[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.

.

[truman]

I want 30Vdc @10A this is the simple power supply on Phils site

[gar]

051222-1410 EST USA

truman:

What is the maximum and minimum voltage your load can tolerate? Is the 10 amps a continuous load, or does it seldom occur? Is your average load maybe 5 amps, and once in while for a few seconds it goes to 10 amps?

Transformers, motors, power resistors, generators, etc. can handle short time overload. Semiconductors have to be selected on peak load because they have short thermal time constants relative to things like transformers.

Your 24 VAC 10 A transformer loaded so the output voltage is 24 VAC will produce approximately 24 * 1.414 = 34 V peak into the bridge rectifier. Then you will have 2 diode drops in the bridge rectifier. So nominally you may be slightly above 30 V across the filter capacitor. Transformer internal impedance and how the transformer is rated and what your input line voltage is will determine the output DC voltage at 10 A load.

The equivalent internal resistance (low frequency) of your power supply looking at the power supply from the capacitor terminals may be approximated as a linear resistor, but from zero to full load it is not very linear. You make this determination by varying the load from 0 to maximum current and measuring the voltage at various points. Like 0, 1, 2, 5, 10, 20, 50, and 100%, and maybe some in between these depending upon the change you see. While making this measurement the input AC supply voltage must be constant. Next plot the curve V vs I. The slope of the curve at any point is the resistance to small changes. Take two points on the curve and determine the difference in voltage and current. Then the internal resistance to a change in load in the region of the two points is Rint = delta V / delta I where delta means the difference. Use the absolute value of both differences to make R positive.

.

[truman]

Ok this is the setup I want to put in my control box it is not wired and the breakout board is not there I want it to be mounted to the side of the box. The drives are mounted on a large aluminum heat sink but I can't figure out how to thermally connect the drives to the heatsink these are the 3977 boards on Phils site they have two 0.090 holes to thermally connect them to a heat sink which I believe is also the ground I really need detailed advice so I do this right. I know I havn't fused the power supply or anything yet I just put everything in its place to show the layout any advice is greatly appreciated safety or otherwise.

thank you

Rob

[ViperTX]

Have you contacted Phil? If the corner holes are for thermal dissipation then I suspect that the heatsink itself might need to be isolated....best to contact Phil.

[truman]

Ok I have the information for the heatsinks that I needed thats a done deal what I did was cut the big heatsink into three then mounted them with nylon nuts and bolts with a plastic spacer inbetween the heatsink and the metal plate everything is mounted to this, should be enough to isolate them I hope. Here is a picture I was hoping someone could tell me what to do with the ground from the plug cord

[truman]

oops I see one mistake allready I connected ground to live will change that right away! One other question I have is how do I know polarity is right I heard caps don't like the polarity reversed?

[truman]

some better pics with corrected wiring

[ViperTX]

Well the caps have a + and a - sign.....or a POS and NEG label.....Does your bridge have the following ~,~,+,-....if so then the ~'s go to the transformer and the + goes to the + or pos of the capacitor and the - negative goes to the - or neg of the capacitor.