# Capacitors

Discussion in 'General Electronics Discussion' started by Dopey, Nov 27, 2010.

1. ### Dopey

5
0
Nov 18, 2010
Ok I have a small problem with capacitors, namely a lack or knowledge, at this moment I’m building a small power supply for a speed controlled electric motor using a LM338 which has a cap on the input side 10,000uf 40v, I got a 10,000uf 35v and a 10,000uf 50v. I was wondering if either of these would do, the supply is 2x12v=24v but I have no understanding of how these things are worked out, is it done by simple rule of thumb or is there some critical calculation involved here, and in what way does it affect the circuit if I get this wrong?

Many thanks for being there for us.
Mark

2. ### barathbushan

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0
Sep 26, 2009
The 40v and the 50v on the caps , just mean that the voltages across the respective capacitors should not exceed 40v and 50v .

I think for a 24v situation , either one of these can be used

3. ### Resqueline

2,848
2
Jul 31, 2009
If the supply is a 2*12V transformer then the rectified voltage is 24V*1.414=32V. Add some % for no-load conditions and you could exceed the 35V rating; so use the 50V.
I don't understand why you ask though, looking at your previous posts you know you're getting 34V from that transformer..

Last edited: Nov 27, 2010
4. ### Dopey

5
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Nov 18, 2010
Sorry it was probably a badly worded question

Actually it wasn’t so much a question about the voltages but more about how to work out their capacitance and how the 10,000 bit is arrived at, or to put it another way can one put to big a capacitor in such a circuit?.

Mark

5. ### Resqueline

2,848
2
Jul 31, 2009
The neccessary capacitance depends on the allowable ripple voltage. You need to stay within regulation. A 1F capacitor discharges with 1V/second at 1A load current.
At 60Hz there's 8.3ms between each peak. Let's say that 8ms is spent discharging and 0.33ms charging the capacitor, and that you can have a maximum ripple of 5V.
Max ripple here = 32V - (24V + 3V) = 5V. Thus I'd figure you'd need (roughly calculated) at least 0.008s / 5V = 1600uF for each Amp of load current.
A rule of thumb however says that 10000uF is good for around 10 Amps load current.
A bigger capacitor results in less ripple voltage, and hence bigger peak currents through the transformer & rectifier, resulting in more heating of those.
It also results in a greater inrush current at turn-on that may give issues with fuses blowing. That can be remedied with a delay fuse or a soft-start circuit though.

6. ### Dopey

5
0
Nov 18, 2010
Wow thanks Resqueline, am out of hospital now for a bit so hopefully will get to grips with all this soon, this is only one part of the power supply I’m building at the moment to power cooling fans and LED’s in the meters and the speed control for the motor in the CNC

Many thanks for the time
Mark