what to do with center tap?

[truman]

here is a pic of my transformer it has two red wires for mains supply and two blue and a green out the otherside I know the green is the center tap but how would I wire that and how would I know if I have the polarity correct?

thank you

Rob

[900steve]

do you need to use the center tap to get 1/2 the output voltage on the secondary side? if not, just wire nut the center tap and tuck it out of the way.

for example, if you have primary at mains, and secondaries at 12v, if you use the center tap and one secondary, you will get 6v. so if your goal is to get the most out of the secondaries, just don't use the centertap.

someone please correct me if i'm wrong.

-steve

[truman]

ok so if I wired it like this it would be correct keep in mind I havn't added the bleed resistor or diodes yet.

[truman]

the transformer is 24VCT 10A so does that mean I would have to use the center tap to get 24V?

[900steve]

the wiring looks good for 24v. if you wanted 12v, you would have to use the center tap.

do you have a multimeter to check your output?

[truman]

I have a multimeter what would I use to test the voltage accross?

[900steve]

well, if you have a multimeter and probes, you can measure the output voltage of the transformer and it will tell you if you have it hooked up correct or not.

you familiar with safely dealing with exposed mains voltages and how your meter works?

[ViperTX]

24 VCT at 10 amps means that if you use full wave rectification (a diode on each blue wire to the + on the electrolytic and the center-tap to the - on the electrolytic) you can get 12 VAC x 1.414 DC volts at 10 amps.

If you wire the same transformer using a full-wave bridge rectifier (center tap is unused) you'll get 24 VAC x 1.414 at 10 amps.

[truman]

I am familiar with the safety using live power just not that experienced and I know to measure voltage you must measure accross a component I just don't know what is a suitable component to use. any advise will be greatly appreciated. isn't the saying volts jolt current kills I don't know something like that. I am also making a trip to the electronics store tommorrow does anyone have any suggestions of what size diode I would need fuse holders bleed resistor or anything you could think of that would save me another trip it is quite a drive

thank you

Rob

[abasir]

To check the voltage, set your multimeter to measure DC voltage and measure across the capacitor.

[ViperTX]

Rob...what DC voltage do you need?

[gar]

051222-0842 EST USA

ViperTX:

A good approximation for DC load current is as follows:

Full wave bridge rectifier with a capacitor input filter --- DC VoltAmps = AC VoltAmps
Thus in this 24 VAC 10 A transformer application the approximation is 240 / (24 x 1.414 ) = 10 / 1.414 = 7.07 A.

Full wave rectifier center tapped transformer with a capacitor input filter --- DC VoltAmps = AC VoltAmps * Derating
Thus in this 24 VAC 10 A transformer application the approximation is ( 240 / (12 x 1.414 ) ) * maybe 90% = ( 20 / 1.414 ) * 0.9 = 14.14 * 0.9 = 12.7 A.
In full wave center tapped rectification there is more RMS heating of the secondary and thus the need for derating.

The approximation of Output VA = VA rating of the transformer somewhat takes care of the increased heating of the transformer secondary from the peak current resulting from the capacitor input filter.

To determine actual maximum load current one should run transformer temperature rise measurements. One way to get an internal average estimate is to measure the DC resistance of the inner most coil at room temperature and then again after full load for an adequate time, and use the the temperature coefficient of copper wire to calculate temperature rise.

.