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Is it possible to use my (3d printers 12V 20A power supply) in series with 12V unknown amperage pc power supply to gain at least 24V 20A for my printers new hotend?

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    \$\begingroup\$ If the powersupplies have isolated outputs, then yes, no problem. Do you have a model number of the pc power supply? \$\endgroup\$ – David Drysdale Jun 2 '16 at 22:55
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    \$\begingroup\$ Yes you can do this as everyone has said. I only add that I have personally done this myself! The two things to note is that whatever current the heated bed will pull, EACH power supply needs to be able to provide this. In my case, each 12V power supply needed to be able to provide 20A of current. To add more on isolating the outputs, the negative terminals of the PSU are connected to the Earth ground on the plug. If you connect them in series, then you will end up shorting one of the PSUs and it will turn off. When isolating the PSU, you disconnect the negative from the Earth ground. \$\endgroup\$ – Addison Jun 3 '16 at 2:14
  • \$\begingroup\$ Insufficient information provided to properly answer this question. Do you REALLY have a PC power supply that can source 20A on the 12V output? Is that what it says on the nameplate? Is your existing printer power supply fully FLOATING so that you can use it in series? Have you actually confirmed this by measurement? \$\endgroup\$ – Richard Crowley Jun 3 '16 at 5:03
  • \$\begingroup\$ @Richard Crowley The 3d printer has its own psu providing 12V at 20A. \$\endgroup\$ – Chris Manning Jun 3 '16 at 8:28
  • \$\begingroup\$ Sorry, you only repeated the incomplete original information and did not answer either of the clarifications: (1) Do you REALLY have a "PC power supply" that puts out 20A @ 12V? (2) Is your existing printer PSU actually FLOATING? The answer to those TWO questions is "Yes" or "No". \$\endgroup\$ – Richard Crowley Jun 3 '16 at 12:01
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Probably.

The PC power supply (-) lead will be connected to the Earth terminal of the AC Mains plug. The other supply may be isolated from the Earth terminal - or may not.

If the (-) terminal of the other supply is in fact connected to the Earth terminal, it's usually pretty easy to open the case and separate the (-) terminal from the Earth terminal.

Then just connect the two supplies in series.

I'd be tempted to install large diodes in anti-parallel with each of the outputs of the power supplies. That is: Cathode to (+), Anode to (-). The diodes should be rated for the full short-current rating of the power supplies.

The purpose of the diodes is to limit the amount of reverse voltage applied to the output of whichever power supply goes into current-limit first.

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Can you connect two power supplies in series to add their voltage? Yes, IF at least one of them is FULLY FLOATING, i.e. not referenced anywhere to ground.

Can you use a PC power supply for one of the sources? Seems very doubtful. Virtually all PC power supplies are GROUND referenced and you would have to be very careful to make a ground-referenced power supply the "lower" (0-12V) half of the series string, with the "upper" half (12-24V) being the fully floating supply.

Since we don't know anything about your unidentified existing printer power supply, this seems rather doubtful.

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    \$\begingroup\$ It seems really silly to say there aren't PC powersupplies that can drive 12V at 20A. -- Have you seen the graphics card market lately? -- My random off the shelf powersupply that I'm running my 3D printer on right now says it can put 15A on the 12V rail, and it's only a 400W supply -- Here's one on NewEgg that says it can do 12V @ 54.1A, and that's not even the top of the line, it's just a mid range 650W gaming PSU. The top of the line 1600W one says it can do 12V @ 133.3A... \$\endgroup\$ – BrainSlugs83 Oct 30 '16 at 21:22
  • \$\begingroup\$ The current capacity of the 12V bus is rather irrelevant when you aren't likely to find any PC power supplies that aren't ground-referenced. So it is rather a moot point. You cannot put two ground-referenced power sources in SERIES without completely shorting out the "lower" of the power supplies. Simple as that. \$\endgroup\$ – Richard Crowley Oct 31 '16 at 2:55
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You are not going to get 20 amps out of that arrangement. You will get 24 volts ok, but the current will be equal to the lower of the two, which will only be around 2 or 3 amps, depending on the PC supply.

Check the label on the PC supply to determine how much current you can draw. It might be as high as 18 amps, but might also be under one amp.

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    \$\begingroup\$ Modern PC power supplies can supply much more than 2 or 3 Amps. One supply that I have worked with is rated at 600 Watts - all of that is at 12 Vdc. The 5V & 3.3V rails use dc-dc converters powered from the 12V rail. \$\endgroup\$ – Dwayne Reid Jun 3 '16 at 1:46
  • \$\begingroup\$ @Dwayne Reid - Your comment made me curious, so I checked the machines in my lab. The big Dell workstations did indeed have high 12 volt ratings of 18 amps. Two other workstations were rated at 16 amps. But a couple of machines had only 0.8 amps of 12 volts, and one 140W supply had only 0.3 amps of 12 volts. A few had 6 amps. So it pays to check the label. \$\endgroup\$ – Mark Jun 3 '16 at 4:57
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    \$\begingroup\$ I suspect that you are seeing the current rating for the (-) 12V rail. 3.5" Hard Drives need both 5V & 12V at reasonable current. But the (-) 12V needs to be only a few hundred mA. \$\endgroup\$ – Dwayne Reid Jun 3 '16 at 6:29
  • \$\begingroup\$ @Dwayne Reid - No, It was definitely the +12 volt rail. The -12 volt rail was much lower. The +12 volt requirement for 3.5" HDDs is rarely over an amp. \$\endgroup\$ – Mark Jun 3 '16 at 7:43
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    \$\begingroup\$ Yes, Chris. But if you need 20 amps of 24 volts, then BOTH power supplies need to be capable of 20 amps. Your available current will be determined by the weakest link. \$\endgroup\$ – Mark Jun 4 '16 at 5:20

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