AC / DC voltage drops across Diode

Zerox10 · Mar 21, 2022

I am unable to find voltage drop of normal silicon diode(0.7v) on Voltmeters during Proteus Simulation. Also we consider 0.7v drop of diode equivalent to DC Source of 0.7v but DC voltmeter is not showing any output and AC Voltmeter shows around 5 volts drop across diode instead of 0.7V.Kindly help me in this confusion.

[ElectriciansForums.net] AC / DC voltage drops across Diode

marconi · Mar 22, 2022

Greeting from London.

The questions you ask would require some understanding of waveform analysis and also how electrical meters measure quantities like voltage and current and then display a number related to these measurements.

By way of some insight, if you measured the voltage produced by a sensor measuring ambient temperature during a 24 hour period and then plotted it against time you might see something like what I have drawn in the attachment. If I asked you 'what the temperature(voltage) was?' you would immediately ask me 'when?'. Or, if I asked you 'what was the average temperature(voltage)', you would have to ask me 'over what period?'.

The same sought of thinking applies to the voltmeters and what they display in the Proteus circuit you have drawn. What does the number on the Proteus voltmeter actually mean or represent? Is it an average value over some period or is it an instantaneous value or is it something else?

Generally, electrical meters like voltmeters and ammeters measure and display some sort of average value over some particular time. Since most commonly the waveform for the voltage or current is sinusoidal in alternating current circuits or a direct current by which I mean a current which only flows in one direction, then the meters are designed to show a value which is either instantaneous or 'the rms value - the square root of the time averaged (aka mean) of the instantaneous square values over some definite time period T'. It so happens that the rms value of an ac waveform is the equivalent constant dc current/voltage which delivers the same power. The rms value is not the same as the simple time averaged(mean) value. The ac electric meters in your proteous example will 'assume' you want to measure and then display the rms or equivalent dc value of the waveform applied across their terminals. Here is some analysis of rms and average values of some standard waveforms:

I think you may be attempting to 'run before you can walk'.

Take a look at this youtube clip on Proteous and instead of using a sinewave excitation voltage use a dc sweep as it does going positive and then negative. This will give you a picture of the actual waveform of current which flows for the positive polarity case and then the negative polarity case. You will see that the waveforms are not exactly the same as the excitation waveforms. After using a dc sweep try a square wave excitation voltage and then a sinusoidal one. Next, for these three cases add a resistor in series with the diode. Then we can talk again if you wish.