12v PS in series for 24v and what to do about 12v bed

Hi, first post. Just now getting in to making my own 3d printer. I'm not new to working on electronic (mpcnc, custom guitar amps, pedal, etc), but I am far from an expert.

So short story is that I ordered most of my parts for a prusa mk2.5 build and now realized the two mini-rambos I have are bad (less than 5v on the 5v rail, maybe other things). Ultimachine has been kind enough to provide some guidance even though the boards are most likely knockoffs (got them in a parts bundle, no returns). Still waiting to hear back, but last response was that they suspected bad component somewhere.

All the printer parts I have are 12v, but now that I have to order a new board I thought I'd go with SKR v1.4 or einsy rambo for quieter printing. However, I do not want to spend more on a new 24v heatbed, fans and power supply at this moment. What i have are two identical 12v 20a power supplies. I understand I can wire in series to get 24v 20a to power the board/drivers/hotend heater, but that leaves the 12v bed and fans.

Question is, after wiring the power supplies in series, can I still use one the the 12v power supplies to power the 12v bed? I have drawn up what I think could be a solution, but it is just a guess. Would anyone with more knowledge that I, care to review the attachment?

Be very careful hooking power supplies up in series. This requires that the power supplies be isolated supplies. I just checked mine which is a computer supply and the negative side of the 12volt rail is shorted to the ground connector on my plug. So if I hooked up 2 of theses in serial the 12 volts would be shorted to ground.

Not sure how to tell. They are the tall type provided in cheap 3d printer kits, like meanwells. They are not atx computer power supplies or laptop power supplies.

BTW, correction: these are 30a instead of 20a, not that it matters.

Ah, just re-read what you said. I measured and there is no continuity between ground terminal and any other terminal. So guessing these are isolated, right?

Ok!.

Start with one 12v power supply (" note the bottom supply "), You will have ground and 12 volts.
This should power all 12 volt sources.

Take your second 12v power supply (" note the top supply "), connect the negative lead to the 1st supply positive lead the second supply.
The 1st supply in combination with the 2nd supply will supply the 24V. You should not wire the 2nd power supply ground for anything else.



You will note the Ground lead is broken on the bottom supply, in your case this is not necessary.
Switching supplies like yours rarely require this though if you would like added protection you could add a 200v .1uf capacitor inline with the break to this supply...

Edited 3 time(s). Last edit at 04/03/2020 08:05PM by Roberts_Clif.

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Computer Programmer / Electronics Technician

Thank you so much for the explanation. Can I use 1 mains cord for both? I'm trying to put your schematic in a physical layout similar to my power supply. They have 3 sets of V+/V-. Here is my updated image, please let me know if I got this correct?




Edited 2 time(s). Last edit at 04/03/2020 10:46PM by munkeyboy.

Yes you can use one main for both.
The wire color codes appears to be correct.

It appear you are using 220vac is this correct..

Edited 1 time(s). Last edit at 04/04/2020 12:17AM by Roberts_Clif.

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Computer Programmer / Electronics Technician

Quote
Roberts_Clif
Yes you can use one main for both.
The wire color codes appears to be correct.

It appear you are using 220vac is this correct..

No, just the colors of the 110vac cable that came with some parts. It was an odd coloring.

Quote
Roberts_Clif
You will note the Ground lead is broken on the bottom supply, in your case this is not necessary.

It's not only unnecessary to disconnect that ground lead, it's dangerous. Don't do it.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].