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Old 23rd March 2006, 09:43 PM   #21
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Quote:
Originally posted by dnsey
Even if it did work, putting 240V into one or both 120V primaries would result in a higher secondary voltage, not lower!

Exactly!

If you want to lower the secondary voltage a little, you can add turns to the primary. Take some hookup wire (18ga?) and wrap 10 or twenty turns around the core. Wire the existing primaries in series and in series with your added turns. Measure the voltage. Add or remove turns as needed.

Unfortunately, this is not practical if you want to change the secondary voltage a lot, which seems to be your case.
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Old 23rd March 2006, 10:02 PM   #22
Bazukaz is offline Bazukaz  Lithuania
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Quote:
Originally posted by AndrewT
Hi,
unwinding the secondary will reduce your VA rating in direct proportion to the turns reduction.
It will reduce power somewhat , but this is mainly related to wire resistance.
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Old 23rd March 2006, 10:48 PM   #23
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is it possible to find the centre tap of a transformer and cut it? how could you tell it the link you are thinking of cutting is half way?
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Old 24th March 2006, 07:18 AM   #24
AndrewT is online now AndrewT  Scotland
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Hi,
I have proposed this in other threads.

Try splitting the two existing windings into four secondary windings.
Add enough turns to each winding to bring the output voltage back up to your needs.

A slight increase in VA rating will result from the increased winding copper, but it is only slight (I think).
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regards Andrew T.
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Old 24th March 2006, 10:05 AM   #25
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The unit has only one secondary, to be honest i think i will just foget it. The transformers while huge, brand new and stupid cheap just arent correct for this application. Ill try and find some other transfromers, annoyingly my local store, maplin has now stoppped selling transformers so its mail order, a right pain when you just want to try something out. Oh well.

Just a thouhg, does anyone living in London know of any decent component shops. Just interested, for such hueg place i havent found anywhere apart from maplin where i cna walk in a buy stuff


Phil
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Old 24th March 2006, 11:24 AM   #26
paulb is offline paulb  Canada
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The purists will get upset, but you could use a single-supply version of a gainclone. You need an output capacitor, which is much less audible than some people would have you believe.
See Fig. 2 in http://cache.national.com/ds/LM/LM3875.pdf
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Old 24th March 2006, 11:44 AM   #27
dnsey is offline dnsey  United Kingdom
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Quote:
Just a thouhg, does anyone living in London know of any decent component shops. Just interested, for such hueg place i havent found anywhere apart from maplin where i cna walk in a buy stuff
It's a great shame - 20 years ago, almost every other shop in Tottenham Court Road and Edgware Road were real electonics suppiers, wher you could get any component you wanted, and a lot of very obscure stuff too. Now, as you say, they've all but gone
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Old 25th March 2006, 03:38 PM   #28
dnsey is offline dnsey  United Kingdom
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Just occurred to me - there are three RS trade counters in the London area.
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Old 25th March 2006, 04:46 PM   #29
Nordic is offline Nordic  South Africa
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They are not the cheapest, but I do support them as they are the only people willing to bring decent stuff into our country and even have a tradecounter in my town.
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Old 25th March 2006, 06:31 PM   #30
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I like Paulb's idea. We did not have split supplies in the old days so most old amps had output capacitors.
Another way is to use a bridge configuration, so that you do not have much current flowing in the OV line. You then electronically generate a mid point voltage with 2 equal value resistors in series across the supplies.
What is the transformer going to be used for?
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