Clarifications on the PWM signal

[Harald Kapp]

You can get PWM out of AC, if you put a capacitor in the line, you get different amounts of on-off time if u change the capacitance, but the wave isnt a perfect square, its a discharge curve. (a quarter of a harmonic)

I'd love to see a schematic of the circuit that would perform as you say. I strongly doubt this would work as advertised.
You can't solve all your electronic problems by adding capacitors - your favorite component it seems.

 

[Harald Kapp]

when the pwm is high the voltage decreases and when it is low the voltage increases

What do you mean by "pwm is high" and "pwm is low"? A pwm signal is characterized by:

• amplitude of the signal ("on" voltage)
• frequency of the signal
• duty cycle

It helps to use the right terms to arrive at a common understanding.

i put a scope at the point as per post #4 i can only see the pwm signals, i cannot actually see the average voltage applied to the resistor, it is only based on calculations.

That is to be expected.

Does this voltage apply at the end of PWM cycle as shown at the End of Region1 or during the Region1?

Average or rms values always are to be taken with respect to a full period of the signal. In your image this is region 1.

what is the reason for calculating the rms value of pwm signal,

The rms value of any signal, not only sinusoidal or pwm, is by definition the value that would create the same power dissipation as a DC signal of the same value as the rms value. In the case of a pwm signal the usefulness of the rms value depends on the application:

• use pwm to control the speed of a motor, then the rms is an indicator of the power available to the motor.
• use pwm to drive an oven, then the rms is an dindicator of the heat the oven will be able to generate.
• use pwm to drive lighting, then the rms is an indicator of the brightness that can be achieved.

 

[ratstar]

I'd love to see a schematic of the circuit that would perform as you say. I strongly doubt this would work as advertised.
You can't solve all your electronic problems by adding capacitors - your favorite component it seems.

I love capacitors, yes.

The thing I said, I learnt it off applied science, where you can put a capacitor in the line of ac, and then it reduces amps like a resistor, except with a capacitor.

If you did it with a variable capacitor, it would have to be really high frequency PWM, cause the farads are so low on them.

 

[Harald Kapp]

This method uses the reactive impedance of the capacitor to limit the current. The capacitor acts in a way similar to a resistor in this application (similar, not equal, be aware). This has nothing at all to do with pmw!

 

[ratstar]

This method uses the reactive impedance of the capacitor to limit the current. The capacitor acts in a way similar to a resistor in this application (similar, not equal, be aware). This has nothing at all to do with pmw!

Its putting out pulses, the same, I think its similar anyhow...

 

[Harald Kapp]

Its putting out pulses,

No, it doesn't. The capacitor shifts the phase between voltage and current, thus creating an impedance that limits the current. It is not similar in any way. Learn how it works.

 

[ratstar]

Oh I might be mistaken.

 

[electronicsLearner77]

This is my understanding of how the pwm will effect the output loads

Sorry for not drawing the waveform properly and if any mistakes. Is my understanding correct?

 

[Harald Kapp]

It will be +3.75 V and +5 V not "-".
Also the voltage will not be constant. The voltage will be pulsed like in the upper trace. The values you wrote will be the effective voltage.

Note that for some applications there will be a difference between applying a pwm signal of an equivalent DC signal. An electric motor for example will develop more torque from a 50 % pwm signal than from an equivalent DC voltage (in your example 2.5 V). This is due to the fact that during the on time a higher voltage is applied resulting in more torque during this period. See e.g. here for more detail.​

 

[electronicsLearner77]

It will be +3.75 V and +5 V not "-".

Sorry my mistake i meant only + value.

The values you wrote will be the effective voltage.

Ok, from now on for AC signals i only talk about RMS values as it will help in calculation of power dissipation. Is it ok?
I am trying to understand more about PWM as i am studying about space vector modulation and i could not follow some of the concepts. Can i ask them, since i need to write a detailed mail explaining my queries?

 

[Harald Kapp]

Can i ask them,

yes

i need to write a detailed mail

no mails, please. Use the forum and open a new thread for each topic..

 

[electronicsLearner77]

yes

no mails, please. Use the forum and open a new thread for each topic..

Ok, i did not mean mail, it is forum.

 

[ratstar]

Its easy!!!

PWM is switching hi-voltage on and off to make for less total current, A resistor, is bottlenecking the electricity down a small pipe, to make for less total current.

Two ways to do it.

 

[Audioguru]

Its easy!!!

PWM is switching hi-voltage on and off to make for less total current, A resistor, is bottlenecking the electricity down a small pipe, to make for less total current.

Two ways to do it.

Do you know why PWM is used to reduce the current instead of using a resistor?

 

[ratstar]

I know its handy to have an output without a resistor on it, its good for a power supply, for one thing. other than that I dont know exactly.

 

[Harald Kapp]

other than that I dont know exactly.

So don't speculate, read the available info and learn.

 

[ratstar]

Do you know why PWM is used to reduce the current instead of using a resistor?

Then why?

 

[Harald Kapp]

Read this.

 

[ratstar]

It would be cool to do your analogue signals this way because it would be universably repairable analogue information (once analogue info is lost, without knowledge of what the data was representing its impossible to get the loss back.), but ud have to sacrifice hz to do it.

 

[Nanren888]

If you mean you can encode information, including sampled analogue level as pulse width, then yes.