Simple PWM Generator

[NC Cams]

SG3625 was a Silicon General/Linfinity number. LM3526 was National and LT3526 is a Linear Technology number.

The 3524 verion is pretty much the stripped down, universal use fits-all model. It has the smallest pin count and the bare minimum to create a PWM power supply (error amp, shutdown, O/C drivers, that's about it).

The 3526 adds pulse by pulse current limit, another op amp and more robust, direct drive capable totem pole drivers.

There are several variations of tech manuals available for the IC"s and some are easier to read/understand than the others, especially for the neophyte electronics tinkerer.

The nice part about the 3526 is that you can run it on 10vdc and drive logic level fets directly from it. Yes, you should use a fet driver but, for low to moderate current (3-5 amps/fet to a single IRLZ44 per pole), they seem to become enhanced adequately using the totem poles themselves. We used the 3526 in some pretty abusive hobby applications and NEVER lost an IC although we fried all sorts of support surrounding IC's from abuse.

All things consdered, creating a PWM from discretes is complexity in search of a need once you learn how to use the PWM IC's mentioned.

[epineh]

Well I have opted for a 8038CCPD - Precision Waveform Generator

As purchasing electroinics can be a little painful here I searched through a supplier I use often and found the above chip, it also generates sine and sawtooth waveforms so it should be a handy little chip for testing, and for five bucks I can live with the cost

Russell.

[NC Cams]

You can do the same as as a/the wave form generator much cheaper albeit with a few more parts - mostly resistors and capacitors.

Take a cheap LM324 and use a stage to create an multivibrator/oscillator circuit - this stage also establishes your pulse frequency.

Feed than signal into another stage of the 324 and turn it into a triangle wave.

Feed that into a third stage of 324 which acts as a comparator. Using a simple divider circuit on one input and the triange on the other, you now affect duty cycle.

The fourth stage can be used to sense current and shut down any signal deemed appropriate to limit current.

The oscillator only needs a couple of resistors and diodes and the circuit for the oscillator/comparator/triangle generator is easily found in the National Semiconductor application notes library - the method for calcing the resitors/caps to set frequency are there as well.

A PWM for MUCH MUCH less than $5.

We used this simple scheme to make PWM train throttles and numerious other PWM circuits until we learned about the dedicated PWM's mentioned earlier. The PWM take way less parts albeit are/may be a bit more expensive.

[Mickster]

Don't know if this is any help to you Russell, but have a mooch around here:

http://www.cpemma.co.uk/pwm.html

Regards,

Mick.

[samco]

you can actually use emc2 HAL (hardware abstraction layar) to do some cool stuff like that. It has a fregen module which does sin/cos/square and such. this can be run into the pwmgen module to create pwm/pdm ouput. Or you could set the input to the pwmgen manually say 1 is 100% pwm and 0 is 0%pwm.

If you look at emc's Hal manual - it gives some good examples.

sam

[NC Cams]

Thanks to the MICKSTER - the word picture PWM outlined in post #15 is linked and "how to build it" in his post - the selection of the ancillary resistors and caps around the 324 set the pulse frequency easily and cheaply.

Model trains and other DC motors work nicely at 3Khx as do most DC circuits that will accept PWM'd current.

Get too high a pulse frequency and too low a pole count on the motor and funny things start to happen with the motor (buzzing, cogging and other bizarre issues).

[Mickster]

Thanks NC,

I actually had to build one of these circuits, to convince a previous boss that what we recorded on the 'scope, on a trouble-shooting exercise, was NOT what we should have seen.

Once he had seen what he SHOULD have seen, the part was changed and the vehicle went out the doors with happy faces all round. Sadly, due to many management and employee differences in regard to trouble-shooting methods, I ended up severing ties with that particular company.

I am now looking at a career change and am quite happily learning all things CNC and woodworking in general.

"If you love doing 'It', do 'It' until the B@stards wear you down, then find a new 'It'... Ad Infinitum.

[NC Cams]

It is simply amazing how dense some so-called intelligent people can be/get - especially when you can show current flow on a scope wave form. If something in the way of an errant wave form is or is not doing what it's supposed to, the damn thing typically won't work right.

And yet, people will argue "that can't be" when Helen Keller can clearly see that a circuit ISN"T acting RIGHT. DUH.

When you do find a "home" where you can do your thing and get the respect you deserve, you've reached a special platteau - nirvannah to some respect.

IF you find yourself continually fighting with people who are ignorant and/or poorly educated or worse yet, stupid (ignorance is fixable, stupidity is engrained), you just might as well move on if stupidity is rampant - life is too short to live with that much un-needed frustration.

You might not make as much elsewheres, but, as I'm learning from rowing my own boat, it is just much more satisfying doing things to satisfy yourself and to make your clients happy/satisfied with the work you do complete successfully for them.

All in all, the LM 324 pwm is pretty cheap and easy but the LM352x series PWM's take up much less board space and offer a bit more in the way of SMPS prortection that you have to conjure up separately when you go the discrete route. All in all, the PWM IC's are pretty handy to know about and implement all things considered and relatively cheap considering what they offer.

[epineh]

Thanks for the input guys, the 324 circuit looks like a winner, makes me wish I didn't order the waveform generator now, ah well for five bucks I think I can wear the cost, though in the week or two it takes to get the chip, I can fab a small board and I found some 324's in the junk pile at work so I may have a crack at it while I wait.

Sam, my PC with EMC on is now dead but the HAL stuff is definately worth a shot, I really like the built in scope and meter objects, will come in handy if I ever ressurect the electronics

Russell.

[epineh]

The only thing needed is to convert the 12volt waveform to a 5volt logic level, I am thinking of using a simple resistor divider network, is this good enough or should I also add a zener to "clamp" the 5v level ?

Russell.