atx psu conversion (noob question) - Page 1
Forum section: electronics / projects

author: aftafoya
author_rank: Newbie
timestamp: October 08, 2012, 06:04:45 pm
content: Been searching all over to find a way to convert an atx power supply to provide ~30-40vdc. All I have found involves connecting a bunch of old PSU's together. I have a fairly new (maybe 2006) 750w Dell that I pulled from a scrap xps comp. Haven't cracked it open yet, but the label specifies 4-12v rails rated at 16A each. Is it possible to connect these rails together in series in order to produce the voltage that I'm looking for? Ultimately this is for another project I'll be working on and am trying to go as cheap ad possible. It will be used to charge two capacitor banks for a dual-pulse CD welder. Any help would be appreciated.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 08, 2012, 06:15:49 pm
content: You can't put the 12v rails in series because they have a common ground. Raising the output voltage of one of the 12v rails is also not easy because you will have to rewind the transformer, replace the caps with higher voltage versions and change the voltage control loop.There is a reason you can't find a way to do it... it's too difficult. I don't know what current you need but just a transformer, a bridge rectifier and a capacitor is all you need.

author: aftafoya
author_rank: Newbie
timestamp: October 08, 2012, 06:28:48 pm
content: Alright, thanks for the help. Looks like this will be my future bench PSU then.

author: johnwa
author_rank: Frequent Contributor
timestamp: October 09, 2012, 08:27:45 am
content: Does it need to be well regulated? I have found that these supplies actually have a fair bit of headroom, and it is possible to get a fair bit more voltage out by running them open loop.I did this once to make a power supply for some garden lights. The voltage drop through the long cable out in the garden was going to be too great at 12V, and I didn't want to run 240V cables outside. I upgraded the output filter capacitors to 35V rated units, and disabled the overvoltage protection and the voltage control loop. This gave 25V unloaded from the old 12V rail, which sagged to about 20V under load. I then upgraded the rectifiers and filter inductor for the -12V rail. This then gave me a supply of +/- 20V at several amps, or 40V total. After losing a bit in the wiring, this was perfect for running 3x 12V 50W halogen lamps in series.This probably isn't much help to you, but possibly someone may have a use for this technique at some stage.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 09, 2012, 11:31:56 am
content: @johnwaInteresting idea, I've never even considered this, I guess for some (unregulated) purposes it could be a really neat solution.

author: T4P
author_rank: Super Contributor
timestamp: October 09, 2012, 11:34:06 am
content: 750W ... from a Dell ... Go figureYou can build several boost converters if you like to boost up the voltage but that's a little bit of a hassle

author: ejeffrey
author_rank: Super Contributor
timestamp: October 09, 2012, 06:20:17 pm
content: Quote from: johnwa on October 09, 2012, 08:27:45 amThat is going to be dependent on the internal design and your line voltage. A wide range power supply operating at the top end of its input voltage range will put out a higher open-loop voltage than when operating at the bottom. That is good news if you are on 230, but not 120 VAC. Newer PSUs with active PFC are designed to operate at a fixed DC voltage, so they probably have a lot less headroom. In principle you could change the PFC stage to increase the DC input to the main converter, but that is probably ~390 volt, and can't get much higher without exceeding the voltage limits of the components.

author: johnwa
author_rank: Frequent Contributor
timestamp: October 10, 2012, 10:00:07 am
content: Quote from: ejeffrey on October 09, 2012, 06:20:17 pmYes, you are right, the PFC circuitry will probably stuff up this scheme somewhat. An 'autoswitching' universal input power supply would probably not be able to put out much extra voltage on 120V input. However, the ones with a manual switch usually use a voltage doubler rectifier on 120V, so the primary DC rail should still end up at around 340V.The supply that I performed this modification on was quite an old one without active PFC, I think it might have actually been an AT type.

author: ptricks
author_rank: Frequent Contributor
timestamp: October 10, 2012, 11:36:57 am
content: What you want to do is possible but I wouldn't suggest it without either having the schematic to the supply or reverse engineering the supply yourself to create a schematic. There are just too many variables that need to be looked at to start changing component values to increase the voltage.

author: metalphreak
author_rank: Frequent Contributor
timestamp: October 10, 2012, 12:49:49 pm
content: Most (apart from a few high end exceptions) "multi-rail" power supplies still only have one voltage regulation circuit for each voltage output. The "rails" are simply individually controlled outputs for over current protection. Some of the big supplies can output 100+ Amps on the 12V rail to power multiple devices (multiple GFX cards, CPU etc). However no single device needs more than ~20A (per cable). If the over-current was for one single rail, it'd only trip with >100A loads whereas a single shorted cable will catch fire long before the over current trips (if at all).Long story short: You can't add together the (same) voltage rails to get a higher output because they aren't really independent supplies.Also, as PA0PBZ mentioned, they share a common ground. Now, if you use multiple power supplies, you STILL can't put them in series, because they are mains earth referenced (as in the metal casing). Some supplies let you remove the chassis earth connection to get floating outputs but I don't recommend it as it may affect the safety features.