Help with Transformer Values

[JMB]

I am building a VI Curve tracer and have two new transformers, one rated at 115V 28VCT 12VA and the other at 115V 24VCT 12VA. I am using a Wavetek True RMS multimeter and verifying results with a Protek true RMS multimeter. When I hook the transformer up to the wall (which is measuring about 124 V on both meters), I am getting readings that I would expect to be the peak coming out of the transformers (about 40V for the 28 VCT and about 32V for the 24VCT Transformer).

I was thinking that perhaps neither is really doing true RMS but the wall reading seems realistic.

Please help me to understand.

Thanks,
Jay

[h_a]

You run the transformer without load; the spec'd voltage shows up only at the maximum load. The difference in between is the regulation of the transformer and it is pretty bad for smaller transformers and also for EI-types.

A 300VA toroid usually has only 3-4% in comparison.

Have fun, Hannes

[JMB]

Thanks, Hannes, for your explanation. I woke up early this morning with this on my mind and it came to me after realizing how we see the drop in voltage with a load. Now I understand people's enthusiasm for toroids though I guess that schematics involving non toroidal transformers take this increased voltage into account?

Jay

[h_a]

A voltage increase of one third is considerable und stock circuits will not be too happy about that. Any real AB-amp circuit will draw a few hundred mA and the voltage should be much lower then. But of course it's a good idea to check that first.

Have fun, Hannes

[JMB]

I am building a VI curve tracer similar to the one at http://www.seattlerobotics.org/encod...4/VItracer.htm. It has a voltage divider portion that attempts to keep the output voltage relatively constant. I believe that this will be okay as long as I select lower voltage selections for delicate components in circuit? It has a built in current limiting circuit.

Jay