I have been using a formula to compute RMS voltage for full wave

rectified and filtered power for eons. I don't know where it came from,

or if it is even accurate, but the numbers I get are usually in the ball

park.

It seems to work great if the input capacitor is sufficiently large that

the ripple voltage is small. But if you make the capacitor to small the

numbers are negative, and hence not valid. Even if you have no capacitor

there is an RMS DC voltage. I googled and googled and could not find a

better formula. Does anyone have one?

What I am using is.

F=Frequency (120hz in us)

C = capacitor value

V = Peek Voltage

R = Load Resistor

Ripple Voltage = (F^-1 / (2*SQRT(3)*V) ) / (RC)

VDC = (1-((2F)^-1 / (RC))) * V

This works great for large caps and/or light loads but is not accurate

for small caps or large loads (you get negative numbers)

Obviously I need a better formula. Can anyone help?

Thanx

Hawker

If you have access to a computer while you're doing your calcs, I'd

recommend using Spice to look at the problem. LTSpice/SwitcherCAD is

free, and easily up to the task.

If you insist on calculating things "by hand," then consider that

there are two time periods: the first is when the capacitor is

charging from the source, and the second is when it is discharging

through the resistive load. The first will be close to a segment of a

sinewave, offset by the voltage drop in the rectifier diodes, which

can be significant, and also modified by the drop in the resistance of

the source (transformer winding resistance, mainly)--which should be

fairly small and often is ignored. The second is an exponential

decay, determined just by C and R. One problem is that it's common to

assume the exponential decay starts at the peak voltage, but with

small capacitance, that's not true, and not even a good

approximation. The output voltage will follow the sinusoid beyond the

peak voltage. The point at which the circuit transitions from the

first region to the second is when the negative slope of the sinusoid

becomes greater than the slope of the exponential decay: V/RC.

You can put all that together and come up with a pretty accurate

estimate of the waveform, and find the average DC from that, but it's

a whole lot easier to just let Spice do it for you. Then it's easy to

add in transformer winding resistance, leakage inductance, the

inductance of filter inductors you specifically add, etc.

Cheers,

Tom