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Selectronic Rcore transformer
Selectronic Rcore transformer
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Old 13th August 2017, 08:08 AM   #11
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
I can't see any reason to adopt a non standard VA rating method.

If they are telling you they voltage rate at 50% of the specified 30VA, then that means the voltages are when delivering only 15VA i.e. 500mAac into 60r dummy load.

I agree with your assumption that the 10% apparent regulation for the 50% duty becomes ~20% regulation when specified by the usual manufacturers' standard.

That means the transformer is a dual 13.75Vac @ 1Aac. But that is only 27.5VA. The current would have to be increased to achieve 30VA and how hot will the transformer be now?
How hot is it at 27.5VA? Why are they testing at 50% of rated VA?
Buy one if it's cheap enough and measure it.
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Old 1st September 2017, 07:16 PM   #12
scoobis is offline scoobis
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Quote:
Originally Posted by AndrewT View Post
Buy one if it's cheap enough and measure it.
Got one on the way.

i total don't trust Chinese ebay sellers to know what there talking about unless there the manufacturer frankly.

i have a constant current load I can run it into, I'll need to rectify the output and filter it as its a dc load, but that is easy enough. i can monitor the current/voltage on the ac side and graph it.

I can probably even do a temp measurement.

once it arrives, i'll test it and post.

I've got a use for it for something totally way under spec for it. So as long as its not doing something totally weird, its not a total waste of money.

Last edited by scoobis; 1st September 2017 at 07:16 PM. Reason: cleanup
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Old 2nd September 2017, 06:09 PM   #13
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
If you want to check whether the transformer meets specification, then you do not test through a rectifier. Connect a resistor of the correct value and feed in the correct primary voltage. Measure the AC voltage and how high the temperature increases.
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Old 3rd September 2017, 02:52 AM   #14
scoobis is offline scoobis
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To quote one of my favorite aussie's "She'll be right" ...

But really, it should be...I'll be using a true RMS bench meters on the AC side so any distortion of the sinewave at the transformer output caused by the rectifier/capacitor loading will not be a problem. I'll still get correct AC rms voltages and current measurements from the output of the transformer.

so power should be equivalent, and therefor the temperature also should track identically.

Don't worry, I did not plan on using the readings off the constant current load on the DC side.

i actually did give this some thought before i decided to do it this way.
but don't get me wrong, i do agree with you, and if i was a transformer manufacturer, sure id spec it out with a restive load. It is certainly the easiest load on the transformer.

The reality is, anyone using the transformers are going to be running them into rectifiers and not purely resistive / linear loads, so its a more real world benchmark anyway.

But, the real reason i even aprached it this way is that i don't have a bunch of load resistors handy to do such a test, and don't plan on investing in any.. So the DC constant current load will have to do.

You do bring up an important point. which is using the "correct primary voltage" since these are spec'ed at 115v/230v (2 windings series or parallel wired) my line voltage here is 120 though.

So i'll run the test at spec, 115v

i could run multiple tests, just to see but i expect that once one test is run, it would be easy to calculate the results of other voltages.
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