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What Is Pulse Width Modulation?

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Ozark Sage
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To ALL Above
Ozark Sage   1/27/2012 3:53:24 PM
The site http://en.wikipedia.org/wiki/Modulation I have used for presentations from 6th grade thru advanced university study groups.  It has never left me down and I offer it here in the hope you might all shre it with others and refer to it when term, use and modulation short hands conflict.  This site and other related sites inclure numerous video example applications. 

As AM-FM-PM-PPM-TDM-QAM-SSB-SM-PAM-PWM-PCM and others were used and mixed during WWII on US Navy vessels, and later in CCS-DSSS-FHS-THSS and other spread spectrum aps the world, and technology began to speed up.  I have personally found these techniques some of the most useful tools of invention and advanced engineering of our time.  Your time in exploration of same will be well spent  


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Re: what is PWM
wquiles   1/27/2012 2:57:27 PM
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To expand a little on the subject of PWM signals with regards to an inductive load, I wanted to share how "we" in the flashlight community use PWM and incandescent bulbs to achieve voltage regulation.

In the case of an incandescent bulb, to provide brightness at the right level and color, each bulb has a range of voltage where that particular bulb works best. A fixed voltage regulator, or a bench power supply would supply the right voltage to the bulb, but that is not practical in a hand held/portable device, where we usually have batteries as a power source. Not only that, but a traditional voltage source might not deal too well with the high inrush current that results from a cold filament being hit with its rated voltage (notice how incandescent bulbs in our house only die when we first turn them ON).

So with a simple/small micro controller (something like Tiny85) you can output a PWM signal to a MOSFET, therefore turning ON/OFF the bulb at a specific duty cycle, which will result in the bulb seeing an average voltage – a RMS voltage. By measuring the voltage across the bulb with the Tiny85's ADC, you can regulate voltage to the bulb (by adjusting the PWM's duty cycle) as the battery drains. So instead of having a bright/beautiful bulb when the pack is freshly charged and an ugly/yellow beam then the pack discharges, with PWM voltage regulation you can have a constant output throughout the cells discharge cycle. The efficiency is very high (very little loss on the MOSFET), and since you are measuring the voltage in real time, you can:

  • monitor the cell's voltage and either "signal" the end user about the cell's status, and/or shutdown in order to prevent over-discharging.

  • the final "bonus" in this approach is that you can also implement a software-based soft-start so that during the first 50-100mS, the bulb gets a slowly increasing voltage, therefore greatly extending the life of the bulb as the cold filament gets a little bit of extra time to warm up.

My implementation uses a Tiny85, but it was an engineer from SureFire (Willie Hunt) who came up with the original idea we use today:




sensor pro
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Re: Practical PWM info
sensor pro   1/27/2012 1:01:24 PM
Very nice site. Clear and to the point. It is a great lab projct for students learning Micro and electronics.


Curt Carpenter
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Re: what is PWM
Curt Carpenter   1/27/2012 12:50:15 PM
This certainly isn't intended as a PWM tutorial for goodness sake.  The point, it seems to me, is in the last sentence.  Much can be gained by remembering and revisiting the fundamental analytical (and not overly complex) models that underly the scheme.  We've been reminded that PWM has such models, easily accessible to us as engineers with that education -- and that we shouldn't forget to use them:  a good point, I think!

Jon Titus
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Practical PWM info
Jon Titus   1/27/2012 12:49:30 PM
Here's a link to a paper I wrote to help kids and teachers understand how to use PWM to dim an LED.  It includes code for a PIC16G690 microcontroller.  http://igen.eetimes.com/tutorials/Vary_LED_Brightness.pdf.

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Useful lesson ... Not!
brett_cgb   1/27/2012 11:43:57 AM
The original article barely describes one type of PWM and why it's so usefull. You certainly wouldn't implement a PWM system based on on just this article.


sensor pro
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Re: what is PWM
sensor pro   1/26/2012 10:49:10 PM
Dear Alexander, If you are looking for practical examples where the PWM is used, I can refer you to a very simple and practical example of a pure industrial use of PWM in measurement of bi-directional tilt. The product is EZ-TILT-5000 at www.aositilt.com and it is a dual axis angle measurement inclinometer. In addition to standard RS232 and analog outputs, it has two PWM channels that output tilt information The scheme is that at mid point of tilt (horizontal) the unit output a signal with 50% duty cycle. Then the signal will change from 10 to 90 percent of duty cycle from the most negative tilt angle to the most positive tilt angle. The output frequency is constant at about 37Hz. Very useful for industrial applications

If more examples are needed, I will be happy to provide.

Alexander Wolfe
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Re: what is PWM
Alexander Wolfe   1/26/2012 6:03:58 PM
There are so many different modulation schemes, but usually they're talking about in the context of encoding information on an rf signal (i.e., radio transmission.) Here, Kevin is talking about applicability to baseband applications, such as power supply. So, yes, it's definitely interesting, and I'd be interested to learn what the full range of modulation schemes of utility in the manufacturing and automation spheres includes. I'm mostly familar with the radio stuff, and I don't suppose QAM is something that what Kevin's talking about would include.

sensor pro
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what is PWM
sensor pro   1/26/2012 10:56:24 AM
Very informative and clearly written article. Thank you.

In my experience I used PWM to drive Peltier coolers and had great results in keeping Avalanch phodo diodes at about -50-- -60C Worked well. I also used it as an output of a tilt monitor to provide tilt infor as a duty cycle, however forund that industry was not very receptive to it. They liked regular DC proportional to tilt or RS-232/422/485  etc...


Thank for the great info.

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