Measuring Pulse width modulation PWM

[Russlk]

That does not download for me.

Walters: The graticle on the scope is typically 1cm lines. The sweep knob is calibrated in time/cm. Do you have a problem understanding what duty cycle, rise time and pulse width are?

 

[walters]

problem understanding what duty cycle, rise time and pulse width are?

Yes because i don't know how they look on the oscilloscope what does

Whats does Duty Cycle look like on the oscilloscope and how do i measure it on the oscilloscope?

Whats does Time constant look like on the oscilloscope and how do i measure it on the oscilloscope?

Whats does rise time and fall time look like on the oscilloscope and how do i measure it on the oscilloscope?

Whats does pulse look like on the oscilloscope and how do i measure it on the oscilloscope?

 

[Russlk]

Perhaps this will help.

 

[walters]

Thanks alot Russlk for the picture

Is rise time the same as the Time constants?

How do i count the gradules and do the formulas to get the values?

Because the picture shows what it looks but how do you get the
values for this stuff? thats the hard part i don't get

 

[Russlk]

Rise time is the time from 10% to 90% of the rising edge. Time constant is the time to go from 0 to 63.2% of the rising edge. No one goes to 100% because that theoretically takes forever.

The squares are 1 cm and there are 5 graduations in each cm. If the time base is 1 uS per cm, each graduation is 0.2 uS. Just count them.

 

[walters]

thanks alot for you helping me

This is the hard part for me is the counting them

The squares are 1 cm and there are 5 graduations in each cm. If the time base is 1 uS per cm, each graduation is 0.2 uS. Just count them.


Each Sqare is 1cm but when you change the sweep time it changes the
1cm gradules value this is the hard part for me

Each Square is 1cm but when your change the sensentive know it changes
the 1cm gradules to another value

and have to do some multiply i think ?

 

[Russlk]

Yes, you need to get a 5th or 6th grade math book and study it.

 

[walters]

Russlk what am i doing wrong how do i do this right?

 

[Nigel Goodwin]

It's simple school maths!.

An oscilloscope is nothing more than a piece of graph paper!.

It displays a graph of voltage (y axix) against time (x axis).

The y axis is calibrated it volts per division, and can be read from the input attenuator control, and the x axis is in milliseconds per division, and can be read from the timebase control. Obviously different settings of the controls move from volts to millivolts, and from milliseconds to microseconds etc.

You should have learnt to read (and draw) a graph at school, reading a scope is not really any different.

 

[walters]

Yea but when you have a waveform on the scope and what to
measure duty cycle,time constant, etc
you have to know WHERE to count at thats my problem where

 

[Nigel Goodwin]

Yea but when you have a waveform on the scope and what to
measure duty cycle,time constant, etc
you have to know WHERE to count at thats my problem where

In exactly the same place as you would on a piece of graph paper?.

If you mean you don't know where on the waveform to count from?, that depends entirely on the waveshape - for a start it needs to be a repetitive shape! - simply look for the repetitions. As an example, a sinewave will 'start' at zero, go high, back through zero, go low, and back to zero again - that's one cycle.

 

[walters]

This is what im really really looking for

Voltage = distance in cm × volts/cm
Example: peak-peak voltage = 4.2cm × 2V/cm = 8.4V
amplitude (peak voltage) = ½ × peak-peak voltage = 4.2V

Time = distance in cm × time/cm
Example: time period = 4.0cm × 5ms/cm = 20ms
and frequency = 1/time period = 1/20ms = 50Hz

Can your please give me examples from these formulas i need please

1.) Pulse width Modulation PWM?

Pulse widith Modulation= distance in cm X time/cm?
example?

2.) Duty Cycle?

Duty Cycle = distance in cm X time/cm?
example?

3.) Pulse width?

Pulse width = distance in cm X time/cm?
example?

4.) Time constant?

Time constant= distance in cm X time/cm?
example?

5.) Rise time and Fall time?

Rise time= distance in cm X time/cm?
example?


Fall time= distance in cm X time/cm?
example?

 

[Styx]

sod that you are over-complicating things!!

majority of scopes are 10big squares across and 8big squares high.

These squares may or may not be 1cm I dont really care, but they are called devisions ~(with each devision having 5 graticles to each square)


so if i set it up to be 5volts per div and 10us per div

so if I have a PWM squarewave that has a period of 7 squares I know the period is 70us (sod the cm measuring stuff)

 

[Russlk]

I think Walters is pulling our collective legs.

 

[Styx]

I think Walters is pulling our collective legs.

if he is, he is a very bad man...

 

[walters]

No im not pulling your legs i need help with this stuff

 

[Nigel Goodwin]

No im not pulling your legs i need help with this stuff

It's all very basic stuff, you need to learn electronics from the beginning, not try to jump in past your current abilities - most of the questions you ask should be addressed at school in maths and science.

 

[walters]

I just need those oscilloscope formulas and i think i got what im looking
for those oscilloscope formulas i need help me please

 

[Styx]

They are not really formula's, you just have to know what you are looking for and the definition of stuff

ie "rise-time"
the time for a voltage signal to chage from 10% to 90%

You just change the timebase to show this with enough accuracy and then just measure then time and then measure the voltage change. Since risetime/slewrate is just dv/dt it's just that

I think the real problems are as I stated and Nigel stated

1) lack of experience with a scope - not a problem we all started somewhere
2) lack of understanding, as Nigel stated.


The lack of understanding is the real killer hear I am afraid. Having a list of so-called formulas will just complicate matters, understand what you are trying to do and get use to the scope (all scope's are different) and it will all fall into place

 

[walters]

you just have to know what you are looking for and the definition of stuff

Yes this is my problem how do i know what i am looking for?

What is the definition of this stuff i listed please?