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Old 26th November 2013, 12:31 PM   #1
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Default 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?

Click the image to open in full size.

Thanks,

Blair
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Old 26th November 2013, 12:36 PM   #2
AndrewT is offline AndrewT  Scotland
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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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Old 26th November 2013, 12:44 PM   #3
DUG is offline DUG  Canada
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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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Old 26th November 2013, 01:05 PM   #4
sreten is offline sreten  United Kingdom
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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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Old 26th November 2013, 02:03 PM   #5
Ketje is offline Ketje  Belgium
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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 lites-up almost normal the connection is wrong.
Mona
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Old 26th November 2013, 03:20 PM   #6
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Quote:
Originally Posted by DUG View Post
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.

DUG,

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.
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Old 26th November 2013, 03:48 PM   #7
AndrewT is offline AndrewT  Scotland
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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:
If you are using a 115V supply then 2,4 should be connected together and 1,3 should be connected together.
You need to power up with a Bulb Tester just in case the manufacturer got the wires swapped.
Use the Bulb Tester for every power up until no more modifications are made.
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Old 26th November 2013, 05:43 PM   #8
DF96 is offline DF96  England
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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.
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Old 27th November 2013, 04:24 AM   #9
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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
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Old 27th November 2013, 08:51 AM   #10
AndrewT is offline AndrewT  Scotland
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Did you use the Mains Bulb Tester you have lying on the shelf?
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