The relation between output current & filter capacitor v

[CMOS]

Filter capacitor value ?

What value of filter capacitor should be used after bridge rectifier for an unregulated power supply of 12V/2A.
Right now I am using 4700uF capacitor but voltage begins to drop as current increases beyond 1A.

What is the relation between output current and filter capacitor value? How can I calculate it?

 

[klug]

Re: Filter capacitor value ?

Capacitor is intended to filter pulsations - it will not affect on constant voltage dropping.

To decrease voltage dropping you need to use more powerful transformer and suitable diodes (try to use shotky diodes) in rectifier. Anyway, voltage will be decreasing when current will increase - use voltage stabilizator to avoid it.

If you are talking about pulsations when you apply load, then you can calculate C, as:

C=H1/(r*Kpuls)

where,
H1 - coefficient (Ohm*mkF) -- it is 1200 for 1 A (you can take 2300 for 2A(for 50 Hz AC), or 1900 for 2 A (for 60 Hz AC))
r - resistance of transformer coil and open rectifier branch, it is =Rtransf+Rdiode for onehalf-period rectifier, and =Rtransf+2*Rdiode for rectifier bridge.
Kpuls - coefficient of pulsations in output =Valt/Vconst

 

[wil]

Filter capacitor value ?

Capacitors are used to decrease "ripple" efects after retifiers in power supplies.
4700uF are enough to your power supply. It is not necessary to put a bigger one.
It happens because your circuit needs more current from the transformer.