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26th November 2013, 12:31 PM  #1 
diyAudio Member
Join Date: Aug 2008

Transformers with dual primaries?
Does the phase relationship matter when wiring a transformer with dual primaries? I have this small transformer I was going to use for a preamp. I'm wiring it with 3,2 and 4,1 tied together because that's the way it looked on the PCB. I wired up a capacitor (CRCRC) filter to it with a regulated filament supply that has a B+ reference at roughly 1/4 B+. If I ramp it up on my variac, the transformer starts to growl. If I eliminate everything but the first cap section of the B+ supply and it seems fine. Can the cap size actually overload the power transformer?
Is it a phase issue with the primaries? Thanks, Blair 
26th November 2013, 12:36 PM  #2 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Yes, the phase matters.
Use a Mains Bulb Tester to check your wiring at every stage of your assembly. Don't use the Variac until after you know the wiring is correct. A major fault causing excess transformer current could burn out the transformer and with luck also burn out your Variac.
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26th November 2013, 12:44 PM  #3 
diyAudio Member
Join Date: Jan 2003
Location: mississauga ontario canada

The dual primary, wired in series, is for 230V input.
If you only put in 115 then you will only get 1/2 of the 15.94V and only half of the 200V. If you are using a 115V supply then 2,4 should be connected together and 1,3 should be connected together. Be safe. You did not mention a diode bridge...perhaps a schematic of what you are doing would help us help you.
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26th November 2013, 01:05 PM  #4 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

Hi,
It looks like for 230VAC AC in is 1 and 4 and you connect 2&3 together. Consequently for 115VAC AC in is 3&1 and 4&2. The dots indicated the phase by the look of it. rgds, sreten.
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26th November 2013, 02:03 PM  #5 
diyAudio Member
Join Date: Dec 2012
Location: Flanders

There is a dot near the connection of the start of the winding.For 115V connect the two dots together.If you are not shure put a lightbulb (<15W) in series,when it litesup almost normal the connection is wrong.
Mona 
26th November 2013, 03:20 PM  #6  
diyAudio Member
Join Date: Jan 2012
Location: Canada

Quote:
You seem to know what you are talking about and I am just curious about the other combinations to this unusual transformer. I hope that this is not too much to bother you with, but I can't find an answer on the internet (probably using the wrong terminology for the transformer and search) and I want to learn without blowing myself up one day. What if 1 & 2 were only connected (and 3 & 4 not connected to mains)? You would still get the half of the voltage output, but would the current be halved? And without connecting 1 & 3 and 2 & 4, but each plugged in to mains separately as 1 & 2 and 3 & 4, would it still yield the same output with respect to voltage and current as 1 & 3 and 2 & 4? Or would something bad happen? Thanks. 

26th November 2013, 03:48 PM  #7  
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Connect 1&2 to 115Vac mains and you get the correct output voltage. But your input is strangled. You end up with ~ half the current.
Same if you connect just 3&4 to 115Vac mains , you get ~ half current. Dug got this bit right Quote:
Use the Bulb Tester for every power up until no more modifications are made.
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26th November 2013, 05:43 PM  #8 
diyAudio Member
Join Date: May 2007

If you only use half the primary on 115V then you get twice the primary voltage drop and twice the primary heating. In effect, you significantly reduce the VA rating.

27th November 2013, 04:24 AM  #9 
diyAudio Member
Join Date: Aug 2008

Sorry I did not chime back in. Our home had a hot water heater rupture, and we are in a hotel. Yay!
I swear I traced the transformer primaries out to be the way I described, but the transformer growls like two primaries out of phase. It's an oddball with a C core style and dual windings. I doubt, "Signal" marked the phase wrong. I will swap them and see. I am familiar with tube power supplies, so there is a FWB and a proper execution. Thanks! Blair 
27th November 2013, 08:51 AM  #10 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Did you use the Mains Bulb Tester you have lying on the shelf?
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