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Author Topic: Can I run two power supplies in series?  (Read 621 times)
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Haverhill, Ma
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I have some electromagnets that are rated for 24 volt. Can I run two atx PSU's in series to make 24 volts?
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Haverhill, Ma
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Never mind. I found the answer here http://power-topics.blogspot.com/2009/07/operating-power-supplies-in-series.html

You guys have so much knowledge I think about asking you instead of looking it up on google sometimes.
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Johannesburg. UTC+2
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My old ATX PSU has -12V so presumably one would get 24V across the -12 and +12?
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My old ATX PSU has -12V so presumably one would get 24V across the -12 and +12?

Yes, but current draw available from the resulting 24vdc would be current limited to the value of the -12vdc source which would be very much less then what the +12vdc can supply.

Lefty
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You can only connect PSUs in series if there is no connection between the PSU output and mains ground. Some common power supplies (e.g. ATX PSUs and laptop PSUs) do have a connection between the 0V output connection and mains ground.
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Haverhill, Ma
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My ATX PSU's must be a common one because I let the magic smoke out of one of them. It also took out two jumper wires on its way out. The other one seems fine so far....
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You can only connect PSUs in series if there is no connection between the PSU output and mains ground. Some common power supplies (e.g. ATX PSUs and laptop PSUs) do have a connection between the 0V output connection and mains ground.

That's good to know.... I think (but will confirm) that when I hacked that ATX PSU recently for table-top use I measured continuity between 0VDC and mains ground. Reason I was checking was to see if there would be a problem if the 0VDC accidentally earthed to the chassis (which I saw was at mains ground since the incoming power cable ground lead was bolted thereto.)
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Yay! Another reason to get a "proper" Lab power supply instead of hacking a computer ATX power supply.

With Lab power supplies, you can connect them in series because their outputs are usually floating/isolated. They're designed for that. These PSUs are made to be fooled around with on your workbench as you hook/unhook your power.  There's usually an isolation transformer, or step down transformer there to protect you, the user.

Computer ATX power supplies are high-voltage/high-current switching power supplies, there is no isolation or step-down transformer in there, it's hooked up directly to your 110/220V source.  They're meant to power a computer motherboard, with the PSU sealed inside the computer case, and only the main power switch as the user access. If you want to fool around, stick your fingers and touch those output wires to power your projects, and you have high-confidence in the safety of these "Made in China" power supply units, then good luck!
« Last Edit: January 20, 2013, 11:00:24 pm by vasquo » Logged

Johannesburg. UTC+2
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Yay! Another reason to get a "proper" Lab power supply instead of hacking a computer ATX power supply.

Yep I'm thinking next purchase = "proper" lab ps
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Me too.  I would really love to have a PSU that would just tap me on the shoulder and say "hey, ya gotta short there" if I bridged its outputs.  For now, I just use big transformers, little fuses, and a Variac.
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If $100 is steep for you guys (and you can buy brand new Lab PSUs, CV/CC, digital readout volts, amps, etc),
then you can DIY your own for about $50 or less.

You need to use a dual primary/dual secondary power transformer. You can buy a 30/60VA for less than $20. The dual primary lets you wire it for 110 or 220V operation, and the dual secondary let's you make split V+ and V- outputs, with a floating common.  Use 317 and 337 voltage regulator chips for the V+ and V- outputs (max 1.5A per rail). This PSU should be good for up to 1.5amps total (using a 60VA transformer), or 3Amps total (using a 120VA transformer). 

The isolation offered by the power transformer assures you that no part of your circuit is connected to the primary 110/220 volts AC line.

Have a couple of these DIY PSUs, and you can wire these puppies in series too for even more voltage (tap the V+ and V- terminals), or in parallel (for more current). -- Just use diodes if wiring two PSUs in parallel.
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