Series Computer Power Supplies to get 24 volts - Page 1
Forum section: electronics / beginners

author: SG-1
author_rank: Regular Contributor
timestamp: June 07, 2011, 06:42:04 am
content: 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 circuitfor the second power supply +12 connected to it.

author: Simon
author_rank: Unknown
timestamp: June 07, 2011, 06:49:01 am
content: 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

author: phil_jp1
author_rank: Regular Contributor
timestamp: June 07, 2011, 07:47:40 am
content: 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).

author: RayJones
author_rank: Frequent Contributor
timestamp: June 07, 2011, 08:27:32 am
content: 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)

author: Psi
author_rank: Super Contributor
timestamp: June 07, 2011, 08:54:29 am
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 07, 2011, 09:11:05 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: June 07, 2011, 11:42:43 am
content: Quote from: Psi on June 07, 2011, 08:54:29 ambut surely they share a common ground ?

author: stl
author_rank: Contributor
timestamp: June 07, 2011, 04:52:06 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: June 07, 2011, 05:01:49 pm
content: yes it could possibly be done, I'd remove both PSU's from their cases and put them in a new case

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2011, 06:05:35 pm
content: 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.

author: SG-1
author_rank: Regular Contributor
timestamp: June 07, 2011, 06:14:50 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: June 07, 2011, 06:20:57 pm
content: Quote from: SG-1 on June 07, 2011, 06:14:50 pmWhy 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

author: RayJones
author_rank: Frequent Contributor
timestamp: June 07, 2011, 07:44:09 pm
content: 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.

author: SG-1
author_rank: Regular Contributor
timestamp: June 08, 2011, 06:08:38 am
content: Quote from: Simon on June 07, 2011, 06:20:57 pmWe have been using NON type fuses.Thanks for the caveat, I am a big fan of Eustace C. Soares PE.

author: SG-1
author_rank: Regular Contributor
timestamp: June 08, 2011, 06:20:22 am
content: Quote from: RayJones on June 07, 2011, 07:44:09 pmI 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

author: phil_jp1
author_rank: Regular Contributor
timestamp: June 08, 2011, 09:40:20 am
content: Quote from: Psi on June 07, 2011, 08:54:29 amDual rail (triple rail) computer PSU means that there is separate current protection on each "rail", but there's only one transformer output for 12v. So you can't actually do that - because that's impossible (if I get it right what you mean).

author: Simon
author_rank: Unknown
timestamp: June 08, 2011, 11:49:22 am
content: Quote from: phil_jp1 on June 08, 2011, 09:40:20 amnot quite, at the very least they will have their own controllers. Part of the idea of dual rail I believe (could be wrong) is that the "noisy" motors of the hard drives are kept separate from other supplies.

author: Unknown
author_rank: Unknown
timestamp: June 09, 2011, 11:19:57 pm
content: At the very least the wires from the different 'rails' are soldered together on the PCB. At the most they may have an individual current limit. Individual controller would be quite pointless with only one transformer, you have only one duty cycle to control anyway.Disconnecting grounded equipment from ground is stupid and a safety (and EMC) hazard, like Hero999 mentioned. Computer supplies are good for... computers. If you want a 24V supply, one of those cheap switchers you can probably find on ebay for a few bucks is likely to be much better.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 10, 2011, 12:18:11 am
content: MeanWell turn out some very nice 24 volt supplies which can be wired in series,parallel or whatever.They come as standard 19 inch rack units,& would be a more professional way to do this job.At my last job,we were using them adjusted up to 26.5 volts,& drawing 65 amps from the parallel pair.VK6ZGO

author: SG-1
author_rank: Regular Contributor
timestamp: June 16, 2011, 06:14:10 am
content: Thanks, I like the MeanWell power supply. Now to sell it to the boss...