# basics of transformer

• posted

I have a 40VCT .25A transformer and I'm wondering what those numbers mean..

does the 40VCT mean 40VAC from the center tap? If so across its full secondary its 80VAC? and the .25A means that it can handle at most .25A across its secondary coil(since its a step down from 120VAC the max current in its primary should be smaller... ~80/120*.25?).

Also should I assume these are all RMS values? When I put my variac up to

100VAC and hook it up to my transformer I get about 40 volts across the hole secondary(hence 20V's across from the center tap). But this means that at the mains(117VAC?) I'll be getting more than 40VAC across the full secondary? Is this ok to have the transformer have a little more than its voltage rating?

Thanks, Jon

• posted

Oh, at at those settings(40VAC across full secondary) the output off my caps is ~30VDC "half way" and ~60VDC full (I'm using a split bridge so I can supply 2).

something like

V+ -+- V1o | ---+ --- | +---G | ---+ --- | V- -+- V2o

Thanks, Jon

• posted

```--
40VCT means that with the rated voltage into the primary and the
rated load on the secondary, the secondary voltage will be 40V```
• posted

cool...

Thanks a lot, Jon

• posted

No, the secpondary is 40 volts end-to end with a tap in the centre.

sometimes it's written 20-0-20

yes

under load the voltage will "sag" down from 43V to about 40V

hopefully that 'G' is connected to the centre tap.

Bye. Jasen

• posted

No it means 40 Volts 'centre tapped'. e.g. 0-20-40 or 20-0-20 if you prefer

Graham

• posted

No, as others have said, it's 40 end-to-end, 20-0-20. and the .25A means that it can handle at most .25A

No, this is a misnomer, the current isn't _across_ the coil, it's _through_ the coil. It's the voltage that's across it. Voltage is pressure, current is flow. :-)

Yes, the no-load output voltage is the peak of the input, which for a sine wave, is 1.414 times the RMS.

Yes, as Jasen said, connect that G to the center-tap. You could also get just the +30, by turning the bottom diode around, lose the bottom capacitor, and tie the two diodes' anodes together. That's called a "full-wave center-tap". What you have here is stacked half-wave rectifiers. If you want to double the power- handling capability of that, and get the DC bias out of the secondary, you can add two diodes like this:

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