PWM- what does it control?

[shortbus=]

this will be a dumb question to those of you that know, but after Googling and reading all the information in my modest electronics library, i can't seem to find the answer.

what does PWM control, volts or amps? most of the references say power, so i assume it changes both? would someone please explain what or why it does what it does.

thanks, cary

 

[MikeMl]

You question is just the same as asking " If I adjust the knob on my bench power supply, do I control volts, amps or power?"

The answer depends on the load. PWM is a method of controlling the AVERAGE (effective) volts, current, or power delivered to some load.

 

[MrAl]

PWM controls what you want it to control. What is actually controlled depends on the
feedback system, not on PWM itself.
Normally PWM switches a voltage source on and off and that way it controls the
current flowing from the input to the output, but i wouldnt limit it to that type of
operation.
PWM basically interrupts the flow of current so that the output can be controlled
to some preset value.

For signal generation, PWM is used to vary the signal level so that the output
follows the input near perfectly, except for being chopped up which is later
usually filtered.

The difference between PWM and pure analog is that PWM can often be done using
very little power for complex signals whereas analog would consume lots of power.
Amplifiers built using this technique can produce very clean sine waves while using
much less power than an analog circuit would use.

PWM, pulse width modulation, varies the width of the pulse so that the amplitude can
be controlled. When a low level signal is to be output, a very short pulse width is
generated, but when a high level signal is to be output, a comparatively long pulse width
is generated. The output filter filters these pulses into a smoothly varying wave that
is much like the original input.

 

[MikeMl]

Mr AL, you seem to have expanded the definition of PWM to include feedback control systems. I tend to think of the feedback controller as separate from the technique of switching some load on/off at a rate high enough so that the natural damping of the load itself acts as a low pass filter (Pulse Width Modulation). Open-loop PWM is a useful technique, for example lamp dimming (no feed back at all).

 

[electronspeed]

let me give you an eg. you have two motors with different voltage requirement.Now you need to reduce the voltage for the 1 which req less voltage.For doing this we have two options either use a voltage divider or a PWM.Certainly voltage divider will be less preferred due to more power dissipation.So we go for a PWM.Hence evidently we can modify the pulse supplied.

 

[MrAl]

Mr AL, you seem to have expanded the definition of PWM to include feedback control systems. I tend to think of the feedback controller as separate from the technique of switching some load on/off at a rate high enough so that the natural damping of the load itself acts as a low pass filter (Pulse Width Modulation). Open-loop PWM is a useful technique, for example lamp dimming (no feed back at all).

Hello again,


I didnt mean to imply that you could only use PWM with feedback, just
that many systems do use feedback.
There are also LED dimmers that do not use feedback for example.

 

[shortbus=]

thank you for your replies guys. so it does control both volts and amps.

i have a project that has a 100VDC at 20A supply being pulsed
(5KHz to 40Khz) variable duty cycle of 5% to 95%, into a very small spark gap(a purely resistive load at approx. 1.5Ω) gap is 0.001 to 0.002 inch.

i need the 100VDC to ionize the the gap to get the spark to "jump" the gap then the working volts drop to approx. 35VDC to transfer the current. i don't always need or want 20amps and was looking for a way to drop the amperage to what ever i need.

at this point i have been ready to use "dropping resistors" to set different amperage limits. i thought there might be a different way to do this such as pwm. do any of you know of a way to keep the voltage level and limit the amps? say 5, 10, 15, or the full 20 as needed?

i would really like any help anyone can give me on this! cary

 

[Thunderchild]

how about increasing the gap !

 

[shortbus=]

how about increasing the gap !

that is one way i guess. the switched dropping resistors is another way too.

this circuit is for a pulsed EDM machine, kind of like a ark welder in reverse. instead of adding metal to a work piece' it removes metal in what ever shape the electrode for the spark gap is.

controlling the amperage in one of these machines has been done electronically since the late 1970's. i just can't find out how it's done!

as soon as i mention making small sparks my threads die! people try not to have sparks in there projects, but a EDM is designed from the start to do this!

cary

 

[Speakerguy]

So your spark repetition rate is 5KHz to 50KHz? How long is the spark duration?

I have some experience with this stuff, I think I might actually be able to help

 

[shortbus=]

So your spark repetition rate is 5KHz to 50KHz? How long is the spark duration?

I have some experience with this stuff, I think I might actually be able to help

WOW!!!! finally some one that understands!!

the duration is the duty cycle, it will be varied ti suit the amperage and to keep the discharges consistent.

i was a die maker before retiring, ran a edm for 13 of my 40 years of my trade. been working at building a home shop size EDM(sinker) for about 6 years. got all the mechanical parts built but don't know that much about electronics.

any help you can give me about current(amperage) control is greatly appreciated!! cary