DC voltages from PC power supply

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Larry Henderson
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DC voltages from PC power supply

Help? I'm trying to find out about wiring old computer power supplies to supply DC voltages other than 12V, 5V & 3.3V. 
A few years ago I needed 19V DC for an LCD monitor with no power adapter so I found a "paperclip trick" which makes the power supply think it's connected to a motherboard (and is powered on). Then I think I connected a 12V, 5V and 3.3V to create what I thought would be approx. 20Volt power source. Also connected the corresponding ground wires together. Wired to a plug which fit the monitor and Voila! It worked for about a year.
My problem now is that when I googled DC voltages, it doesn't seem to be recommended to connect differing voltages in parallel so I'm confused. I did something that apparently shouldn't work OR is not recommended or safe to do. Now part of my confusion is series and parallel circuits. If I place 12V and 5V power sources in series, the resulting voltage should be 17V (potential difference). BUT by joining two positive wires together and the 2 negative ground wires together, is this a parallel connection? I don't even know how I'd make a series circuit (if this isn't one?)
Can someone clarify this situation for me? Thanks in advance for any advice.

Larry Henderson
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Oops! solved the mystery.. I

Oops! solved the mystery.. I jerry-rigged the PSU a few years ago.. the monitor is rated for 12V 3.8amps.. tested the voltage with multi-meter and it reads 11.91V.
Now my next idea is using 2 PSUs. How would I connect 2 12V lines in series to get 24Volts? Connect 12V line to a ground of the other PSU? Could anything go wrong with that? ^_^

evil
evil's picture
Computer power supplies are

Computer power supplies are not a very good source as a railed power supply. Let me discuss batteries for a sec, wiring all positives and negatives equals the voltage of a single battery with the ability of sustaining the current/amperage of all the batteries placed in parallel. Parallel equals small voltage but large amperage. Placing the batteries in series(positive to negative â??) does just the opposite, large voltage and small amperage. When you are dealing with power supplies there are a few things to take into consideration. Isolation, maintaining regulation with no load(what I suspect you may have been doing with the paper clip,only wrongly with a short circuit). Generally all the power supply return lines are tied together(no independent returns)(With non isolated outputs) which means what you (probably did) was essentially short the non dominant voltage(5,3.3) with the (or back feed/biased them with)dominant voltage(12). Again connecting an output to any grnd without isolation will result in not so desired effects for you or the supplies. Also tying them in parallel will not work as only one of the supplies will take it upon it's self to carry the load the other will just not do anything and that's if it doesn't short out because the other one.... What are you trying to power?

Larry Henderson
Larry Henderson's picture
evil said: Computer power

evil said: Computer power supplies are not a very good source as a railed power supply. Let me discuss batteries for a sec, wiring all positives and negatives equals the voltage of a single battery with the ability of sustaining the current/amperage of all the batteries placed in parallel. Parallel equals small voltage but large amperage. Placing the batteries in series(positive to negative �??) does just the opposite, large voltage and small amperage. When you are dealing with power supplies there are a few things to take into consideration. Isolation, maintaining regulation with no load(what I suspect you may have been doing with the paper clip,only wrongly with a short circuit). Generally all the power supply return lines are tied together(no independent returns)(With non isolated outputs) which means what you (probably did) was essentially short the non dominant voltage(5,3.3) with the (or back feed/biased them with)dominant voltage(12). Again connecting an output to any grnd without isolation will result in not so desired effects for you or the supplies. Also tying them in parallel will not work as only one of the supplies will take it upon it's self to carry the load the other will just not do anything and that's if it doesn't short out because the other one.... What are you trying to power?

Thanks for replying Evil.. I'm trying to make a power supply that rates about 19V to power most laptops.. I'm thinking that I may be able to use a nominal 24V using 12V (yellow) and -12V (blue) lines.. or wiring two 12V lines from 2 PSUs in series.. btw the "paperclip trick" is not really a short circuit.. pin 16 (the green wire) is "power on" detection.. "Power on is pulled up to +5V by the PSU and must be driven low to power on the PSU" .. this is achieved just by "shorting" the green wire to any one of the black ground wires. I used just the 12V line to power an LCD monitor and it worked fine for about a year. (I mistakenly thought it needed 19V but it was rated at 12V)

evil
evil's picture
It would seem to me that it

It would seem to me that it would be much simpler and safer for the laptop to, depending on where you live, go to a thrift store or Goodwill or the like and they will likely have a power unit that is closely related to the voltage and current that you need and for cheap.

Larry Henderson
Larry Henderson's picture
The laptops I get are usually

The laptops I get are usually write-offs (out of warranty) and need different power jacks so I often just splice wires to get something temporary til I figure out if I can fix the laptop or not. If I can get it working, then I may look at a better power supply solution. I have quite a few AT or ATX power supplies just kicking around so I'm just experimenting with ways to make use of them. The paperclip shortcut is the easiest way I've heard of for checking if the power supplies are even working cuz defective PSUs are one of the most common problems with PCs (after motherboards?).
I'm going to try putting 2 PSUs in series. Seems I have to insulate one PSU from grounding to the casing.. also suggested to add a resistance across a 5V line of both psus (for more consistent power?)

evil
evil's picture
Putting 2 PSU in series is a

Putting 2 PSU in series is a bad idea, it only works with batteries because they are floating current or isolated, no earth ground potential. Putting a earth grounded power supply positive to a different earth grounded power supply ground will create nothing more than a short and smoke your power supply. Putting them in series is a bad idea unless you have plenty of them and just like the smell:)

Larry Henderson
Larry Henderson's picture
evil said: Putting 2 PSU in

evil said: Putting 2 PSU in series is a bad idea, it only works with batteries because they are floating current or isolated, no earth ground potential. Putting a earth grounded power supply positive to a different earth grounded power supply ground will create nothing more than a short and smoke your power supply. Putting them in series is a bad idea unless you have plenty of them and just like the smell:)

There are a few guys who have done it (maybe we'll never hear from the ones that didn't go well?) ;)

I'll let you know how it goes.. Unless you don't hear from me..................... lol!

Larry Henderson
Larry Henderson's picture
evil said: Still Curiously

evil said: Still Curiously awaiting results!?!

Just purchased a universal power supply/8 adaptors for laptops/notebooks for under $10 on ebay.. have a half dozen laptops and no power/plugs to test 'em :(

Still alive though ;)

Shandaash
Shandaash's picture
I have my problem with my

I have my problem with my power supply..

 

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