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-   -   Need help with dual secondaries in series (http://www.diyaudio.com/forums/chip-amps/111688-need-help-dual-secondaries-series.html)

glt 10th November 2007 07:38 PM

Need help with dual secondaries in series
 
Hi, first pot here. I'm building the chipamp.com PS and used a transformer with dual 30V secondaries. However, the secondaries are in series (only 3 leads.It is not center tapped as the voltage across both secondaries is 60V). As you may know, I get separate virtual grounds in the chimpamp.com PS. I am able to get +40 and -40 v. However there is 30V between the two virtual grounds of the PS, so when I shorted them (thinking they could be shorted) I burned a bunch of fuses.

After thinking about it, it kind of make sense since the virtual ground of the secondaries (the zero volt of the sine wave) must be some volts apart since the secondaries are in series.

So the question is: is there any way I can get the grounds of PS at the same level (since I need to connect them to the ground of the amplifier. (I am using the Hypex UCD180)

Thanks in advance for any help.

head_spaz 11th November 2007 11:04 AM

Hi, and welcome to DIYAudio.

This is how to do it.

http://home.swbell.net/deewm/images/XFMR.jpg

Please keep in mind that +/- 40V is a bit much for a National chipamp. IIRC... 84 V is max for Overture Series. Don't forget to budget approximately 10% safety margin for mainline overvoltage fluctuations. When I lived in Anaheim (moved away in 99), we sometimes got overvoltages up to 176 volts!!! Could last for weeks at a time. Can't even begin to tell you how many appliances failed over the years, but I will say that my guitar equip was always blowing up. Since moving to Arkansas, haven't lost a single device. Line voltage here peaks at 124VAC during peak hours. Even with lightening storms of biblical proportions I've not lost a single appliance. I don't regret moving.

Nordic 11th November 2007 11:18 AM

If it only has 3 leads for secondaries , it IS centre tapped, and you should use only one rectifier.

head_spaz 11th November 2007 11:35 AM

Excuse me, I seem to have misread the original post. It suggests separate dual secondaries, and then a contradiction indicating it has three leads. I'm so easily confused my nickname is head_spaz.

Indeed Nordic is right IF you have only three leads on the secondary. One is a center tap (or an off-center tap, but that's kind of rare) and you'll need to incorporate a single bridge rectifier.

Audio sector.com, or Chipamp.com, or maybe both, provide instructions for using a CT transformer with their PSU boards.
Rather than search for the link, I'll just post a pic that I archived one day.

http://home.swbell.net/deewm/images/CT_Trans.jpg

Credit wherever credit is due... etc...

Hope this helps.

glt 12th November 2007 01:48 AM

Thanks so much for the replies. The transformer is indeed dual secondaries but the secondaries are connected in series. I checked http://www.avellindberg.com/transfor...onnections.htm and indeed there is a configuration where the secondaries are in series.

I think the difference between CT and series secondaries is that in CT one side is half wave and the other side is half wave. In dual secondaries in series, both sides are full wave.

I did some more experiments changing the phase of the primaries (which are connected in parallel) and blew more fuses. So there is only one the only way to connect the primaries. I also experimented by changing the connections of the secondaries and I couldn't get the virtual grounds to be at the same voltage

So I think the transformer behaves like a CT transformer and only one bridge can be used. However, when I measure the voltage across the outer leads (not the "middle" lead), I get 60V AC (84VDC)

Not sure what's the purpose of such transformer, but I got it cheap and did not come with any kind of spec. It was an avel lindberg though... I guess I need to pay full price and get a "real" dual secondary transformer... Thanks again for the replies.

glt 12th November 2007 06:41 AM

Salvaged the transformer
 
Update: Just after giving up on using the transformer, I decided to open up the outer insulation and look inside. I then disconnected the two secondaries and soldered a separate wire to the other wire. Now I have a well behaved separate dual secondaries transformer. Patched it up with shrink tubing, scotch tape and some plastic sheets....


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