50 AM   #9
haldor is offline haldor  United States
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Another alternative (at higher cost and bother) is to get two smaller transformers. Plitron sells a 300VA toroid with 18V secondaries for $45 each. A pair of them will cost you $24 more than one 625VA transformer would, but saves having to modify the beast.

Phil
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