PSUDII transformer question

[rotaspec]

For my first attempt at PSUDII, I am trying to design a PSU for my Aikido preamp.
EZ80 FW rectifier, CRC filter.

The real transformer I am using measures 300v AC per side (600vCT unloaded) according to both my DMM and Conway FET input analog multimeter. I don't know the trafo rating as it came out of a console, but it's plenty heavy for 4 small tubes.

If I tell PSUD that it is 600v RMS, I get ridiculous voltages out. If I tell it 300v RMS I get much closer to an actual measurement I made with a breadboarded circuit (30mA load gives me 360v DC).

Why does PSUD want only half the measured RMS? Or does it assume that the RMS is just one half of the secondary.
Perhaps I am dense and can't see the obvious here, but if you buy a trafo rated at 600v CT at say 100mA, surely what you measure is 600v across the total secondary.

Put another way, if I design a PSU that needs a trafo of 300v RMS according to PSUD, what do I order? 300v or 600v?

Gary

[aardvarkash10]

Its not a 600VCT transformer, that's why. Its 300V CT - in essence two 300V transformers operating out of phase. If you specify a CT transformer in PSUD, it assumes the voltage is per phase, not across the two phases.

[rotaspec]

It would have been nice to have seen that assumption somewhere in the help file for PSUD.

It seems that even the manufacturers are/were unsure of this. I have here a Beacon R139 transformer clearly stamped SEC: 72v CT 1 AMP. By your argument it should be called 36v CT.

If I measure it I see (a little over off load) 72v. So it looks like a 72v, I'm told by the label it is 72v, but I have to tell PSUD it is 36v. Correct?

[Cale06]

When you are on the edit screen for the transformer voltage and source resistance click on the help button. It says:

"Enter the voltage for the secondary of the transformer. Note that this must be the off-load voltage, which is generally higher than the rated voltage of the transformer.

For full wave rectifiers, use the voltage for one side only. For example, if the transformer off load voltage is 350-0-350V (700V end to end), the correct value to enter is 350.

Enter the source impedance of the transformer. If you don't know this, you can click on the [...] buttons to invoke the off load voltage calculator, or source impedance calculator respectively."

My only question is, do you also halve the source resistance if it's a full wave rectifier? Common sense would say yes but I'm not positive.

[aardvarkash10]

its in the help file...

[rotaspec]

I see it now under the edit transformer help. Thanks for that.
What I was reading (and note what I said about the Beacon tranny) was this under:
Nominal RMS output voltage

"The RMS output for which the transformer is designed during normal operation. This is normally the voltage used by the transformer manufacturer to describe the product."

Now you can see where things get a bit hazy for us old farts who have forgotten all the stuff taught 40 years ago and are only just taking it up again.

[aardvarkash10]

regardless, you have to think about how you are going to use it. You MAY decide to use it bridge or half wave, in which case you will be measuring across both windings.

But a CT is pretty obviously going to divide the output end to end in half.

Like you, brain fade of early middle age is setting in - I often find myself doodling a sketch of my set-ups to clarify in my mind what I need to achieve and what I have. It soon clears up the nagging doubts...

[boywonder]

Originally Posted by rotaspec

It would have been nice to have seen that assumption somewhere in the help file for PSUD.

It seems that even the manufacturers are/were unsure of this. I have here a Beacon R139 transformer clearly stamped SEC: 72v CT 1 AMP. By your argument it should be called 36v CT.

If I measure it I see (a little over off load) 72v. So it looks like a 72v, I'm told by the label it is 72v, but I have to tell PSUD it is 36v. Correct?

I'm assuming that you measured 72V end-to-end, correct? 72V CT = 36V-0-36V. Either end to the CT will measure 36V.

If you are modeling a FW CT PS in PSUD then you would input 36V, since the CT is grounded.

If you are modeling a bridge rectifier, then you would enter 36V if you are using 1/2 the secondary (one end and the CT), or 72V if you are using both ends of the secondary (leave the CT disconnected)

[vitalstates]

I've asked this question before, and just this week I asked the same question on another forum....I'm still scratching my head!

I recently built a ps for a 2A3 circuit, using hammond 302ax(300-0-300). I entered 300v transformer into psud. Originally psud told me to expect 310ish volts but I actually got 370ish.

psud is very critical about internal resistance on secondaries and as I've never found any published figures for Hammond txs I've just measured dcr and entered that into psud.

to cut a long story short, after investigation, the only way I can get psud to agree with reality is by entering a 300-0-300 transformer as 350v together with the actual measured secondary resistance......I am confused!!


I've always added the secondary dcr to the value of the limiters on the rectifier anodes and entered the result into the transformer resistance box, does this sound correct?

has anybody figured out what psud value to enter for Hammond transformers?

Ed

edit: my mains is slightly over standard...248-250 when using 240 transformers......but this doesn't account for the large discrepancy.

[kavermei]

For correct internal resistance, you need to add the reflected DCR of the primary winding, too.