Electronic – First PSU – need help with capacitor size

capacitorfilterpower supply

I am building my first power supply unit (my first project too) and need help in figuring the size of filter capacitor (and type, though I presume Electrolytic) I need to use after the rectifier diodes.

Specs:-
2 channels of 1.5VDC – 9VDC adjustable outputs with max 2.5A over both channels

Components sourced:-
229D12 Transformer – output is 12.6VCT (3.8A) at 60 Hz (65% = 2.5A) …. 4 x 1n5404 rectifier diodes (rated at 3A with Vf=1V) …. LM350 adjustable voltage regulator (rated at 3A with Vf=1.25V)

What I know so far:-
60Hz rectified is 120 cycles/sec so T=0.00833sec …. 12.6V – (2*1V diode drop) = 10.6V rectified – 1.25V (Vf of LM350) = 9.35VDC max …. Max current is going to be 2.5A (limited through LM350 and 3.74 Ohms of resistance)

I have tried sourcing the formula to calculate the size of capacitor needed, and I have seen many formulas regarding capacitors. Each one of them confused the heck outta me with funky symbols and abbreviations that I have yet to learn and each of the formulas was for something other than what I was wanting.

I speak basic English, understand average Mathematics, but still have no idea on most of the symbols and abbreviations used in EE. Please keep your answers simplified for this noob. 😀

I appreciate any help ya'll can give.

Best Answer

  1. Define your maximum ripple voltage: ΔV (also called dV, V ripple). Look at image below and I guess you should figure it out.

  2. Use this formula to determine minimum capacitor capacity.

    \$C = \frac{I}{Vpp * 2f}\$

    where: C is capacitor capacity in Farads (1F = 1000000uF), I - current, Vpp (or delta V) - voltage change on capacitor when it is being discharged, f - frequency before bridge

    This method can be found in wikipedia article about ripple, just in diffrent form (formula on wikipedia calculates ripple at given capacitor).

enter image description here

Image source: wikipedia article mentioned above + me

  1. Use capacitor with voltage rating at least a bit higher than your transformer amplitude, not RMS voltage. Amplitude for sinuoid after full bridge is:

    \$Vpp = Vrms * \sqrt{2}\$

In your case it will be about 17,8V, so you cant use 16V capacitor.

Also notice, that transformer open circuit (no load) voltage may be much higher than nominal.

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