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51fordf2
05-20-2009, 11:48 AM
Hi, all...

I have a 12.5 - 0 - 12.5, 3 amp VCT transformer. My question is, is 3 amps the maximum combined for the transformer, so each 1/2 would be capable of 1.5 amp? If I run just one side, still 1.5 amp?

I can't find this anywhere, I thought it would have been easy to find.

thanks in advance!

Roger

P. Short
05-20-2009, 04:10 PM
Each 'half' should be capable of 3A RMS while maintaining 12.5V RMS across its outputs with a nominal 110VAC input @ 50/60 Hz at some temperature or other.

51fordf2
05-20-2009, 05:06 PM
Cool! Thanks, Phil!! That's what I was "hoping", but knowing my luck, figured it was 1.5 per half...

Guess I should go buy a lottery ticket...

R

WireWrap
05-21-2009, 09:47 AM
Cool! Thanks, Phil!! That's what I was "hoping", but knowing my luck, figured it was 1.5 per half...


The limit on output current from a transformer (simplifying several factors) is the size of the secondary (output) winding. The larger the wire, the larger the current capability. Since your transformer is rated at 3 amps, the secondary must be capable of that current. Even though you have a center-tapped secondary, it is capable of 25 volts at 3 amps if you aren't using the center-tap. In that instance, the transformer is passing the same 3 amp current throughout the entire secondary winding. So it WILL be sized to handle that current, whether you draw it all or not. You could use this for two separate supplies, and draw different currents through each half of the secondary, but the limit is the same for each -- 3 amps.

If I haven't managed to totally confuse you, let me know & I'll try to make it more incomprehensible... ;) :confused: :!:

.

51fordf2
05-21-2009, 10:09 AM
You could use this for two separate supplies, and draw different currents through each half of the secondary, but the limit is the same for each -- 3 amps.

"the same for each" has me confused. Scenario - I am using the CT, for two 6 volt power sources. Both will pull two amps for a total of 4 amps (2 amps per side) - can I do that?

I guess the question is, is the current rating per side, or total? If I understand it right, when you are using the center tap, you basically have two separate, but equal transformers, and since the winding is capable of 3 amps, either side would be capable of 3 amps. Otherwise, if you don't use both sides, you get the full voltage, but still at 3 amps.

Eh?

thanks,

R

WireWrap
05-21-2009, 01:38 PM
"the same for each" has me confused. Scenario - I am using the CT, for two 6 volt power sources. Both will pull two amps for a total of 4 amps (2 amps per side) - can I do that?

I guess the question is, is the current rating per side, or total? If I understand it right, when you are using the center tap, you basically have two separate, but equal transformers, and since the winding is capable of 3 amps, either side would be capable of 3 amps. Otherwise, if you don't use both sides, you get the full voltage, but still at 3 amps.

Eh?

thanks,

R


"By George, he's got it!!" You're right on the money.

P. Short
05-21-2009, 11:24 PM
"the same for each" has me confused. Scenario - I am using the CT, for two 6 volt power sources. Both will pull two amps for a total of 4 amps (2 amps per side) - can I do that?

I guess the question is, is the current rating per side, or total? If I understand it right, when you are using the center tap, you basically have two separate, but equal transformers, and since the winding is capable of 3 amps, either side would be capable of 3 amps. Otherwise, if you don't use both sides, you get the full voltage, but still at 3 amps.

Eh?

thanks,

R

The two transformers are not fully separate, which can complicate things.

It is possible to get 3A out of both sides of the transformer, but it has to be set up right. If the two regulators are of opposite polarity (probably by using a full-wave bridge rectifier) then you can get two 3A outputs. If one power supply is inverting (such as a buck-boost switching regulator) and the other is non-inverting then you can get two 3A outputs with the same output polarity.

On the other hand, I don't think that it is possible to use two linear regulators to make two 3A outputs of the same polarity.

WireWrap
05-22-2009, 11:56 AM
The two transformers are not fully separate, which can complicate things.
...
On the other hand, I don't think that it is possible to use two linear regulators to make two 3A outputs of the same polarity.

A very important point that I may have glossed over while discussing current.

The center-tap prevents you from treating the secondary as two isolated windings. For any given half-cycle, the center-tap will be half-way between the end terminal voltages.

For example -- if the center-tap is grounded, for one half-cycle the end-points will be of opposite polarities: +12.5V and -12.5V, reversing on the next half-cycle. If one end is grounded, the CT will be +12.5V and the other end +25V, then both will be negative on the next half-cycle.

While it is possible to create two outputs of the same polarity, they each must go to isolated loads that have no common reference, including (especially) ground. You could, for example, have each light a bulb at up to 3A as long as there are no common connections between the lights.


...
Scenario - I am using the CT, for two 6 volt power sources. Both will pull two amps for a total of 4 amps (2 amps per side) - can I do that?

Yes, but only if the two loads are completely isolated from each other and from ground.


.:)

51fordf2
05-22-2009, 12:50 PM
OK, what I am doing, is powering my FireGod boards. They both use 12 vac input, and are rectified on the board. Must be ac for the ZC, if I understand it correctly. The original parts BOM uses two wall warts, but one didn't work out of the box, and the other smoked in the 2nd day of testing. So, I am using a 25 volt VCT tranformer, to provide 12 vac to each of the boards. I also have two 12 vac transformers, I can use, but thought I'd check on the VCT first. Here's the way I plan to hook them up...will it work?? The wall warts were 1000 Ma, and this transformer is 25 v, 2 amp. I don't think I was pulling an amp on the boards, as they work fine, but rather think the wall warts were bad to begin with. But, if this works, and I read you all correctly, I could provide 2 amps to each circuit board, if necessary...

Thanks,

Roger

P. Short
05-22-2009, 01:42 PM
OK, what I am doing, is powering my FireGod boards. They both use 12 vac input, and are rectified on the board. Must be ac for the ZC, if I understand it correctly. The original parts BOM uses two wall warts, but one didn't work out of the box, and the other smoked in the 2nd day of testing. So, I am using a 25 volt VCT tranformer, to provide 12 vac to each of the boards. I also have two 12 vac transformers, I can use, but thought I'd check on the VCT first. Here's the way I plan to hook them up...will it work?? The wall warts were 1000 Ma, and this transformer is 25 v, 2 amp. I don't think I was pulling an amp on the boards, as they work fine, but rather think the wall warts were bad to begin with. But, if this works, and I read you all correctly, I could provide 2 amps to each circuit board, if necessary...

Thanks,

Roger

I don't have the schematic for the Firegod board, but I suspect that what you propose will not work. If you follow the ground signal from one power supply through the Firegod boards through the communications cables through the other FG board back to the power supply for that board, you will probably find that you have shorted out one of the transformer windings (either directly or through some forward-biased diodes).

51fordf2
05-22-2009, 02:03 PM
The two FG boards are independent of each other. They are two FG systems, each having 128 channels. There are no interconnections between them whatsoever, other than the CT on the transformer. The only reason I had two wall warts, is that I had two systems, not two wall warts used on each system.

Still won't work?

R

WireWrap
05-22-2009, 06:11 PM
I don't have the schematic for the Firegod board, but I suspect that what you propose will not work. If you follow the ground signal from one power supply through the Firegod boards through the communications cables through the other FG board back to the power supply for that board, you will probably find that you have shorted out one of the transformer windings (either directly or through some forward-biased diodes).


The two FG boards are independent of each other. They are two FG systems, each having 128 channels. There are no interconnections between them whatsoever, other than the CT on the transformer. The only reason I had two wall warts, is that I had two systems, not two wall warts used on each system.

Still won't work?

R

Like Phil, I don't have a Firegod schematic, but I think what he's saying is correct. You have to send your data to the FG board to control the lights. Unless it's a wireless path, you will have a ground connection in your data cable. This ground will be at different potentials in the two FG boards, causing a short when they are connected to your PC output. :(


.

FireGod
05-22-2009, 11:21 PM
Roger, If I were to try what you are proposing, I would use the center tap of the transformer as a common ground. This common ground would need to be the same potential as the ground on your PC (AC neutral).

It would be best if you had two isolated transformers.

Why don't you just use one of the 12.5 VAC outputs from the transformer for both of the PC interfaces? Since the transformer is capable of 2 amps it should be fine. I would buy an inline fuse holder and stick a fuse in each circuit to each PC interface just for safety.

The only time the load is at the maximum is when all 128 channels are turned on to 100%, the rest of the time the load is quite low. The transformer should be fine only using one output.

51fordf2
05-23-2009, 09:01 AM
David - thanks...why is the simplest solutions ofttimes the most illusive?? Forest and trees syndrome, I think...

I'll use the two 12 vac transformers, and put the 25 vct away for now.

Thanks again, to all! I always learn something, on the forums. I'm rather glad I asked, instead of just jumping in, and using the vct.

R