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Author Topic: 48V power supply caps  (Read 2406 times)
AI4NS
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« on: April 11, 2008, 07:56:42 PM »

What would be a good choice for caps in a 48V power supply that I hope will develop about 40-50A? I see that Astron uses 100K mfd in the RS-50. Would that be ok, or overkill? I am building a 600W solid state HF amp, and would maybe later go to 1KW.

Mike
AI4NS
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WB2WIK
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« Reply #1 on: April 11, 2008, 08:20:45 PM »

Depends on the ripple frequency.

Are you talking about a linear power supply (120 Hz ripple)?
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KE3HO
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« Reply #2 on: April 11, 2008, 08:27:46 PM »

It depends on a lot of things. For example, if you are talking about the caps before the regulator, then it depends on line frequency (assuming you are working with an AC powered transformer), voltage "headroom" (difference between peak voltage at the input to the regulator and the minimum necessary input voltage to the regulator), and to some extent, the ESR of the capacitors.

You can get a pretty good idea how much capacitance you need with the following equation:

I = C x dV/dT

where I is current, C is capacitance in Farads, dV is the voltage change, and dT is the time span over which the voltage will be dropping (which is typically 1/2 of a line cycle for a full wave rectifier).

So, if the peak output voltage of your transformer is 72V and your regulator requires 52V to maintain regulation at 48V output and you want to draw 50 amps,

50 amps = C x (72V - 52V)/.008 sec

or C = 50 amps x .008 sec/20V = 0.02 Farads.

(the 0.008 sec comes from 1/2 a line cycle at 60Hz, rounded of course)

So in this example you would need at least 20,000uF capacitance to keep the regulator running nicely. Remember, this is an absolute bare minimum value. In practice you would need more because the capacitors have their ESR (equivalent series resistance), line voltage can fluctuate and have momentary drop-outs, etc. Looking at the ESR, If your capacitor has an ESR of 0.1 ohms, then at 50A you would have an additional 5V drop under load. This would have to be subtracted from the voltage headroom. So in this example, the 20V would be reduced to 15V and C would then be 0.0267 Farads, or 26,700uF.

Be aware, also, that this ESR results in heating in the capacitor (in the example above it would amount to 25 watts which is too much for a capacitor to handle). A common way to help this situation is to use several low ESR capacitors in parallel. This splits the current between the caps, which is equivalent to adding all of the ESR values in parallel. Rather than using one 50,000uF capacitor, you might use 4 12,000uF capacitors in parallel, or even 8 6000uF capacitors in parallel. At this current level, more is better.


If you are talking about the capacitor on the output, then you have a whole different set of things to consider, and that depends a lot on your regulator design.

I hope this is helpful.
73 - Jim
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W5KG
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« Reply #3 on: April 12, 2008, 09:51:16 AM »

In my ALS-600 power supply, Amertron used a
15000uF 80v filter capasitor. It is a 50vdc linear PS not the switcher.
Don, W5KG
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N9GXA
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« Reply #4 on: May 14, 2008, 09:16:14 AM »

I think the ALS-600 also incorporates a choke. I believe this lowers the required uF needed. I am in the planning stages of building a 600 watt amp powered by a 50V/25A power supply. I planning on no choke but filter caps only after the regulator. In this case, I have read 6000-7000 uF per average amper needed (per K0GKD's article). I will start with a total of 88,000 uF, but wondered if I can drop that back some.

  Good luck...
73
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AA3M
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« Reply #5 on: May 14, 2008, 04:05:36 PM »

Large phone systems use 48 volts as the power source.
We replaced our PBX about five years ago and the new one is still designed around 48 volts.
Try searching for telephone PBX power supplies 48 volt.

73

John
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