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ehous 16th February 2013 02:53 PM

Need help with ac transformer application
I have a step up transformer for a high voltage tube application.

The primary is .4 ohms measured with a multimeter. I am blowing fuses on a variac instantly. The power supply was bought new surplus and the manufacturer is very cooperative, however, I do not have an understanding of transformers to talk to them.

The primary turns is 18 single coil. The specified amp draw is 1.2 amps at 50 volts in, "not 120"

Any idea what I need to say when I call them Monday? I have spoken to them twice and they the plugged it into a variac to test prior to shipping to me. There has to be something I am missing for .4 ohms not to short the input.

Thanks, Ed

KatieandDad 16th February 2013 03:05 PM

Are you using the transformer at the correct frequency ?

It sounds like it might be an SMPS transformer.

ehous 16th February 2013 03:22 PM

That I do not understand. I bought it assuming it was going to work off a 50vac transformer at 60 hz.


DF96 16th February 2013 04:10 PM

18 turns is not much for a 60Hz primary, even for 50V. Could it be intended for an aircraft 400Hz supply?

DUG 16th February 2013 04:18 PM

Pictures? Manufacturer? Part number? Schematic/pinout?

ehous 16th February 2013 04:41 PM

It is a 5kdc supply for an electrostatic application. As stated the manufacturer is willing to talk, after 2 phone calls, I figure I am not asking the right questions.

First time uploading photos, excuse my ignorance if it does not work.

Flickr: ehous' Photostream


ehous 16th February 2013 04:45 PM

Follow up, they assured me in their testing, the low voltage components were control circuits and were not used in the test, simply 50 vac in, 5000vdc out.


Enzo 16th February 2013 05:24 PM

It is not as simple as you describe. 50vAC in but 5000v DC out, means we have all those other components - the rectifier diodes,capacitors and such. You are focusing on the transformer alone, but a shorted diode would surely blow fuses. Check all those parts for shorts.

If the primary is only 18 turns of wire, it will naturally measure VERY low resistance. It is after all just a piece of wire. But it is a transformer, the circuit doesn't see it as a DC resistance.

Wait, are you dialing a variac down to 50VAC to power this thing? Instead of powering it from a 50VAC transformer? Do you have an isolation transformer? COuld you power the isolation transformer from your variac and connect the power supply to THAT? I am concerned it may be a result of your setup.

ehous 17th February 2013 01:32 AM

Thanks for the responses, I used a 117 to 50 volt toroidal, with a 10 amp fuse between the 50 volt secondary and the high voltage primary. The fuse failed instantly using a variac on the main.

I will follow up if I get any further help from the manufacturer.

These posts have directed me to ask them what frequency the primary is expecting? And if its switch mode, do they have the power supply to offer me or a schematic to build from?

Does anyone know if a amateur can prototype board a dependable smps in a point to point DIY fashion off a data sheet? Or is there engineering involved?

johansen 17th February 2013 03:48 AM

3 volts per turn @60hz is on the order of a 25 kilogram transformer--and that's just the iron core.

the capacitors shown in that schematic are adaquate for something on the order of 20Khz--which would make more sense.

as far as building your own 5000 volt power supply, that really depends on how much regulation you need. does it need to stay within 50 volts? or is +/- 500 ok?

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