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Simple pwm control 2013-01-17

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alec_t

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alec_t submitted a new article:

Simple PWM control - Here's a simple PWM control circuit using inexpensive readily-available components.

The circuit component values shown are for a 12V supply but could be altered for other supply voltages less than 15V.
Most of the work is done by a CD40106 hex Schmitt inverter IC.
U1a/R1/C3 form an oscillator running at ~22kHz. Its square-wave output is integrated by R2/C4 to give an approximation of a triangular wave. This wave is summed at the input of gate U1b with a...[/quote]

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alec_t

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alec_t

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Most Helpful Member
You are more likely to get help if you start your thread inside the appropriate forum; not in an Article.
 

Kevin O'Brien

New Member
Nice circuit. I could not understand this for a while. But I think I get it now, very neat. I am assuming that the power to drive the oscillator is provided by the power within the Schmitt comparator? So at t=0 the output of it is high which starts the oscillations going. Is that right?
 

Gasboss775

Member
I tried to make a PWM circuit using the same basic structure as you, but I must have been off with my component values as I couldn't get the same range of duty cycle that you've achieved. I will try out your circuit soon.
 

Gasboss775

Member
I've had similar results as you with the second circuit, about 10~90% some variation of the operating frequency too.
 

Gasboss775

Member
Today I tried your first circuit, using your exact component values ( all 1% including the two 1n capacitors ) These were my results:

PWM circuit results.png

I can only assume that our differences are due to different thresholds in our 40106's. I think the tolerance of the set points is quite wide. I might mess around with component values again to see if I can do any better.
 

Gasboss775

Member
Just had a look at the CD40106 datasheet, The spread of the trigger points is very wide. For VDD=15 V;


Upper threshold can be from 6.8 to 10.8 V That's +/- 23%!! Lower threshold can be from 4.0 to 7.4, That's +/-30%

So anyone building this may need to play around with the component values. The circuit makes a good led dimmer ( you wouldnt need the output transistors except for the case of power LED's. )
 
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