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Old 13th March 2014, 08:35 AM   #1
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Default Linear PSU's in Parallel?

Hello! I have an open-frame linear PSU with two seperate outputs, adjustable over a range of 12 to 15 VDC @ 1.25 A max. I would like to to connect the outputs, through identical blocking diodes, to have available 2.5 A.

Way back when I was working at Ford Aerospace in a satellite tracking/command facility, the many data buffers present all had two Lambda PSU's connected as I described. But I know that the primary purpose of this configuration was to provide redundancy in case one PSU failed. (Never happened). Each PSU had the capacity to power the equipment on its own.

I would be using this PSU to replace another open-frame unit with 1.5 A max output, which is adjusted for 13.5 V to float my RV battery and power a small number of 12V lamps.

I am hoping a member/members here has experience and knowledge on the subject. I've alwys gotten excellent advice here, and thanks in advance.

Rich-

Last edited by Deaf as a Bat; 13th March 2014 at 08:36 AM. Reason: Loose nut behind mouse.
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Old 13th March 2014, 09:00 AM   #2
infinia is offline infinia  United States
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you could only combine currents if they both have current limits using constant current mode, which is unlikely for lower cost units. look up the data sheets, they should mention parallel operation or constant current if they have this feature, otherwise tread very carefully.
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Last edited by infinia; 13th March 2014 at 09:05 AM.
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Old 13th March 2014, 09:07 AM   #3
Mooly is offline Mooly  United Kingdom
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I can tell you what will happen with this... because its something I've done in the past with two 0-30 volt lab supplies when I've needed to charge a VRLA battery more quickly.

You will find that one or other supply will deliver all the current up to the point it either limits (as with my supplies which are current limited) or something fails. The other supply will deliver zero current to the load because of the slight difference in output voltage of one compared to the other.

Again in my case, I could continually tweak the voltage of one supply such that the load was balanced between the two. We are talking values of only a few mv making the difference between one supply delivering all the current up to its limit, both delivering equal current up their limit, or one delivering no current at all.

What you might try... is adding a small resistance to each supply to increase the output resistance significantly. That might help with current sharing but you need both supplies to be nominally identical in output voltage. If they are linear supplies (such as with a pass transistor) then blocking diodes may not be needed (I certainly don't use them with my supplies).
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Old 13th March 2014, 09:14 AM   #4
Mooly is offline Mooly  United Kingdom
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Bit more info... the effect I describe happens because a partially discharged battery will draw either no current with an applied charging voltage up to the "open circuit" terminal voltage of the battery, and current limited only by the supply when that terminal voltage is exceeded.

Other loading (your bulbs) will affect that behaviour, but what you must do is to ensure that whatever you do arrange works under all conditions. Worst case for sharing I would imagine is charging a partially discharged battery with no other resistive loading across the charger.
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Old 13th March 2014, 09:23 AM   #5
infinia is offline infinia  United States
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my advice > keep the trickle charger and consider installing a batt / house switch and buy the right sized 120V to 12 V converter they are cheap.
RV and Boat owners have done this a million times so they have a better solution I'm sure.
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Last edited by infinia; 13th March 2014 at 09:30 AM.
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Old 13th March 2014, 09:33 AM   #6
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Thanks all, my fears and suspicions you have confirmed.

I'll be shopping for a ~5A linear PSU to fill the need.

Also confirmed is the expertise here. Thanks!

BTW, it has been my experience that SMPS have not made the grade for floating lead-acid batteries. Even with the use of blocking diodes, I have lost two, one a Lambda. Linears have hung in there fine.

Also I listen to AM radio at night, and the SMPS' do make RF noise which is intolerable.

Rich-
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Old 13th March 2014, 09:36 AM   #7
infinia is offline infinia  United States
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really ask in good RV forum > they have these smart battery chargers up 10A that might kill two birds with one stone.
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