In a desire to better understand transformers, I have purchased a simple power transformer:

enter image description here

It is the model 187C24, manufactured by Hammond. I'm having trouble understanding the data sheet. I have added the relevant part of the data sheet above. The complete data sheet can be found here.

If I understand the data sheet correctly, the primary coil has a single primary coil between terminals 2 and 3. It also has a secondary coil between terminals 5 and 8. There is a center tap on the secondary coil, terminal 7.

The data sheet also says that if I put 115 VAC in, the transformer will output 24 VAC. According to this previous question, that means if I put 115 VAC across 2 and 3, I will get 24 VAC across 5 and 8.

I don't want to work with 115 VAC for safety reasons. Therefore, I instead used a signal generator to create a 60 Hz, 1.15 Vrms signal. I then applied this signal to terminals 2 and 3. I expected that there would be 0.24 Vrms across the 5 and 8.

I used an oscilloscope to view voltage across terminals 5 and 8, and found the output to be 0.298 Vrms. This is 25% larger than I expected the output to be.

Why do my results differ substantially from the data sheet?

I don't know enough to judge the reason, but I have two guesses:

  1. This transformer behaves differently at low voltages/low currents
  2. The tolerance of this transformer is really low
  3. My particular transformer is defective.

2 Answers 2


The rated output voltage of a transformer secondary is given under a rated load current.

The unloaded output voltage will be higher.


In addition to the unloaded voltage being higher than the voltage at rated load I notice that you have quoted your voltage measurement to only one significant digit, 0.3 Vrms.

Most digital meters will have a rated accuracy of a few percent on VAC RMS ±2 or 3 digits so you can't rely on this reading. If you are measuring on, for example, the 2.000 VAC range and reading 0.300 V then your reading should be more accurate than measuring on the 200.0 VAC range and getting 0.3 V.

  • \$\begingroup\$ Thanks, I will update. I measured it on the oscilloscope. It was actually 298 mVrms or so. \$\endgroup\$ Jun 28, 2021 at 0:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.