# smoothing capacitor

Discussion in 'The Projects Forum' started by ueo, Mar 11, 2012.

1. ### ueo Thread Starter New Member

Mar 11, 2012
1
0
Hello everyone, I am a graduate of BS accountancy and working related to my profession.
I am installing 6 pcs surveillance cam at home. For power supply, somebody told me to buy a 12 volts transformer with 6 amperes, so i did it. But it didnt work because its output is still 12v AC. It needs to be converted from AC to DC. My problem now is that: when i search through internet I found diagrams for full wave rectifier with 4 diodes, but no capacitor rating for smoothing. I am now begging from you to tell me the smoothing capacitor rating value usable for my transformer with input of 220v AC output of 12v AC 6Amperes connected with 4 diodes bridgetype rectifier for full wave, so that it can produce an output of 12v DC full wave. thanks.

Jun 5, 2009
116
5
It all depends on the total current pulled? I should think you selected a 6amp psu just so you have enough rather then to little, do u know much much current would actully be pulled from th psu?

3. ### #12 Expert

Nov 30, 2010
16,704
7,354
radical2 C Er F = I
1.414 x capacitance x Voltage ripple(peak to peak) x frequency = current

Assuming a full wave rectifier and 50 Hz power with 10% ripple voltage, C = 6amps/(1.414 x 1.2 x 100) and that's about 35,000 uf

But you still have the problem that the output voltage will not be 12 when you rectify the AC. It will be more like 15.3V

That's why the reluctance. If you didn't know that cameras work on DC, you aren't going to like designing and building a voltage regulator. I hope you still have the receipt so you can go back to the store and buy a 12 volt DC power supply.

ps, read the labels on the cameras to find out how much power (amps) they really use.

4. ### wayneh Expert

Sep 9, 2010
12,392
3,246
You may consider just pulling a PSU out of an old (ie. free) computer. It'll have lots of nice features that would be hard in a DIY supply; things like over-current protection, a fuse, a nice switch and power cord, overheat shutdown, tight voltage regulation at 12v, and on and on. Higher efficiency, too.

If you want to build your own, that's fine. For your filtering cap, there's no such thing as too big. The only reason to go small is cost. More is better in all other respects.

As noted, you'll need to know more about what your cameras need. You may need to regulate voltage, to protect them from over-voltage or too much noise from the supply. On the other hand they may have been designed to operate on such a supply and may not need much filtering on the power.