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Wiring transformers in parallel

Zeebit

Zeebit

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Can I hook up two identical transformers in parallel to increase the maximum current draw?

The transformers are of the same part number, 220V to 12V, center-tap. I plan to use it in tapped config.
 





T

Things

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Yes, but you'll need to make sure you get the phasing right or they'll cancel each other out. Hook them up and if you get very little voltage, reverse the mains input on the primary on one of them.
 
Zeebit

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Great! I just hooked them up and they both output the same voltage on the secondary with a few mV difference.
 
lasersbee

lasersbee

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You'll also need to phase the output...
Remember that they are AC devices...


Jerry
 
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Leodahsan

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Zeebit said:
How do I do that?
Click to expand...

This is what Things meant in the post #2. If you hook the transformers with phases not matched, their outputs will cancel themselves, and you can see that measuring the paralleled output. It should measure millivolts if hooked unphased.

If you hook it ok, it should measure around 12v.
 
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Cyparagon

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Things' method will work for NSTs or other current-limited transformers, but if you're backwards on a regular type, it will burn up rather quickly. Try this instead:

Connect one wire from the secondary to a wire from the secondary of the other transformer.
Apply mains voltage to the primaries of both transformers.
Measure the voltage between the two remaining secondary wires.
If the voltage is zero or close to zero, you may connect those two remaining wires together.
If the voltage is twice what you expect, switch the polarity (for lack of a better term).
 
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Sigurthr

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Excellent methodology ^

I would recommend wiring the primaries identically (in reference to physical orientation of wire in relation to the core) and reversing the phasing of the secondary in the event the transformers are out of phase (for common low voltage transformers - not MOTs), and not the primaries as I have come across several mains stepdown transformers that have one end of the primary connected to the core/chasis (autotransformer style). This end is supposed to be neutral/ground. Reversing the primary on this type of transformer will result in electrocution hazard as it will cause the chasis to become live.

On a MOT this is not advised as the secondary instead has one side tied to the core/chasis which is grounded.

I found this out the hard way one day but thankfully I was using a current limited isolation transformer so I did not suffer more than a rude wake up call.
 
Zeebit

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The transformers I have have labels on them. I have tested them out and I got the proper phasing.

OT: I tried to do a continuity test on a TO-220 LM317 between the tab and the pins. Both input and output pins are connected to the tab. Based on the datasheets, only the output pin should be connected to the tab. Is there something wrong with the one I have?
 
Zeebit

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Bump.

I need answers. The store will close soon and they won't be open this weekend.
 
Cyparagon

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When in doubt, compare to a known-good one. Don't use continuity unless you know what you're doing. Different meters will test for continuity differently, and a continuity beep does not mean zero ohms. Use resistance instead. Only the output pin should have zero resistance to the tab.
 
Zeebit

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I just measure the resistances and this are the values I got.

Adj-tab = 60ohms
Out-tab = 0 ohms
in-tab = 1.5 ohms

It looks to be okay. Thanks for your help!
 
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Zeebit

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One more thing. I need to use both +/-12V rails instead of +12 and the tap.

I will be using a bridge rectifier to get +/-12V. Where do I need to put the smoothing caps? Between the +/-12V rails or one at +12V to gnd and one at -12 to gnd?
 
Sigurthr

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Just remember that with one on each when using it as a 24V supply you will only have half the capacitance and thus twice the Vripple for the same amount of current draw. I too put one on each but then I put another of equal capacitance (but double voltage rating) across the + and - lines to equalize the Vripple.
 
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