> From:         Russell McMahon[SMTP:apptech@PARADISE.NET.NZ]
> Sent:         Friday, July 09, 2004 9:12 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE:] Design Challenge - Lowest cost PWM

>> You can use a 556 or a quad opamp.
>> Google works for this, really.

>I'm aware of Google :-)

> And the 324, which is what I'm using in my actually working circuit that I
> mentioned, is a quad op amp - horrible results - but could be good enough.

> As I noted, I'm aware of 555, 556 and similar. The issue is turning them
> into a working solution at fair price. Linearity tends to suffer where you
> are using exponential cap charge - but may be OK. Getting a full 0%-100% PWM
> range is often more challenging. I can play with them myself (and yet may) -
> i was interested in seeing if anyone popped up with a working superior
> solution rather than just the skeleton ideas.

Linearize the 555 output by charging a capacitor through a constant current
diode (or use a JFET with its gate connected to its source). Put a normal
diode in parallel with the constant current diode so that the capacitor
discharges rapidly when the 555 output goes low. The resulting voltage on
the capacitor will be very linear. Use an LM311 comparator to compare this
voltage with the input voltage.


> So far the LPC901 seems the most promising processor based solution. I like
> my 4060/339 idea for a non-processor alternative 0 the digital ramp would
> overcome the cheap opamp setup problems.

>        Russell McMahon

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