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
Define your maximum ripple voltage: ΔV (also called dV, V ripple). Look at image below and I guess you should figure it out.
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).
Image source: wikipedia article mentioned above + me
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.