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Old Jan 02, 2003, 04:44 PM
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Putting pc power supplies in paralell

I have been using a pc power supply for several months now to feed my Supernova. I followed the instructions here and it worked great. Having read many of the threads here, I've gathered that I'm getting all the watts I can from it since it's one of the cheap 250W Taiwan specials. The problem is that when I try to charge my big 1300+ SCR 10 cell packs, I have to limit the Supernova to 3 amps. If I punch Auto mode the charger starts trying to pump 5 amps at 16-18 volts into the packs and then shuts off complaining of "input voltage too low".

Someone mentioned putting two pc supplies in paralell to increase the available watts. Does anyone have some specific instructions on how to do this? I've done a search and read several threads without finding the answer.

Thanks in advance!!
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Old Jan 02, 2003, 10:18 PM
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Try to increase your load on the 5v should boost the amps on the 12v output.
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Old Jan 03, 2003, 12:21 AM
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I believe a power resistor on the 5V line is what I put on. I bought it at Radio Shack. 8 Ohm maybe. Some just use a auto tail light bulb.
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Old Jan 03, 2003, 02:18 AM
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The document I read has said to use a 1 or 2 ohm resister. I have seen that at Radio Shack and it looks like a long rectangular white ceramic stick.
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Old Jan 03, 2003, 10:29 AM
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Quote:
Originally posted by aau007
The document I read has said to use a 1 or 2 ohm resister. I have seen that at Radio Shack and it looks like a long rectangular white ceramic stick.
The one I bought was the only one my local Radio Shack had. I just confirmed that mine is an 8 ohm 20 watt power resister. It is as you say a long (mine-approx 1/2" x 1/2" x 2-1/2") rectangular stick with a bare wire coming out each end.
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Old Jan 03, 2003, 11:19 AM
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Those are called sandstone resistors.

Radio Shack usually has some lower values, 1 or 2 ohm. I think dropping to a lower resistor will put more load on the 5V line and increase your 12V output. But you'll never get more than the rating, how many amps does it say for 12V.

You could put several in parallel to get the resistance you need. Two of the 8 ohm in parallel will be 4 ohms, that might be low enough, three would probably be better if you have them.

You can order them online from RS or one of the electronic discount houses, but you'll have to wait for delivery of course. Look in the phone book for electronic parts dealers. Here in Norfolk I use Priest Electronics, no comparison to RS though, these guys have just about everything.

Jimmy
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Old Jan 03, 2003, 06:33 PM
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If you have a 250W PC power supply, the 12v should be rated at about 10A to 12A. The amp on 12v increases as you increase the load of the 5v lead. How close can you get to the rated amp. on the 12v depends on the quality of the PS. Don't go too crazy on the load since the resister may get hot and then you will need to put a heat sink on the resistor.
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Old Jan 06, 2003, 02:50 PM
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I guess what everyone is saying is that the 5v line is not loaded enough so I'm not wringing all the watts out of the 12v tap. That would make sense because even at 5.5 amps and 19v the power supply is only being asked to supply 105 watts and it says it's a 250watt supply. What doesn't make sense is that I loaded the 5v line from the beginning because I read all the posts on converting the power supplies. I put 4 of the RS power resistors in series paralell for a total of 1ohm resistance at a 20watt rating. Shouldn't that be enough load to get all the watts out of the 12v side?

I do notice that just before my supernova quits and starts complaining that the fan slows down noticably and I hear a faint click. Then it speeds back up for a second and then boom....it shuts down. It's almost like the supply is doing fine and then decides to quit supplying...

aau007,

If that's the standard for most PS's then were talking 120 watts. So if I'm trying to pull 104 and it's cheap PS I could be exceeding its capacity as I suspected earlier. So that brings me back to my original question. How do you put two PS's in paralell to get more available watts. I have another one laying around gathering dust....
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Old Jan 06, 2003, 07:14 PM
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Um.

You may want to check if these are linear or switching power supplies.

If they are switching power supplies, it is probably a bad idea to put them in parallel or series, because if I understand correctly it can place a short-circuit across the AC (because there's no isolation transformer) and it will smoke both power supplies.

Toshi
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Old Jan 07, 2003, 09:58 AM
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Ditto on NOT putting them in parrallel or series...

Typical PC switching power supplies are NOT isolated and will fry if you connect the outputs together in any way. The only type of PS I would even consider tieing together are two IDENTICAL TRANSFORMER-ISOLATED power supplies.

Be carefull

Pullin'
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Old Jan 07, 2003, 12:41 PM
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Jim Walker,

Did you say 5.5A @19v? You must be approaching the limit on the PS then? The 12v lead supplies 12v only. If your FMA is charging at 5.5A at 19v, it is using the internal step up to supply the required charging voltage and that means it is drawing more than 5.5A current from the PS. 5.5A @19v is 104.5W, devide that by 12v and you are drawing over 8.7A from the PS. Consider some loss and power to operate the fan in the FMA, you can be drawing over 9A effectively. If you have an old PS, it may not have the rated 10A anymore on the 12v lead. See if you can put a WhattMeter between the PS and the FMA and see what you get as input current to the FMA.

Although, it is a 250W power supply, your recommended rating on the 12v should only be about 10A. The remaining wattage goes to supply power on the 5v leads and the fan on the power supply.

Keep in mind that you may never want to use 250W from a 250W PS, especially from a safty standpoint. When the 250W PS is put on a PC, the PC will usually not be drawing anything over half the wattage. That's also the reason file server can have power supply of up to 500W since some people put 6 hard drives, 4 network cards, tape backup, etc. in it.

And no, you should not parallel the PS's since they are switching PS's. All PC power supplies are switching. I think (and don't quote me on that) you can connect them parallel if the PS's are each plugged into an outlet with its own isolated ground.
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Old Jan 07, 2003, 02:05 PM
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Thanks everbody for their input.

aau007,

Yes, I knew that the Supernova was definitely using it's step up circuitry to get the necessary volts to push 5.5 amps into my 10cell SCR pack. That's why I started thinking in watts after I read your post and you have answered my question, thanks! It looks like I've pushed the computer power supply concept to it's limits and should get a purpose built PS if I want more performance than that.........
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Old Jan 07, 2003, 09:40 PM
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Here is the answer!

I noticed this discussion about using PC supplies in series and parallel and just happened to have a very good web site URL that I believe will answer all the questions. Its about how to connect 2, 4, 6, .... PC power supplies in parallel or series http://www.procooling.com/articles/h..._s_as_one_.php

One major thing to watch out for with a series connection is whether the supply's output is connected to the case. If it is you either have to make sure the cases do not touch or connect in any way or you have to isolate the outputs in the manner described. Fixing them separately to a wooden board would work of course for our purposes.

Rod
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Old Jan 08, 2003, 08:33 AM
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Thanks Hotrod!

That's exactly what I was looking for!!!
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