I have two 24 volt DC power supplies (that each look like this - according to documentation).

PS + AC in ---->|       |-----   + 24 volt out
                |       |
PS - AC in ---->|       |
                |_______|-----   - 24 volt out

I have been instructed to cross connect to form a common return (apologies if bad nomenclature/vocabulary is being used)

PS 1 + AC in ---->|       |-----   + 24 volt out
                  |       |
PS 1 - AC in ---->|       |
                  |_______|-----   - 24 volt out  (return)
                             / (PS1 - out is cross connected to PS2 + out)
                   _______  /
PS 2 + AC in ---->|       |-----   + 24 volt out (return)
                  |       |
PS 2 - AC in ---->|       |
                  |_______|-----   - volt out

The device driver card I am plugging into requires me to pin together each of these 4 outputs and the specifications says provide 11 Amps for routine use, with a possible maximum of 22 Amps.

An existing setup in the lab is using 4.5 - 6.2 Amp power supplies configured in a similar configuration and it works perfectly.

I am assuming that the 4.8 Amp currents of these power supplies additive (i.e. nearly 10A provided)? But I don't have a good idea of the consequence of saturating the signal request by under powering the driver card. In general, since I can't provide information about the card itself, what kind of risks are there in this kind of signal saturation, and power plateauing?


1 Answer 1


There's quite a bit of confusion here, sorry.

The way you show the supplies you have only +24 available, not 48. There are two +24 outputs and one return, so you can connect 24V loads to either (or both) supplies.

Whether they are 'floating' or not depends on how the supplies are made internally, nothing to do with how they are connected.

If you can use a resistor for the fan, the resistor will depend on the actual (not nameplate) fan current draw, not on the rating of the supplies. If the fan is anything like amperes current draw, using a resistor is not very practical (a 2A 12V fan run from 24V will require a large power-wasting 24W+ resistor). Better to get another supply to run the fan.

  • \$\begingroup\$ Thanks for getting me started. I thought floating had something to do with a lack of ground? I was also under the impression that DC fans had virtually no current draw ~0.5A. It looks funny now to not have a negative 24V out from my power supply, I will have to double check my diagram. \$\endgroup\$
    – EngBIRD
    Aug 21, 2015 at 14:10
  • \$\begingroup\$ Yes, floating refers to the grounding. If it's grounded inside the supplies that's that, but if it's not you can ground it. If it's not obvious, the voltage between the +24V lines in your configuration is about 0V (just the difference between the two supplies). \$\endgroup\$ Aug 21, 2015 at 15:10
  • \$\begingroup\$ I have no idea how much resistance is required because I don't know how much your fan draws. You would need to put 12V on it and measure the current. Then R = 12/I. So if it's 215mA then the resistance is 56\$\Omega\$ and the power is I^2R = 2.5W (so a 3W or 5W resistor). \$\endgroup\$ Aug 21, 2015 at 16:05
  • \$\begingroup\$ If you actually measured it, yes, but if it's just read off the nameplate then it's probably high, so your resistor will be too low and the voltage across the fan too high. Do you understand ? \$\endgroup\$ Aug 21, 2015 at 16:49
  • \$\begingroup\$ In retrospect, I don't think fans and voltages are at all relevant. I think I was trying to provide too much information about the setup, and got side tracked from the real problem. I have updated my question, and tightened the scope about my power supplies ability to supply ample voltage and current. I appreciate any insights you are able to offer. \$\endgroup\$
    – EngBIRD
    Aug 26, 2015 at 19:04

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