Need help with ac transformer application


2006-07-05 3:01 am
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
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.


2006-07-05 3:01 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?
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?


2011-11-17 3:49 pm
SMPS - Switch Mode Power Supply.

The AC voltage applied is often very high frequency, 100KHz is not unusual these days. At these frequencies the core material ceases to be iron and is usually a ferrite compound, but more importantly, the number of turns per volts are drastically reduced.

Such a transformer cannot be used at 50/60Hz.


2006-07-05 3:01 am
Follow up: it requires 50vac @ 20khz...

I am hoping they can either come up with a supply for me to buy or a schematic of what they use. I imagine designing this my self is nearly impossible from Internet data sheets. Everything I can find is dc output...