Author Topic: Can power supplies be linked together like batteries for higher voltages/amps?  (Read 777 times)

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Offline Beamin

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So while I don't have a 60V power supply I do have 5 12 volt ones of the ATX computer flavor. A sixty volt  few amp power supply = $$$ but ATX power supplies are almost free. Buy an old Compaq at the good will for 10$ throw away the computer and keep the power supply.

Can you put these in parallel or series to get higher voltages or current? I would imagine they would have to be the exact same rating if not the exact same brand and model so that the weakest one is not getting over loaded. But if not why aren't power supplies able to be put in a circuit for greater V or I like batteries? I'm talking DC where you don't have to worry about things being in/out of phase. What would be nice is if I had above 30 volts so I could run tubes. I'm a big fan of the auto shut off the ATX have incase you short something, which has saved my ass many times. I think when I tried this as a kid one power supply kept going into protection mode, but who knows, my goal as a kid was more for the destruction and overload of devices rather then the creation and smooth running within spec of the unit, like now. Gun powder and flash powder was a tool I employed often. When your only tool is a hammer all your problems look like CRT screens.
« Last Edit: January 13, 2019, 04:21:53 am by Beamin »
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Online Brumby

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Can you put power supplies in series for higher voltages?
 * YES - but ONLY if they are floating with respect to each other.

Can you put power supplies in parallel for higher current ?
 * A qualified yes.  There are issues with balancing the load drawn from each supply.  For example, say you have two 12V supplies than can deliver 10 A each and you want to connect them in parallel to drive a 12V 20A circuit.  With them being two independent supplies with the possibility of slightly different characteristics, such as voltage output, you might have one that will try to deliver 15A until it sags enough that the other supply will kick in the extra 5A.  Needless to say the first supply isn't going to be able to do that.
 

Online Brumby

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For series connection, the maximum current available will be the maximum current capacity of the weakest supply.  So if you have 3 supplies of 5V 25A, 12V 10A and 12V 2A, you can have a supply of 29V at a maximum of 2A.
 
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Online Brumby

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For parallel connection, you will certainly want the output voltages to be as well matched as possible - but even so, there will still be small variations.

One option that has been used is to have low value power resistors in series with each power supply before being connected to a common point.  The calculation of the value of these resistors will depend on the situation.
 

Offline Mr. Scram

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Many two channel power supplies offer this feature. An example would be having either two channels of 0-30V of 3A each, or one of 0-30V and 6A, or one of 60V and 3A.
 

Offline Andreas

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So while I don't have a 60V power supply I do have 5 12 volt ones of the ATX computer flavor.

ATX power supplies are not designed for serial connection.
Usually the 0V output is connected to earth ground.
And no reverse polarity protection in case of overloading the output.

But you can usually series connect good lab power supplies.
They have floating outputs and a reverse polarity protection diode for that (in case of over load).

with best regards

Andreas
 

Online Brumby

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... or one of 0-30V and 6A ...

Indeed - but those units have the luxury of having the regulation circuitry appropriately configured.
 

Offline floobydust

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For series power supply connections, add reverse-diodes so the weaker PSU does not get damaged.
It's like two power supplies arm wrestling if the load is near maximum.
 

Offline not1xor1

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So while I don't have a 60V power supply I do have 5 12 volt ones of the ATX computer flavor. A sixty volt  few amp power supply = $$$ but ATX power supplies are almost free. Buy an old Compaq at the good will for 10$ throw away the computer and keep the power supply.

Forget that. Andreas is 1000% right  :).

In any case ATX power supplies are usually quite powerful (at least 200W, in most cases more than 400W).
About 10 years ago the specifications changed so the main (more powerful and feedback tied) rail is the 12V one and that can provide tenths of amperes.

If you want a higher voltage, just buy a cheap V/I adjustable step-up module and connect it to the ATX 12V output.
Of course the maximum available current would decrease as the maximum power is anyway constant.
 

Offline treez

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Quote
For series power supply connections, add reverse-diodes so the weaker PSU does not get damaged.
It's like two power supplies arm wrestling if the load is near maximum.

Yes agreed, this could be the case with two series'd power supplies which both dont have soft start, and there is a big external cap across the "total" (doubled) output....the weaker power supply  could possibly  get reverse polarity'd during the startup surge....and so the diode is needed.....even then there'd still be a diode drop of reverse polarity on the weaker one , which i doubt would do its electrolytics any good. (assuming it had electrolytics).

This is why its good to have the soft start.

You can buy  (or make) “boxes” which have transistors in them which act in the linear mode, and there is error amplifier circuitry in there which makes them all pass the same current….so you connect the various power supplies to this box and it equalises the current output from all of them…….and gives you your one single high current output.
There are more elegant ways to do it, but if you just have a  couple of supplies that you want to put together , then this is a crude solution which will do the job for you.
« Last Edit: January 15, 2019, 07:39:39 pm by treez »
 

Online ArthurDent

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As others have said, unless you absolutely know that the output of a power supply is designed to be floated or put in series/parallel, assume that it can only be used the way it was in the original application. There are some supplies that are designed to be floated but these are generally higher end supplies and it will say, either near the terminals or in the manual, how the supply can be used. Here are some examples.
 

Offline JohnPen

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As others have said Isolated outputs with maximum current equal to the lowest current rating power unit is fine for PSUs in series.  For parallel no need for Isolated but tricky voltage matching adjustment to share the load with relatively equal amounts of current.  However for something like charging a car battery, which pulls the output voltage down, and assuming each power supply has an adjustable constant current output you can just wind the volts to greater than say 16v and set the constant current of each supply to supply the current they can handle quite safely.  I didn't have a battery charger at the time and my home built linear power supply gave out 2 x 2A .  :)
I wouldn't recommended for everyone to try this because some PSU designs might be very unhappy with this type of operation.  However for my PSU this method was used a number times over the years with no problems.  The supply in question is now over 38 years old, used a lot for hobby, and has only needed replacement electrolytics so far. :)
 

Online james_s

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Hot swap server PSUs are designed to operate in parallel and share the load. With modification to float the DC ground they can be operated in series, they're not really designed for that but lots of people including myself do it.

Cheaper supplies like wall warts and such are not designed to run in series or parallel however I've done that with success. I consider that to be one of those things you shouldn't really do but if you don't care about possibly damaging a power supply there's a reasonable chance it will work.
 

Offline Zero999

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Load sharing is unimportant.

In order to be safely connected in parallel, only three things are required from the two power supplies:

  • The power supply will go into constant current mode when overloaded
  • A slightly higher voltage, than its set-point can be connected to its output, without damage or instability.
  • It's stable when unloaded.

The power supply with the higher voltage setting will provide all of the current, up until the current limit is exceeded, its output voltage will fall to the same level as the other power supply which will supply the extra current.
 

Online Brumby

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Load sharing is unimportant.

In order to be safely connected in parallel, only three things are required from the two power supplies:

  • The power supply will go into constant current mode when overloaded
  • A slightly higher voltage, than its set-point can be connected to its output, without damage or instability.
  • It's stable when unloaded.

The power supply with the higher voltage setting will provide all of the current, up until the current limit is exceeded, its output voltage will fall to the same level as the other power supply which will supply the extra current.

The Op was talking about ATX power supplies, not lab style.  Current limiting in the form of which you are speaking would certainly address the load sharing issue - but this is not a feature of ATX supplies, in my experience.
« Last Edit: January 13, 2019, 09:09:42 pm by Brumby »
 


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