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Electronics Forum / Basics / January 2006basics of transformer
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 >I have a 40VCT .25A transformer and I'm wondering what those numbers mean.. > [quoted text clipped - 9 lines] > secondary? Is this ok to have the transformer have a little more than its > voltage rating? 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 >>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 No, the secpondary is 40 volts end-to end with a tap in the centre. sometimes it's written 20-0-20 >> Also should I assume these are all RMS values? yes >> 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? under load the voltage will "sag" down from 43V to about 40V > 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 [quoted text clipped - 13 lines] > | > V- -+- V2o hopefully that 'G' is connected to the centre tap. Bye. Jasen >>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? 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 >> across its secondary coil 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. :-) > 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). Yes, the no-load output voltage is the peak of the input, which for a sine wave, is 1.414 times the RMS. > something like > [quoted text clipped - 9 lines] > | | > `-----|<|-----+- V2o = V- 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: > ,---+-|>|--+--------+- V1o = V+ - | | | | > | +-|<|--|---, | > ( | | ---+ [quoted text clipped - 6 lines] > | | | | > `---+-|<|------+----+- V2o = V- That's just a squared-off full-wave bridge, with 0V from the center-tap, just like you had, but exploits the other half- cycle. :-) We've determined that it's safe to turn the variac up to 120V, and measure the no-load output there; then, if you wanted to , you could plug the tranny right into a the fused, switched mains. Then, if you want, you could put various loads on the output and see what happens to the voltage and current. For a transformer this size, I might even use a .1A fuse, lessee, 40V at .25A is 10 watts, which is like .083A at 120V. :-) Cheers! Rich >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?). --- 40VCT means that with the rated voltage into the primary and the rated load on the secondary, the secondary voltage will be 40V across the entire secondary. (20V from the center tap to either end.) --- >Also should I assume these are all RMS values? --- Yes. --- > 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? --- Yes. It's supposed to because of the fact that since there will be resistive losses in the primary and secondary windings when the transformer's secondary is fully loaded, there will be an inevitable rise in voltage across the primary when it's less than fully loaded. The difference between the no-load and full load voltage is known as the regulation of the transformer, and for small transformers like you have is typically 30%  SignatureJohn Fields Professional Circuit Designer >>I have a 40VCT .25A transformer and I'm wondering what those numbers >>mean.. [quoted text clipped - 34 lines] > the regulation of the transformer, and for small transformers like > you have is typically 30% cool... Thanks a lot, Jon > I have a 40VCT .25A transformer and I'm wondering what those numbers mean.. > > does the 40VCT mean 40VAC from the center tap? No it means 40 Volts 'centre tapped'. e.g. 0-20-40 or 20-0-20 if you prefer Graham |
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