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Electrolytic Capacitors are a very important part of a low voltage power supply. Usually the supply needs about 3000 micro farads of capacitance per amp of current that the supply will be capable of producing. Most of them need to be a safe margin of voltage value more than the voltage of the secondary of the transformer. 30 volt rating will work if your power supply has 18 to 20 volts on the secondary. It would be much safer and last longer if the capacitors were rated at 50 volts. The higher the voltage rating, the more expensive the caps get.
Let us say we are building a 13.8 volt at 20 amp power supply. We will need around 60,000 micro farads at 50 volts rating and there should not be a problem down the way. We can get the rating we need by paralleling caps of less capacitance value. The voltage rating will always stay the same when connecting the caps in parallel. But the micro farad rating adds up.
There are many places to look for these larger value caps. Swap meets, are good for finding them. Old telephone company power supplies have very high quality caps in them. Old broken DC welders have many. Electronic surplus stores sometimes have them. If you order them new, expect them to be expensive.
A lot of old VCR's and Stereo equipment have some smaller caps, but if you have access to many of them, you can connect them all up in parallel and get what value you are after. The really big ones, the size of 16 oz. beer cans are hard to come by, but can be found. Use your imagination as to where to look for them and what to look in.
Happy hunting !
I have several of these big caps. The physical measurements are 3 inches by 5 3/16 inches tall. Each single cap would be good for a 20 amp supply. Send me an email for pricing.
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