Supply-voltage controlled PWM (NE555)

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Hallo,

im trying to develop a voltage controlled PWM signal with the NE555 chip, instead of my AVR. In the datasheet (http://elektronik.kai-uwe-schmid... ), on page 8, is the necessary circuit for this purpose. My problem is, that I want to use the supply voltage, to controll the PWM signal -> about 10..14V supply voltage should be the range for the max/min signal width. Has got anybody an idee, how to modify the ciruit?

thnx
Zoltan

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The enclosed circuit is one such idea.

The PNP can be any small signal PNP; the 2N2907 is a good start.

The diodes can be any small signal type, such as 1N916, 1N4148.

Input signals above 10 volts will cause current to flow in the collector, and that will raise the control voltage on the NE555. The small the resistor, the greater the sensitivity. Start with about 6.8k.

I hope this gives you some ideas to start with.

RickF

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Regulate the 'modulation' (control voltage) input to a fixed voltage. The best voltage is between 5.0 and 6.7; a small zener will do the job.

As the VCC changes, the threshholds will remain the same while the charging current changes. This will modulate the puse width.

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Thanks for the answers, i will test both suggestions.

Meanwile i have tested a citcuit, its similar to mike-s idee. It works now fine, but i didn't expected a so strong temperature drift of the NE555 (see picture above). As per datasheet the temperature stability should be of 0.005% per °C. The Pulslenght (with ~2kHz) varies here in about 1% per °C! (For the temperature testing I used my table lamp, covering only the ne555, to ensure that the temp. drift is coused by the NE555) Is that tmeperatur drift normal?

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Hi,

You are probably heating the capacitor and resistor as well, which will have a high tempco unless you get more expensive ones.

I assume you just grabbed them out of the parts bin... which are usually the not-so-great ones!

-Colin

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Hi Colin,

you are right, the capacitor has a large tem. drift, and this capacitor "creates" the PWM signal. I think, I will also add a temp sensor to the circuit, to compensate the temp drift. Its seems the easiest way to me.

Zoltan

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It would be much better to use a low TC capacitor. In general, life is much less complicated to not make a problem in the first place than to try and compensate for it. In this case, it would be much, much easier.

Low TC capacitors aren't hard to find.

Rick

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By TC do you mean tantalum capacitor?

-- Bill

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T= [T]emperature [C]oefficient = capacitance drift over temperature

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@farang

Thank you for the advice. I will take a look at these "low TC capacitors".

Zoltan