I want to measure mains voltage using a transformer. bought a 9V transformer, removed the secondary and rewind it with 10 turns that gives me 2mV per volt 1:500.

The problem is that the ratio is not constant, the ratio decreases from 1:530 to 1:470 as voltage increases on the primary (primary voltage range is 0-250VAC).

  • Why is this change in ratio happening and how I can compensate for it?

Update: I tried winding 10 turn around the primary but problem still exists, I also decreased the load from 100Ω to 10KΩ.

Update2: When I wind another secondary around the primary, the first secondary was in place and I had two secondaries, I removed the first secondary and left with only one secondary winded around the primary. Now the ratio is much more stable, ranging from 1:470 to 1:490.

  • \$\begingroup\$ What were you expecting from an xfmr with high leakage? \$\endgroup\$ Oct 8, 2023 at 14:47
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    \$\begingroup\$ Increase the coupling between primary and secondary by winding the secondary "on top of" the primary, not next to it. \$\endgroup\$ Oct 8, 2023 at 15:46
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    \$\begingroup\$ @Bryan Using a Variac. \$\endgroup\$ Oct 8, 2023 at 16:07
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    \$\begingroup\$ I did wind another 10 rounds what do you mean "another"? Do you keep the winding shown in the pic and wound "another 10 turns" around the primary? Or did you remove it and re-wind around the primary? You must have one primary and one secondary, and the only secondary should be wound around the primary. \$\endgroup\$ Oct 9, 2023 at 12:17
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    \$\begingroup\$ @ElectronSurf that'll be even better but it's not what I meant because it's practically too difficult and not worth. Wind 150 turns (half) of primary, don't cut the wire, wind all your secondaries around this half primary (spread into the window if you can) and then wind the remaining primary on top of the secondary. Each half of the primary is a "bun", and the secondary is the "beef burger". Google "sandwich transformer winding" for further details. \$\endgroup\$ Oct 9, 2023 at 18:06

1 Answer 1


primary voltage range is 0-250VAC

So, with 250 V RMS on the primary, the secondary voltage will be 500 mV RMS but, depending how you measure the secondary voltage could give you a significant error due to the common-mode voltage due to capacitive coupling between primary and secondary. This doesn't produce an error voltage but, that common mode voltage could cause your measurement equipment to be a lot less accurate.

Why is this change in ratio happening and how I can compensate for it?

There is no change in ratio; just an error in your measurement method. I suggest you ground one of the secondary wires, ground the laminate stack and repeat and, if this doesn't clarify the situation then please disclose your measurement method in detail.

It would have been very helpful to know if you got a linear relationship between input voltage and output voltage before you "adjusted" the secondary so radically. I mean, there was very little reason to rewind it because, using a resistive potential divider would have achieved the same result.


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