# Ideal Diode Model - Voltage time Waveform

Joined Jan 14, 2018
3
Hi,

Im just looking for some info regarding a ac rectifying circuit with an reverse biased diode in, and a capacitor that spends the whole time discharging through out the load.

What would the Voltage Time waveform look like? Is the Vmax the Acmax/pi. Or the 0.45 x Root mean square of the AC voltage?

Also the frequency is that the same?

#### MrChips

Joined Oct 2, 2009
30,821
It looks like this:

Joined Jan 14, 2018
3
It looks like this:

Thanks

How Do I calculate the mean Vdc and Peak Vdc then?

If I have the AC peak to peak, and the RMS?

Whats the relationship with the frequency btw?

#### WBahn

Joined Mar 31, 2012
30,075
Assuming that the input is sinusoidal, the AC peak-to-peak also gives you the RMS voltage and vice-versa.

Which to you think it important in this case?

Why don't you spend some time trying to tease out the answers to your own questions and come back with your best thoughts on them. We can then help correct your thinking if it is going off track.

These seem like the kinds of questions that would be asked on a homework assignment. Is it?

Joined Jan 14, 2018
3
Yes it's homework!!!

The peak to peak voltage is 20v. So the Ac Vmax is 10v, and the rms of the ac is 7.07v i think.

So the dc, is that Vdc = 10v / pi? 3.18? Is that the Vmax or the V average? Or is the V peak still 10v?

#### WBahn

Joined Mar 31, 2012
30,075
MOD NOTE: Moved to Homework Help.

#### WBahn

Joined Mar 31, 2012
30,075
Yes it's homework!!!
Moved to Homework Help.

The peak to peak voltage is 20v. So the Ac Vmax is 10v, and the rms of the ac is 7.07v i think.
Why just think? What defines the RMS voltage of a waveform?

What level course is this? Is it for an engineering degree? High school? What level of math it is being taught with? Any calculus?

Answers to these questions will help us discuss the issues at the appropriate level, because this stuff is taught at all kinds of different levels with different expectations on how deeply you will be able to understand and work with the concepts versus just memorizing and regurgitating facts and formulas.

So the dc, is that Vdc = 10v / pi? 3.18? Is that the Vmax or the V average? Or is the V peak still 10v?
Are you talking about the Vdc of the AC input waveform, or of the rectified signal output?

It would be could for you to provide at least a sketch of the circuit you are taking about?

What is your thinking behind asking if Vdc if 10 V / pi?

Forget about formulas. If you want to determine the average voltage for the rectified waveform, then first make it clear that you understand what the concept of an average voltage means? Then sketch where it is, roughly, on your waveform diagram. Then figure out how to at least get an estimate for it based on the information you have, or at least identify what other information you think you need to be able to do so. Present that and then let's discuss.

#### MrAl

Joined Jun 17, 2014
11,494
Hi,

Im just looking for some info regarding a ac rectifying circuit with an reverse biased diode in, and a capacitor that spends the whole time discharging through out the load.

What would the Voltage Time waveform look like? Is the Vmax the Acmax/pi. Or the 0.45 x Root mean square of the AC voltage?

Also the frequency is that the same?

Hello there,

I would also have to ask a few questions to know better what you want to calculate here.

First, are you allowed to use an ideal diode, which has zero resistance when 'on' and infinite resistance when 'off', or do you need to have some model or resistance for the diode when 'on' ?
Second, is the source impedance allowed to be zero or do you need to have some source impedance?
Third, is there a resistive load and if so do you have some spec on that?
Fourth, do you have a capacitor value?
Fifth, what is the line frequency?
Sixth, any inductance?

These questions are important because they change the output wave shape by a large amount. That also changes the average value.

In the real world, when we lighten the load we see the average DC value go up because the capacitor does not discharge as much between half cycles. This is hard to calculate until we go to a technique that calculates each half cycle one by one until we get to some large time value. That is hard to do analytically so i am not sure what kind of solutions you are allowed to use here. For example, are you allowed to use numerical techniques, because that would probably be the simplest with the best results for whatever models and values you choose. There are some approximate techniques too but they are often just assumed to give the right result, and sometimes dont.