Understanding Specification & Datasheet For Center Tap Power Transformer

Question

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

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.

Answer 1

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

The unloaded output voltage will be higher.

Answer 2

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.