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Old 22nd September 2005, 06:10 PM   #1
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Default Back to Back transformers to get +B of 200V ??

Hi there,
I am currently attempting to get a little headphone amp project going. It needs a HT of 200V at about 60mA.
I am trying to use a pair of 18V-0 18v-0 toroidal transformers to derive the HT. I am using one secondary windings of the first transformer to feed 18V into the second transformers paralled 18V windings. I then feed this into a full wave rectifier (solid state). We have 240V AC so I was expecting something around about 200V DC loaded and about 300V loaded after filtering. However what I end up with is about 160V DC. This is the same whether loaded or unloaded.
I think the problem is that I am putting in 240V AC over the first transfs full primary winding, but am taking out the AC from a full winding with center tap, which will halve the final AC. Am I correct in this assumption, and if so what will the solution to my problem be ?
I know I should expect high losses using this arrangement, but not 100%.
Before anyone tells me not to do this, I have successfully done this in a tube preamp, but this used split rails, which is slightly different. Theoretically it should work perfectly in any setup - just so long as the first transformer is big enough. In my case my first transformer is about 100VA.

Any help is appreciated.

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Old 22nd September 2005, 06:34 PM   #2
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used to be done all the time with a pair of filament transformers --
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Old 22nd September 2005, 08:21 PM   #3
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Use a full-wave bridge (4 diodes) and you'll get about 300V unloaded. With choke input (at least 15H) you'll get around 200V loaded. If you know the current rating or winding resistance of the transformers, it's possible to estimate quite closely using the PSU Designer program from
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Old 22nd September 2005, 09:58 PM   #4
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Having asked the question, it sort of suggested the answer, which was full wave rectification with the "-" referenced to earth. Tried that and hey presto i'am running at 300V - loaded. I have a small choke, which does little else but take the spike off the charging cycle.
All I need to do now is find a 8W 1K5 resistor to gobble up 90V. Now thats a tricky one.

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Old 22nd September 2005, 10:31 PM   #5
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not that tricky
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Old 23rd September 2005, 03:58 AM   #6
billr is offline billr  New Zealand
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why not use a 12-0-12 transformer connected to the 18-0-18 transformer. that will give you 2/3rds of the normal primary voltage to rectify, therefore if you are getting 300 now, you should get around 200v

just a thought.

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