# Series or Double Input Voltage for Tranformer?

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: $$\frac{V_p}{N_p}=\frac{V_s}{N_s}$$ $$V_s=\frac{(2V_{P1})N_s}{N_p}$$

• 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/…
– user16324
Commented Jan 22, 2016 at 10:21
• 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. Commented Jan 22, 2016 at 10:37