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whitelabrat 18th April 2008 06:50 PM

Transformer current question
I'm trying my hand at designing a power supply, but I have a question about center tapped transformers.

I have a Hammond 373BX rated for 700Vac and 175ma. Using a two diode full wave rectifier with the center tap to ground, can I assume that I effectively have 350Vac and 350ma?

tubewade 18th April 2008 07:01 PM

Yes, that has been my experience, provided that the given current rating was across the full winding to begin with. In using centre tapped full wave rectification, each half of the secondary is given some time to cool while the other side is delivering current. The primary winding only cares how many total VA are being drawn and you won't exceed that with half the voltage and twice the current.

Tom Bavis 18th April 2008 07:26 PM

You can get about 175 mA DC with a capacitor input filter, or about 250 mA with a choke input filter (at lower voltage - nothing comes for free).

It is difficult to calculate the ratio of RMS transformer current to DC output, but you can use the table at or get the free program PSU designer form

whitelabrat 18th April 2008 07:58 PM

Ok. To apply the same idea. Let's say I have a target B+ of 345vdc at about 27ma. I've got a 500vct transformer that's rated for 40ma at that voltage. I could then guess that I'll have an effective 57ma with a full wave rectifier and my 10H 500ohm choke. According to PSU Designer II I should be just a bit under the mark, but close enough.


500vct/40ma --> UF4007 full wave --> 47uf --> 10H 500ohm --> 120uf --> 27ma current source = ~325v B+

whitelabrat 18th April 2008 08:08 PM

I'm about to go off topic, but for safety sake I'd like to have a bleeder resistor in the circuit. How about a 1M ohm resistor after second filter cap? I guess that would draw about 0.3ma by ohm's law? I'm a bit math dyslexic so bear with me.

Tom Bavis 18th April 2008 08:39 PM

Yes, though that will take about 3 minutes to discharge to below 100V. I'd use a smaller value - like 220K/1W, or 150K/2W.

The bleeder is important if you power it up without tubes, or switch it off before they warm up. The normal operating current bleeds the supply faster under normal operation.

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