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Old 13th December 2008, 05:24 PM   #1
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Default One more PSU2 question

PSU2 is a fine tool but I think I'm banging on the wrong end.

Most power transformers are rated for potential and current, ie 300-0-300 ct and 100ma. Yet the PSU2 transformer box wants voltage and impedance. I'm using ohms law to calculate impedance: 300v/.1amp = 3000ohms. I gather this is not the way however because the circuits I draw do not produce the rated current and potential at the transformer. More like 30ma @ 200v.

Please clue me in.

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Old 13th December 2008, 05:56 PM   #2
Stixx is offline Stixx  Germany
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Though I can't explain in real scientific terms I can tell you are mixing things up.

Curent draw is dictated by your schematic and is set in PSUD2 with I1.

The transformer box wants the resistance of the secondary winding... I do wonder where you found impedance? You can either take the resistance out of the manufacturer data, or guess it, or have PSUD calculate it (example in the attached image).

That should lead to more realistic results (since PSUD2 is a great tool).

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Old 13th December 2008, 07:23 PM   #3
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Thanks for the help, I get it now.

I just opened up an old tube powered Simpson signal generator I had sitting on the bench and measured transformer and inductor resistance and modeled that circuit and it all jives with reality.

I wonder if this could be used as a tool for estimating current ratings on those old junk bin transformers and inductors?

As for new part builds, I just need to dig a little deeper to find the resistance from the manufacturers specs.
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Old 13th December 2008, 10:08 PM   #4
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I wonder if this could be used as a tool for estimating current ratings on those old junk bin transformers and inductors?
No, afraid not. It's true that a winding with high resistance won't be able to supply high current, but that's about all it tells you. The size of the iron core also limits the available current.
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