How to determine the value of the resistor and capacitor?

[huji]

hi,
After a half-wave rectifier, how do we determine the value of the resistor and capacitor (that is connected parallel with each other)? Is there any formula related to it and how does the value of the capacitance and resistance affect the output? thanks.

 

[flatulent]

Re: How to determine the value of the resistor and capacitor

The resistor is just a safety feature to discharge the capacitor when the power is off and someone might be prone to touch the wires. Select a time constant of a second and the capacitor will discharge to nearly nothing in five seconds.

The capacitor value is determined by the maximum load current, the peak to peak ripple you can live with and the frequency of the voltage cycles from the transformer, usually twice the mains frequency if you are using a bridge rectifier or a full wave rectifier.

Start with the equation Q=VC. Take the derivitave dQ=CdV. Then calculate the charge drawn from the capacitor by the load current times the time interval between input voltage cycles. put that into the second equation along with the peak to peak ripple and solve for the capacitance.

This is the simple way. There are some second order effects from the transformer resistance etc. but this will get you close.

 

[IanP]

Re: How to determine the value of the resistor and capacitor

Try this:
Smoothing capacitor for 10% ripple, C = (5*Io)/(Vs*f)
Io = output current from the supply
Vs = supply voltage (peak value of unsmoothed DC)
f = frequency of the AC supply
If you need more info goto:
http://www.kpsec.freeuk.com/powersup.htm (smoothing).
Regards,
IanP

 

[carrier]

Vr(p-p)=Idc/fC
Vdc=Vm-Vr(p-p)/2
where
Idc=maximum dc current at output
Vm=peak voltage of dc rectified output
Vr(p-p)=maximum of desired ripple output

 

[power-twq]

Re: How to determine the value of the resistor and capacitor

you should choose R and C for that R*C must far greater than line frequency.

if R*C is small, there will be serious ripple.


best regards

hi,
After a half-wave rectifier, how do we determine the value of the resistor and capacitor (that is connected parallel with each other)? Is there any formula related to it and how does the value of the capacitance and resistance affect the output? thanks.