Design filtering capacitor

[Hassan Tariq]

Hello every one , can any body tell me how to design filtering capacitor in variable dc power supply , I have a project to make a variable dc power supply from 1.25 v to 20 v . i want to know how to design filtering cap , if anyone have such formula then please share with me .

 

[CDRIVE]

Are you contemplating actually making one?

Chris

 

[Hassan Tariq]

i'm taking about this capacitor , which is in red circle . Please tell me any formula to calculate its value for my dc power supply .

 

[CDRIVE]

That's a 3,300uF Electrolytic cap. We don't know what the secondary voltage of the Transformer is so we can't tell you what the voltage rating should be. What's the secondary voltage?

Also, the LM317 should have a .1uF cap connected close to the Input pin to GND.

Chris

 

[(*steve*)]

A simple rule of thumb is that 1000uF per amp of current from a full wave rectified ac source will give you about 5V of ripple.

In this case there is 3300uF and if the max load current is 1A, then the ripple will be about 1.5V.

If you have a 12V transformer, and your reg requires 2.5V overhead, this means the max output voltage is (12 * 1.414) - (2 + 1.5 + 2.5) = 10.9V. If you were further to allow for an input mains voltage that is 10% low, 10V would be a safe max voltage.

 

[Hassan Tariq]

this is 12x2 center tap transformer , voltage at sec is v rms = 27.8 volt .

 

[Hassan Tariq]

i want to make a dc power supply , from 1.25 v to 20 v . i have using 12 x 2 center tap transformer , and voltage at sec is vrms (sec) = 27.8 by multimeter .i am facing problem in designing of filtering capacitor . I have search on different websites , every website give me different formula. i'm totally confused . Can anybody tell me exact formula to design filtering cap ?

 

[davenn]

.i am facing problem in designing of filtering capacitor .

OK for a start ... you don't "design " filter capacitors, or any other capacitor for that matter. You just go and buy the one you need

Steve gave you the correct answer in post #5 as to the capacitance value and the voltage rating

and some links to go deeper
http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm
http://my.integritynet.com.au/purdic/power2.html


Dave

 

[Colin Mitchell]

Choosing a filter cap (electrolytic).
The ripple is 5v per 1,000u. If the 3-terminal regulator requires an input voltage that is 6v less than the voltage on the electrolytic, you will never see any ripple. Use the same reasoning for higher currents.