# Capacitor Size

1. Aug 25, 2009

### daveroderick

I want to know if I am figuring correctly. I am putting together a 12 volt 3 amp DC power supply. The formula in the instructions is C = Tc / R. I figured the required cap to be 20,750 uf. When I went to a couple of supply catalogs to check the price for the cap I found that no one had had a cap that size (not even close). The closest cap that I found was 9,800 uf, which was almost 20 bucks. It was also seemingly quite large in size. My supply catalogs have a wall plug 12 volt 3.3 amp DC power supply which does not look any where near large enough to hold the size cap my formula is telling me I need.

Am I missing something here? In an effort to see if I was doing the math correctly, I took a 12 volt 300 ma DC power supply and figured what size the cap should be. I figured the cap size required to be 1,467 uf. I then cracked open the case of the power supply and found a 1,485 uf cap. So it would seem to me that I am figuring this correctly. Is there another way they do some of these power supplies that have larger amperages? It looks to me at this time that I would need to use 10 2,200 uf caps to reach the required capacitance for this power supply cost effectively. This project is not about cost, if it were I would spend the 15 bucks for the wall plug unit. But it seems to me the physical size of the cap would make the wall plug unit almost impossible. Also, if these cap sizes are common in 3 and 4 amp DC power supplies, where does someone go to get them?

Thanks,
Dave

2. Aug 25, 2009

### vk6kro

Large electrolytic capacitors are expensive. They are often discarded with old amplifiers etc, so it is worth building up a junk box of useful stuff.

Your power supply should probably be a regulated one. This would cut down on capacitor costs and give you a better power supply.

A linear regulator would use a transformer, giving about 12 volts, to drive a bridge rectifier and a 2500 uF capacitor to give about 18 volts.
You then run this into a 12 volt regulator on a heat sink to give 12 volts DC out.

This will give 12 volts out as long as you can provide more than, say, 15 volts into it.
So, the 18 volt supply can have 3 volts of hum on it before the 12 volts output starts to show anything but 12 volts.
This gives you quite a bit of latitude with your capacitors and also makes the supply give 12 volts regardless (within limits) of the mains voltage and the load on the regulator.

Wall warts are usually not regulated and many of them are not even filtered, so you can do better by building it yourself.

Typical linear regulators include the 78T12 LM318K or a LM338K which is variable.

There are also switching regulators that you might like to look at. They are a lot more complicated but more efficient.

3. Aug 25, 2009

### negitron

A lot of those >3-amp power supplies nowadays are actually switching supplies and these don't need the large filter caps that linear supplies require because the switching frequency is in the tens of kilohertz rather than 60 Hz.

4. Aug 25, 2009

### Carl Pugh

What voltage capacitor did you get the price for?

Did check using catalog had handy.
Mouser stock number 598-SLPX223M035H4P3
Aluminum Electrolytic Capacitor
35 VDC, 22,000 uf
$4.34 each/quantity 1,$3.26 each in 10 lot quantity.

Other electronic catalogs should have equivalent parts at equivalent prices.