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I need to create an isolation transformer so I can float a Device Under Test (DUT).

The parts I have to do so are two transformers: 120V primary / 6.3V-0-6.3V secondary @ 3A.

I intend to connect the secondaries together to create an (essentially) 1:1 isolation transformer with 120V output.

My question is: do I connect the center taps?

My understanding is that with the center taps connected, the voltages reference them as neutral - essentially creating -6.3V and +6.3V (180 degrees out of phase).

Without the center taps, 12.6V would be the voltage across the remaining two (secondary) leads - right?

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  • \$\begingroup\$ Why use two transformers when it sounds like you only need one? \$\endgroup\$ – Andy aka Jan 22 '15 at 9:54
  • \$\begingroup\$ I need to float a Variac, which takes 120V as the input. \$\endgroup\$ – Ramrod Jan 22 '15 at 9:56
  • \$\begingroup\$ Why use TWO transformers? \$\endgroup\$ – Andy aka Jan 22 '15 at 10:05
  • \$\begingroup\$ Because I do not own a 1:1 transformer. I do however have the two I mentioned above. \$\endgroup\$ – Ramrod Jan 22 '15 at 10:36
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    \$\begingroup\$ Do make sure that the DUT and any of its supporting equipment does not draw more current than the rating of these transformers. The 120V side of these transformers is going to be in the 300mA range. \$\endgroup\$ – Michael Karas Jan 22 '15 at 13:10
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Leave the center taps disconnected, since with floating center taps there'll be no chance of imbalance in the 12.6 volt windings causing damage anywhere.

Also, be sure that your variac drives the transformer array instead of the array driving the variac, since that way the array won't have to supply the variac's losses.

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    \$\begingroup\$ +1 for suggesting that the variac should go in the input line side of the transformer array. \$\endgroup\$ – Michael Karas Jan 22 '15 at 13:07
  • \$\begingroup\$ Ok, so Variac -> transformer -> load. Should I connect the transformer case to the third prong on the variac output, or leave the entire transformer case floating? \$\endgroup\$ – Ramrod Jan 23 '15 at 6:28
  • \$\begingroup\$ Transformer case??? \$\endgroup\$ – EM Fields Jan 23 '15 at 7:23
  • \$\begingroup\$ Well, liquid will be in use. Not nearby the transformers, but I'd prefer to give them some level of protection. I intended to mount them inside an old vented PSU case. \$\endgroup\$ – Ramrod Jan 23 '15 at 15:56
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Your solution will work essentially equally well with or without the centre taps connected.

In your position I'd probably not connect them as if there is and slight voltage imbalance between the halves on either transformer, this would have less ill effect with centre taps NOT connected.

What you are proposing is perfectly viable and seems to be safe enough - and I'm sure I've done things like that somewhere along the way over the years. When operated at full power losses will be higher than usual for a "normal" isolating transformer. And you'll probably get worse regulation / more sagging under load.

More soon ...

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Your idea will work best without the centre taps connected but there's a basic problem if you are expecting to produce an isolated 120V AC supply due to the transformers regulation under load. A lot of transformers will not have the "ideal" turns ratio to produce a given output and on no load a 12VAC output transformer may well be 13 volts AC. Under load conditions the 13V drops to 12V but the basic turns ratio is biased to produce a slightly higher output voltage under load.

This "amended" turns ratio works OK when you step down the 120V input but then works against you when you step up the voltage - the overall effect is that it may look like 120V on no load but this will drop to possible closer to 100V when you use anything near full load current on the output.

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You do not need to connect the centre tap. Leave both unconnected. If you connect both you even risk of overloading the transformer as there could be a small voltage difference between both centre taps.

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As others have recommended, don't connect the center taps.

However, I don't think anybody has dealt properly with the power limitation issue. You have not indicated the current capacity of your transformer secondary. This will directly limit the power which you can draw from your isolated 120 VAC. Let's say your transformer is rated for 1 amp on the secondary. Then this level (1 amp and 6.3 volts, or 6.3 watts) is all you can feed into the second transformer, and it's all you can pull from your isolated 120 volts. For this example, that's 120 volts / 6.3 watts, or about 50 mA.

This may or may not be a problem, but you need to be aware of it. You need to calculate the power drawn by your isolated load, and see if it's compatible with the transformers you've got. Also be aware that the above calculation assumes a power factor of 1. If your load is (for instance) a switching power supply which does not have a good input filter, you will not be able to draw as much power as you think without the transformer limits giving you problems.

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  • \$\begingroup\$ "The parts I have to do so are two transformers: 120V primary / 6.3V-0-6.3V secondary @ 3A." \$\endgroup\$ – EM Fields Jan 22 '15 at 16:35
  • \$\begingroup\$ Right. So 3 amps times 6.3 volts is 19 watts. Is your isolated load less than 19 watts? If so, you're OK. If not, be prepared for (at the least) your transformers getting hot. In the worst case, they may catch fire or fail in some manner that damages your load. \$\endgroup\$ – WhatRoughBeast Jan 22 '15 at 16:43
  • \$\begingroup\$ To modify my previous comment: I suffered a brain fart. The power available is 12.6 times 3 amps, or 38 watts. Sorry. \$\endgroup\$ – WhatRoughBeast Jan 22 '15 at 17:42
  • \$\begingroup\$ I'll be way under 38 watts, and don't intend to even reach 0.5A. \$\endgroup\$ – Ramrod Jan 23 '15 at 6:25

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