Re: [ATM] Looking for the best PWM cycle time for dew heaters

["Lawrence D. Lopez" <lopez@mv.mv.com>]

Allen:

To actually answer your question, I expect that 1 Hz is fine.

But:

The usual N channel mosfet running from the top rail
needs a power supply above the top rail in order to turn on
fully.   This needs to be accommodated.

This occurs because people usually ground the bottom of the
dew zapper element and use the mosfet as a source follower.
If you put the dew zapper from the + rail
to the mosfet the problem does not exist.

Second if it is merely a pulse width modulator you need
to accommodate the hash/noise that might result from switching
the current.

The absolute best to me seems to be to merely make a switching power
supply as it can have very little ripple in the input and output.

If you take a  PWM and filter it's input and output you actually
get a switching power supply.

What I did is I used this:

http://www.astro.uwo.ca/~rasc/HEATER.pdf

But the I discovered a PWM motor controller and warped it into
the same thing.  I got a nice PC board, ...

Do not eliminate the PTC (or you could substitute a fuse).

Larry


AMaroney@sptimes.com wrote:
> Has anyone figured out the best pulse width modulation (PWM) time for a 
> dew heater controller?  Specifically, how long should one 100% cycle be?  
> If it were too long the dew heater would noticeably cool off before 
> heating up again (running at less than 100%), but is too short a problem, 
> say a few dozen cycles per second?  I was thinking of using a PIC16F88 
> (yes, a 555 could do it also) and some IRF510s as the 12v switches. 
> Thanks.
>
> Allen
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>
>   

_______________________________________________
ATM mailing list http://www.atmlist.net/