Author Topic: Series Computer Power Supplies to get 24 volts  (Read 4879 times)

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Offline SG-1

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Series Computer Power Supplies to get 24 volts
« on: June 07, 2011, 04:42:04 PM »
I would like to series two Blue Star 680W ATX/SATA/PCI ( P/N ES-680-GD ) computer power supply 12 volt rails to obtain 24 volts.  Is anything weird going to happen ?  Or should I ask if the output will remain stable.  Current draw will be around 30A for less than 50 mS.  The +12 output is rated 38 amperes.  My biggest concern is if the ground reference for the +12V will be a short circuit  :o for the second  power supply +12 connected to it.
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Offline Simon

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Re: Series Computer Power Supplies to get 24 volts
« Reply #1 on: June 07, 2011, 04:49:01 PM »
I think you need to check if the chassis is connected to the negative rail. I suspect it may be in which case your screwed

Offline phil_jp1

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Re: Series Computer Power Supplies to get 24 volts
« Reply #2 on: June 07, 2011, 05:47:40 PM »
Usually in computer PSU's ground is connected to earth. And if you will connect two of them in series - you will short one of them.
But you can try to disconnect earth from ground (but make sure that mains can't touch the case). And if you will do that you might end up with pretty noisy power supply (but of course it depends on what you need it for).
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Offline RayJones

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Re: Series Computer Power Supplies to get 24 volts
« Reply #3 on: June 07, 2011, 06:27:32 PM »
Do you just need 24V, or must it be 24V above mains ground?
If no real ground reference is required, you could use the -12V rail in conjunction with the +12V rail...

The impulse current is probably best sourced from an external reservoir capacitor given it is only required for 50ms - what's the duty cycle? (note there is only 0.5A continuous on -12V)

Offline Psi

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Re: Series Computer Power Supplies to get 24 volts
« Reply #4 on: June 07, 2011, 06:54:29 PM »
Some high power computer power supplies have dual 12v rails, if you have one that does you might be able to cut some pcb tracks and isolate one of the 12v rails from ground. Then you can put them in series. You would only need one of the power supplies too.
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Offline Mechatrommer

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Re: Series Computer Power Supplies to get 24 volts
« Reply #5 on: June 07, 2011, 07:11:05 PM »
be careful. some atx psu like my cheapo, the 12V is rated 15A but -12A only rated 0.5A. i disconnected the board ground from earth, and they are seems to be isolated now, i guess will be no problem putting them in series. i can even connect the 12V rail to earth without anything weird going on.

Offline Simon

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Re: Series Computer Power Supplies to get 24 volts
« Reply #6 on: June 07, 2011, 09:42:43 PM »
Some high power computer power supplies have dual 12v rails, if you have one that does you might be able to cut some pcb tracks and isolate one of the 12v rails from ground. Then you can put them in series. You would only need one of the power supplies too.

but surely they share a common ground ?

Offline stl

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Re: Series Computer Power Supplies to get 24 volts
« Reply #7 on: June 08, 2011, 02:52:06 AM »
I've never done it but I read in a forum that it can be done, you have to isolate one of the PCB's (from one PSU) from the earth, usually you have a earth connection from the mains connector and the screws that make the contact between the PCB and the chassis.

Offline Simon

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Re: Series Computer Power Supplies to get 24 volts
« Reply #8 on: June 08, 2011, 03:01:49 AM »
yes it could possibly be done, I'd remove both PSU's from their cases and put them in a new case

Online Hero999

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Re: Series Computer Power Supplies to get 24 volts
« Reply #9 on: June 08, 2011, 04:05:35 AM »
Isolating the secondary from earth is risky because it might not be double insulated from the mains so there's a risk of shock. There could also be problems with EMC as one of the reasons for grounding is to short circuit the high frequency current coupled across the ferrite transformer.

Offline SG-1

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Re: Series Computer Power Supplies to get 24 volts
« Reply #10 on: June 08, 2011, 04:14:50 AM »
I would like to thank everyone for your  helpful replies.

The duty cycle could be as short as 6 seconds for a couple of times, but then it would lengthen to 30, 60, or more seconds.  Six second is the duty cycle of the test specimen.   

No ground reference is needed for this application.  These circuits are normally ungrounded with a ground detector built into the battery charger.

I have some 24 volt shunt trip breakers to test, they weigh about 500 lbs each.  Our normal variable power supplies struggle at 48 volts.  Usually a pair of car batteries show up.  I do not like that option, because if somebody screws up they could get a battery acid shower. 

Thanks for the great ideas.
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Offline Simon

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Re: Series Computer Power Supplies to get 24 volts
« Reply #11 on: June 08, 2011, 04:20:57 AM »

I have some 24 volt shunt trip breakers to test, they weigh about 500 lbs each.  Our normal variable power supplies struggle at 48 volts.  Usually a pair of car batteries show up.  I do not like that option, because if somebody screws up they could get a battery acid shower. 

Thanks for the great ideas.

Why not use the batteries with fuses ? and possibly put them in a box should the worse happen, sounds safer than playing with power supplies that were meant to be grounded

Offline RayJones

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Re: Series Computer Power Supplies to get 24 volts
« Reply #12 on: June 08, 2011, 05:44:09 AM »
So as it turns out simply using the already available -12V rail in series may be sufficient when combined with an external discharge capacitor.

50ms / 6000ms works out as a 250mA average current over the interval.

I'd suggest researching capacitor discharge point controllers for model railways. Their design topology sounds exactly like what you are after.
The secondary benefit being the extended discharge current is inherently self limiting.
By the sounds of it you do not need and do not want >720W continuously.

Offline SG-1

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Re: Series Computer Power Supplies to get 24 volts
« Reply #13 on: June 08, 2011, 04:08:38 PM »

I have some 24 volt shunt trip breakers to test, they weigh about 500 lbs each.  Our normal variable power supplies struggle at 48 volts.  Usually a pair of car batteries show up.  I do not like that option, because if somebody screws up they could get a battery acid shower. 

Thanks for the great ideas.

Why not use the batteries with fuses ? and possibly put them in a box should the worse happen, sounds safer than playing with power supplies that were meant to be grounded

We have been using NON type fuses. 

Thanks for the caveat, I am a big fan of Eustace C. Soares PE.
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Offline SG-1

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Re: Series Computer Power Supplies to get 24 volts
« Reply #14 on: June 08, 2011, 04:20:22 PM »
So as it turns out simply using the already available -12V rail in series may be sufficient when combined with an external discharge capacitor.

50ms / 6000ms works out as a 250mA average current over the interval.

I'd suggest researching capacitor discharge point controllers for model railways. Their design topology sounds exactly like what you are after.
The secondary benefit being the extended discharge current is inherently self limiting.
By the sounds of it you do not need and do not want >720W continuously.


I will check out the technology you have mentioned.  You are correct I do not need continuous output.  These type of breakers can clear 1500MVA of fault current in 3 cycles.

http://www.electricalmanuals.net/files/MV-HV-EQUIP/BREAKERS/CUTLER/VCP-W/IB-32-255-1F.pdf
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