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combining dis-similar DC power supplies

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Paul Relf-Davies07/03/2019 17:19:35
74 forum posts
1 photos

Hi all,

I'm building a bench PSU to support a project in relation to my mill (power for a 3-axis DRO and a powered X-Axis (using an old car wiper motor) & my rusty O-level electronics has failed me blush.

I'm making use of an old but usable PC PSU, which will give various voltages: 3.3v as well as +/-5 &12v.

Given these, I should be able to 'make up' other voltages, for instance +12v & -12v should give a 24v supply.

However, my question is this: Were I to combine 2 supply lines with differing max current outputs, how do I calculate the resultant max current?

In the example above, the +12v supply can deliver up to 15A, but the -12v is only rated up to 0.5A. Were I to combine these to give 24v, what would be the max current I could expect to draw?

I'm sure there is a simple just escapes me!



Clive Foster07/03/2019 17:37:46
2245 forum posts
73 photos

For all practical purposes the lowest current rating defines the maximum current that can be drawn at the combined voltage.

What actually happens to voltage and current of the combined pair if you try to draw too much current out of the lower rated supply depends on the type of PSU and the design of its protection circuits. Basic transformer-rectifier-capacitor supply output voltage tend to initially decline fairly gracefully before everything goes seriously pear shape. Linear stabilised power supplies will most likely limit the current to try and hold up the output voltage. Switch modes ones can be complicated but constant voltage or constant power modes are common overload / short circuit protection modes. Generally its not a good idea to try and go outside the design specifications for normal use but what happens if a fault condition tries to draw more than the rated current can be important.


Frances IoM07/03/2019 18:02:39
779 forum posts
26 photos
they share a common 0V (ground) line adding the 12V to the 5V will generally produce copious magic smoke - the -12V supply is very low current as Clive has stated and will be useless for a motor - you may well find the starting current of the motor is too great for the PSU and the inductive load will generally not be liked by PSUs designed for computer use.

If you use two sep PSUs then possible if you remove the ground connexion to one PSU and ensure the metal case never comes into contact with the metal case of the other PSU - NOT a recommended technique - there are cheap chinese boards that given 24V say will produce lower voltages without consuming too much power
Paul Relf-Davies07/03/2019 19:15:46
74 forum posts
1 photos

Thanks for your replies...

I had a sneaking suspicion that would be the outcome. Fortunately, for the project at hand, I only need +5v and +12v, for which my donor PSU can supply far more current than i actually need.

I was simply curious as I can envisage that the additional potential combinations to be gained could have been useful for future projects.

Thanks again


Brian G07/03/2019 19:17:37
705 forum posts
28 photos

Why not just buy a 24 volt LED power supply as used for 3D printers? If you don't trust eBay suppliers, Ooznest do a 24V 10A supply for £25.


John Haine07/03/2019 21:07:26
3169 forum posts
171 photos

Depending on your requirement, LED supplies can be a dog - the regulation is very poor.

Ian P07/03/2019 21:47:26
2406 forum posts
101 photos
Posted by John Haine on 07/03/2019 21:07:26:

Depending on your requirement, LED supplies can be a dog - the regulation is very poor.

As you say, depending on requirement, LED supplies can be highly regulated, not all are dogs.

Ian P

Brian Oldford07/03/2019 22:32:35
673 forum posts
18 photos

Treated with respect ex-PC PSUs can make excellent bench PSUs albeit there may be a little ripple on the O/P. They often need a small base load to operate correctly which can be applied by putting a modest load resistor between +12v and Ground (0v). Some may need to be forced to turn the O/P on by earthing the green wire.

It is even possible to create a decent current O/P 24v or 36v supply by connecting two or three PSUs in series but very special steps must be taken. 

Edited By Brian Oldford on 07/03/2019 22:35:17

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