# Confusion in deriving ripple voltage for an unregulated power supply

Above is a full wave rectifier as an unregulated power supply

In many texts and tutorials the ripple voltage(|AX| in above figure) which is

deltaV = I / (2*f*C)

I think it comes from the fact that

C * dV/dt = I so deltaV = (I/C) * delta_t;

and according to texts since delta_t is T/2 = 1/2*f

then deltaV = I/(2 * f* C) where I is the load current.

I can understand that derivation but if you look at my image above, according to this equation the real delta_t is not T/2 but instead less than that. Hence the ripple is |AX| not |YC|. But the tutorials take delta_t as T/2 where it starts from the point A to point C. But actually delta_t starts at point A and ends at point B.

Im confused at this point. What is the real ripple Voltage here?

• You are correct, but you're missing the point. Nobody cares if ripple voltage is off by 10%. The book approach gives a simple, robust method of getting close enough using methods which are appropriate to the skill level of the students at that point in their education. Commented Sep 21, 2016 at 13:55
• Since this is your 384-th question, how come you didn't learn how to format formulas yet? Commented Sep 21, 2016 at 13:57
• @WhatRoughBeast where does %10 come from? Commented Sep 21, 2016 at 14:05
• When in doubt, use orders of magnitude (powers of 10). 1/10 is 10%. The point is that the book gives a simple, easily-comprehensible answer which the student (you) can understand. As such, it is an example of lies-to-children, and a very good one.If you expect your introductory circuits class to make you an expert, high-precision circuit designer, I'm afraid you're in for a big disappointment. Commented Sep 21, 2016 at 14:10

Here's a picture from a different web source that gives the same answer but, notably with small print at the bottom: -

So it's an approximation and it doesn't take account of diode drops either: -

What is the real ripple Voltage here?

This picture is better because it shows the forward volt drops in the diodes but it's still not exact - how far do you want to go getting to the real answer. Do you want to consider any of these: -

• Non-linear resistance of the "real" load
• Real load effects such as pulses of current it might take
• Self-discharge of electrolytic
• Transformer secondary leakage inductance and resistance
• Volt drops in wires

It's just an approximation at the end of the day and at the point when the formula becomes grossly inaccurate it's probably a good idea to add more capacitance to the bridge output to reduce ripple voltage anyway!