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Author Topic: Power supplies in series  (Read 577 times)
ZS6ARF
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Posts: 31




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« on: December 23, 2007, 11:55:16 AM »

Greetings and a happy festive season to all,

I have three power supplies that is rated 5 volts at 80 ampere.

If I connect them in series with each on adjusted at 4.6 volts to give me a total voltage of 13.8 volts, will it work?

I am concerned that on load the supply closest to the load will draw more current than the rest which can be troublesome for regulation.

Any ideas?

Wynand (ZS6ARF)
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AA4PB
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Posts: 12639




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« Reply #1 on: December 23, 2007, 12:04:08 PM »

You can connect them in series provided they are isolated from ground (no common reference between them). Each will supply the same amount of current.

It is when supplies are connected in parallel that you must be concerned about how they share the current. Parallel connected supplies need to be designed for that use with interconnected regulator circuits. That doesn't apply to supplies in series.
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ZS6ARF
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« Reply #2 on: December 23, 2007, 12:42:22 PM »

Thanks for the reply. That does make sense. I'll add a over-voltage protection on the output to cut the supplies if something do go wrong.

73
Wynand (ZS6ARF)  
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KB9CRY
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« Reply #3 on: December 23, 2007, 12:59:07 PM »

I wouldn't do that.  Just buy an Astron of the right size that you need.

You can try it; I'd want to be around to make sure you can turn off the power quick if any smoke is let out.
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AA4PB
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« Reply #4 on: December 23, 2007, 07:26:25 PM »

If each supply has its own adaquate over-voltage protection you probably don't need another. If one supply goes over-voltage it will cut off and the output voltage will drop by 1/3.

I ran two supplies in series (adjusted for 6.5V each) to get 13VDC for 24/7 several years on a 2M station with no problems.
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HA6SST
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« Reply #5 on: December 24, 2007, 06:12:34 AM »

One thing which may help, wire a 500 ohm resistor across each supply. I found that this improved regulation when I tried this using PC Supplies.

HA6SST
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HA6SST
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« Reply #6 on: December 24, 2007, 06:13:17 AM »

One thing which may help, wire a 500 ohm resistor across each supply. I found that this improved regulation when I tried this using PC Supplies.

HA6SST
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K4SAV
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Posts: 1785




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« Reply #7 on: December 24, 2007, 08:29:23 AM »

There can be a problem here.  If the load is applied and one or two of the supplies are ON, and the other one is OFF, the OFF supply will have a reverse voltage applied across its terminals.  If the supplies have a reverse diode across the supply terminals capabile of handling 80 amps (has to be a big one) then all may be OK.  If it doesn't then there is the possiblilty that the off supply could be damaged.

Jerry, K4SAV
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ZS6ARF
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« Reply #8 on: December 24, 2007, 10:36:01 AM »

Thanks for all the advice.

It look like I can connect the three power supplies if:

I ensure each ones chassis is isolated on both the input and output sides. To be sure there isn't a common ground somewhere.

I have diode protection for reverse voltages

I install resistors to help stabilsing the load

73

Wynand (ZS6ARF)

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KV6O
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« Reply #9 on: December 24, 2007, 02:39:46 PM »

I have done the same thing with 3 5V, 150A supplies with no problem. Just be sure that they are isolated!  

Steve
KV6O
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KE3WD
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« Reply #10 on: December 24, 2007, 03:50:43 PM »

Those resistors are needed to regulate SWITCHING power supplies.

If you have linear supplies, don't use the resistors.  


.
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WA9SVD
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« Reply #11 on: December 25, 2007, 08:12:12 AM »

HA6SST on December 24, 2007       Mail this to a friend!
One thing which may help, wire a 500 ohm resistor across each supply. I found that this improved regulation when I tried this using PC Supplies.

HA6SST
==================

    Almost ALL "PC" switching supplies REQUIRE a minimum load to operate at all, not just for "better regulation."  Without a load, most will simply shut down.
    But each case is different.  And some can be "floated," while other supplies have the negative sude connected to the case in multiple places through mounting  hardware, making their use in series impossible without considerable re-work.
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