Power transformer voltage - is a load required for secondary voltage measurement?

[capacitorless]

I think this subforum is more for stuff like microphone, interstage and output iron, but I have a mundane/newbie power transformer question.

Can I supply wall (115vac here) on the primary, connect to the secondary with a (in this case, low end) meter, and get a reasonably accurate measurement of the secondary? Or, is a load of some kind and/or good quality meter usually a requirement?

Typically, I would measure in a completed circuit, but that would take quite a bit of work, and I have a reasonably large number (read: pile of old chassis) of transformers to measure as well. I don't need current, since I'll be de-rating whatever I have, based on the original tube complement. Anyone been down this road before?

The chassis in question is a Hammond type G as found in a 40's era DR20 cabinet, but I would like to know how to estimate this empirically.

Edit: would a simple rectifier (either a pair of diodes or full wave bridge) and a RC filter, with a small value resistor to convert to DC be any more useful/accurate? Just looking for simplicity.

 

[13engrsapper]

The type of rectification you choose will change your DC reading, as may load. You can measure your secondary straight from the wall (put an appropriate fuse inline). I am sure one of the smarter guys can give you better info. Any meter should be decent enough here. I would probably rig up a little cord with an inline switch and inline fuse I guess, and some alligator clips on one side and a wall plug on the other.

 

[CJ]

Be careful when measuring a high voltage secondary as some of the center tapped versions will have let's say 325-Centertap-325, so if you measure from the outside legs you will get 650 volts which can cause with some meters.

You will probably read about 15% to 20% higher without a load when measuring AC and maybe 30℅ higher if measuring rectified DC that has a filter cap as the cap will charge to peak value of the sawtooth wave coming off the diodes.

 

[ruffrecords]

Transformer output voltage specs are normally quoted when loaded with their rated current. Due to internal resistance of the windings, this measured voltage will be lower than what you read with no load. How much lower is used to give a figure for transformer regulation. A good transformer will achieve 10% regulation. Physically small transformers tend to be nearer 15%. Most of the loss will be in the dc resistance of the secondaries so you can get a reasonable idea of the rated current simply by measuring the dc resistance of the secondary windings. A simple application of Ohm's law on the measured open circuit voltage and dc resistance will give you a rated current estimate based on, say, an assumed 10% regulation.

But as a first guess, you can assume the output voltage at rated current will be about 10% less than the open circuit measurement.

Cheers

Ian

 

[meloamantis]

I'd say go with the second option and use a rectifier with an RC filter. It’ll give you a clearer idea of what the transformer can actually handle under real conditions. Plus, you’ll get a stable DC reading, which is super helpful if you're checking out a bunch of transformers

 

[fragletrollet]

Wouldn't the reading be about 20% higher after rectifier with no load? Would one have to add a load resistor to the caps to make them actually load anything after they've charged?