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If I'm using 2 identical 120V transformers and I have access to 240V input, is it better to put the two transformers in series to double the voltages on the secondaries, or should I just run a single transformer off of the 240V line?

I know that the type of transformers that I'm using (Microwave Oven Transformers) are not very efficient and waste at least 20% of their power to heat/transformer hum/eddy currents and usual core loss reasons. So I'm somewhat hesitant about using another one in series because of this 20% hit twice.

We see that, by the transformer equation, doubling the input voltage doubles the output voltage: $$\frac{V_p}{N_p}=\frac{V_s}{N_s}$$ $$V_s=\frac{(2V_{P1})N_s}{N_p}$$

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    \$\begingroup\$ The ONLY way to make this safe is if you have an American-style 240V supply, which is 120-0-120V with the centre being a good solid Neutral connection, and each transformer connected across a 120V leg. In that case, imbalance between transformers causes current to flow in the Neutral, instead of voltage imbalance which could destroy the transformers. And this is EXACTLY the same answer you accepted in electronics.stackexchange.com/questions/210488/… \$\endgroup\$
    – user16324
    Commented Jan 22, 2016 at 10:21
  • \$\begingroup\$ Sarah, what is it that you didn't understand about these two accepted answers: electronics.stackexchange.com/questions/211963/… and this: electronics.stackexchange.com/questions/211954/…? You cannot put primaries in series without some means of balancing the voltages. Also, if they were in series and adequately balanced, the secondary outputs from each would not be double. \$\endgroup\$
    – Andy aka
    Commented Jan 22, 2016 at 10:37

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If you put a 120V transformer on a 240V line, it will likely blow up. Putting two transformers in series can be tricky, as there are many ways to blow them up that way too. What you are trying to do is potentially very dangerous. There is a chance you could die, from multiple causes, and it appears to me that you don't yet understand your electronic fundamentals well enough to navigate it safely. I would highly recommend that you stop what you are doing, and recruit a professional to help you, or just buy a power supply that will do what you need.

I know this isn't the answer you want, but I think it's the answer you need. Sorry.

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    \$\begingroup\$ @SarahSzabo I'm sorry but you are in no position to judge, and no position to take life-threatening risks with things you don't understand. Please heed the excellent advice you've been given here. \$\endgroup\$
    – user207421
    Commented Jan 22, 2016 at 6:14
  • \$\begingroup\$ The transformer dielectric can be oil, or other substances that are highly flammable. The voltages that you are dealing with will stop your heart in the right circumstances. I design high voltage medical devices. I actually do know what I am talking about here, and I would be partially responsible if something happened to you because I gave you advice that you weren't sufficiently equipped to follow. Electricity is dangerous. Respect it. If you don't, you will lose. \$\endgroup\$
    – Peter Frey
    Commented Jan 22, 2016 at 6:21
  • \$\begingroup\$ Have you tried this in any simulation software@sarah szabo ? if you try this in simulation you may expert what will happen. So you may avoid the unexpected things going happened ! Safety first \$\endgroup\$
    – Photon001
    Commented Jan 22, 2016 at 6:21
  • \$\begingroup\$ @Raj putting two identical primaries in series in a simulation WILL NOT show any problem because they are identical. Your advice is laregly irrelevant and, will give the false impression that primaries can be put in series. You ought to consider this. \$\endgroup\$
    – Andy aka
    Commented Jan 22, 2016 at 10:42
  • \$\begingroup\$ @Raj. Yes. Simulated transformers never explode or burn up. Simulations can't burn you, or stop your heart. The issue in this case isn't whether it's possible to make this work. The problem is that, if you don't really know what you are doing, it can be dangerous. \$\endgroup\$
    – Peter Frey
    Commented Jan 22, 2016 at 13:56

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