# Put outputs of 2 computer power supplies in series?

I've been working to figure out an LED lighting project for my classroom for a while now, readings on this site has helped. One part of the solution is to combine in series computer power supplies (which we can get free) to get specific voltages. This will help us use the ideal voltage for the different needs of the different LED strips. For example, By adjusting resistance to the correct lower value and also set the power supply to 8V, Red LED strips can go from around 50% efficient to better than 80%.

It's well explained on this site that you can not connect say the 3.3V and the 5V rails from a single computer power supply to get 8.3V since they share the same negative terminal. I remember some page here (can't re-find it) that talked about combining separate computer power supplies to get combination voltage. BUT, I wonder, wouldn't the separate power supplies actually have a common ground (the grounded wire on the plug?) If this is problem, any easy work around? Any better way to get about 8 volts using computer power supplies?

UPDATE: PICTURE SHOWS CURRENT SETUP: Note that black wires (ground) are divided into 2 bunches and the yellow (12V) gathered into 1 bunch. Green and Black wire are connected (so that power is on) as per this instructable: I tested with an ohm meter and the mains earth ground IS electrically connected to the output DC power ground. Can I fix this problem?

• Just a note, the ground wire on the plug is mains earth ground. This will be different from the output DC power ground from the supply – DerStrom8 Aug 2 '16 at 19:17
• Look at the following links. If you have the skils and patience and a suitable type of ATX PSU you can modify it to be a variable voltage supply. Usually in the range of about 5-13V on the normal 12V rail. - google.com/search?q=convert+variable+PSU+ATX – KalleMP Aug 2 '16 at 21:53
• @KalleMP, Thanks for the linked search. Looks like I might not be able to hook the 2 power supplies as hoped. Will look into the variable voltage supply -- actually looks like a better solution, though I do wonder about the efficiency of output. As one backs down from 12V does the wastage to resistor increase? If so, how much? – David G Aug 3 '16 at 4:29
• The supply is a switched mode type. The efficiency is generally constant over the bulk of the working range, typically over 80% in consumer gear, over 90% at the sweet spot for each design and dropping at low and max load. – KalleMP Aug 3 '16 at 8:39
• Not ideal to defeat he mains grounding. If the output is not designed to float then it may be designed with very limited insulation capacity and while an extra 10 or 20V should not make much difference it 'might'. In any event you would want to isolate the output ground from the case/mains-protective-ground and keep all the metal bits well earthed for safety. – KalleMP Aug 3 '16 at 8:42

It all comes down to one simple issue. Is the ground (black wires) of the second computer power supply connected to mains power input green-wire safety ground, or is it not connected?

It should be trivial to actually measure this with a cheap meter or even a battery and light conductivity tester. Or even by opening the power supply unit and examining how the green-wire mains safety ground is handled with respect to the power supply black-wire ground node.

If the mains green-wire ground and the output black-wire ground are connected together then you cannot use the power supply in a "floating" configuration as you are proposing.

This assumes that you are using the computer power supplies properly with a grounded mains power cords. Improperly disconnecting the green-wire ground is a safety hazard at best, and likely illegal in a school or other public building setting.

• I did the test and updated the question above with results. @pipe, any ideas? – David G Aug 3 '16 at 4:24
• If the mains earth ground IS electrically connected to the output DC power ground then the power supply is GROUNDED and CANNOT be used in series with another power supply because the mains earth connection will place a DEAD SHORT across the other power supply. The PC power supply is NOT suitable for your proposed circuit. No, there is likely no "fix" for this problem. The best you can do is to search through all your surplus power supplies and try to find one that is floating. – Richard Crowley Aug 3 '16 at 4:56
• Based on peoples overwhelming expectations that the mains earth ground would not be electrically connected to the output DC power ground I did some double-checking. I cut the 2 purple + black wires I had soldered to keep the PS on at all times and found that made no difference. I took the cover off the PS hoping to trace the black wires back to the chassis/mains earth ground but things got too complex quickly. In conclusion, don't know at what point the mains earth ground and output ground get electrically connected, but they are. Thanks everyone. Going to check out VPS. @KalleMP. – David G Aug 3 '16 at 14:48

It seems a bit wasteful to use computer power supplies in this way, but it will work. The output ground is not connected to the input ground (protective earth), so each individual power supply will "float" relative to the other.

As soon as you connect the black lead of one PSU to, say, 5 V on the other PSU, they are obviously not floating anymore, but you can get your 8.3 volts.

The reason I think it's important to mention that they are not floating is that after you do the first connection, you can't continue mixing voltages without careful planning.

• Thanks. Also, if the 3.3V has a 10 amp (33W) capacity and the 5V has 10 amp (50W) capacity, how do I figure out the capacity of them combined? Also, @pipe, as you mention this seems a bit wasteful of computer power supplies, I'm curious how you would go about getting a simple, inexpensive, high capacity 8V power supply. – David G Aug 2 '16 at 20:06
• I have done this very thing in the past as a quick way to get 24 V. I had to float the two chassises from each other since they aren't totally floating with respect to each local minus. – winny Aug 2 '16 at 20:23
• @DavidG The same current will flow through each supply, so you'll have an 8.3 volt, 10 amp supply. The weakest will set the limit for the complete circuit. And I'm not necessarily saying it's a bad idea, just that it's a bit overkill and they're bulky and noisy. If you have a free supply of them and the noise and size is not a problem, there's nothing inherently wrong with the setup. – pipe Aug 2 '16 at 20:39
• @DavidG If something is floating, it's galvanically isolated. Two cars, side by side, each with its own 12 V system. If you try to connect a bulb between plus on one car and minus on the other, nothing will happen because they are both floating with respect to each other and earth/ground. You computer power supply is "pretty much" floating, but not fully. – winny Aug 2 '16 at 20:58
• @DavidG, I'm saying you may have to. Try it fist as is. If the internal leakage though the Y-caps causes problems, float each chassis and try again. – winny Aug 2 '16 at 21:07