Pwm -556 1u3q27

PWM Generators

1 of 2

http://www.m.case.btinternet.co.uk/html/pwm_generators.html

I have been asked by a few people about other methods of producing PWM with other IC’s such as 555 timers and such like. There are of course disadvantages of making your own PWM signal that are easily overcome by using a dedicated IC the main one being current control. The way I see it, the best way to reduce the current flow in the output stage is to reduce the PWM duty cycle rather than to turn it off completely. The effect that this will give you is maximum power and control of your Robot at all times rather than it shutting off then powering up again every time the current limiter kicks in! (and of course when it does kick in it will be at a time when you need most control!!) Anyway I have scoured the text books and have found this circuit. It uses a 556 timer (which is just 2 555’s in one package) that runs from +15v, the DC voltage required as an input in from 0v to 12.5v and will give a PWM output at 80Hz with a duty between 0% and 98%. A max duty of 98% is OK but 100% is better as the output transistors can get quite warm if you require full power for long periods (not very likely though)

Now I have never built this circuit so I cant guarantee that it works good enough for our use, so if you do build it let me know what the PWM wave from looks like so I can post it here! The only component that may be a problem is D1, it is listed as a 1N5297 and is a 1mA current regulator so it can be replaced with anything else that will regulate 1mA to pin 1&2 of the IC. The next circuit I found is actually a concoction of 2 separate designs that cunningly come together to form a 0 - 100% PWM generator. The first stage uses a single 555 timer to generate a constant saw tooth waveform.

Yes I know that we don’t want a saw tooth but it can be easily converted to PWM. If you look at the saw tooth you can see that it is a wonky triangle, but wonkyness aside it has a fat bottom and a thin top (just like most triangles). So lets say we want a 50% duty cycle from this triangle, well if you look half way up this triangle then you will see it has approx 50% of its width half way up, so if we had a device that will chop off the top of the triangle (and the beginning of the ramp) then we would have a square wave with approx 50% duty! The same goes if we wanted 10% (go higher up the triangle) or for 75% go lower down the triangle. The device that can do all of this chopping is an Op-Amp. If we connect the Op-Amp up as a comparitor then it will compare the triangle with a set voltage, if we adjust the set voltage then what we are actually doing is setting the height at which the Op-Amp ‘chopps’ the triangle giving us the desired PWM.

25/06/2008 9:02 PM

PWM Generators

2 of 2

http://www.m.case.btinternet.co.uk/html/pwm_generators.html

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25/06/2008 9:02 PM