I have the following instrument transformer for voltage measurement. The secondary winding is directly connected to the analog inputs at the ADC with \$R_l=300\,\Omega\$ connected across the secondary terminals.

I have found that the transformer distorts the measured voltage. Please see the attached waveforms recorded by the osciloscope

enter image description here

The red trace represents the voltage on the primary side and the purple trace respresents the voltage at the secondary side.

Can anybody tell me why this behavior occurs and how to correct that?


I have studied a literature regarding the instrument transformers and I have found there that each voltage transformer has some inherent phase error and voltage conversion error. The authors of the literature stated that those errors are caused by the resistances and leakage inductances of both the windings. Basically the voltage drops on the resistances and leakage inductances causes those errors. Due to this fact the way how to minimize those errors is to minimize the voltage drops. The voltage drops will be minimized in case only small current will flow through those resistances and leakage inductances i.e. we have to connect high enough load resistance on the secondary winding. Based on that fact I have thought that the transformer operating in the open circuit conditions at the secondary side should achieve the smallest error. So I have done the same measurement as mentioned above in open circuit conditions and I was surprised that the phase error is greater than in case \$R_l=300\,\Omega\$.

enter image description here

  • 3
    \$\begingroup\$ That looks like a mains step down transformer. It's not intended for measurement but deriving 6V ac from the mains. What exactly are you connecting to it, on both sides? Can you post a schematic? \$\endgroup\$
    – danmcb
    May 27, 2021 at 8:36
  • \$\begingroup\$ Poor datasheet but "inherently short-circuit-proof" tells us the leakage inductance and/or resistance is very high and will distort your measurements if you have any load on the secondary. Please draw a schematic of your circuit. \$\endgroup\$
    – winny
    May 27, 2021 at 9:26
  • \$\begingroup\$ you have some Phase shift at the secondary coil of your transformer. in every forms of transformers, you have some error for giving the exact phase. you can solve this problem by putting right value of a capacitor and connect it in series with your output load. i had the same issue and i fix it this way. but for your problem, i haven't many parameters (the other part of your circuit, it's application, electrical characteristics and .... ) \$\endgroup\$ May 27, 2021 at 10:27


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