Dwayne Reid wrote:

> Tjaart van der Walt wrote:
>
> <much snipping>
> >
> >Just over 20kHz! Dwayne! Give us you input!
>
> Sorry for the delay in responding, Tjaart.  I've had my head glued in front
> of my monitor trying to get some board layouts finished for a project that
> ships next week.  Thank God for outfits like apcircuits (just 3 hours to the
> south of me!).
>
> In the meantime, Scott wrote just about what I would have written (and, in
> fact, is quite similar to an 8 step 25KHz PWM motor speed control I
> presently use).   One thing to watch out for is just WHERE (in time) you
> sample the overcurrent.

Ha! you saw the pitfall! Because I am just comparing the average drop, Ican check for overcurrent
(or undercurrent) anywhere in the loop. I had to
average the drop with a resistor and cap because I switch the MOSFET
hard on.

> My motor speed control monitors for shorted load
> and cuts back to minimum pulse width if a short is detected.  I couldn't
> sample immediately after turning on the FET drive because of delays in my
> gate driver.  If I waited too long to sample, I couldn't get my minimum
> pulse width low enough to prevent destruction if the short persisted too
> long.  You may have a similar problem unless you add some form of peak hold
> to the overcurrent sense.

I am thinking about forcing the PIC into some sort of a waiting state. If the pulsewidth runs down
to zero, it must wait a few seconds before trying again. I will
have to play with this idea first because I think an inrush current could confuse
the poor 12C508 of of it's tiny mind...

> I looked at your schematic and was wondering what you are using to control
> the battery voltage.  Is Vsupply regulated or do you have something else to
> set the battery float voltage.

Yep, the supply is pretty well regulated at 6.7V (or something, I'll have to check)with a switcher.

>  Do you worry about equalize mode?  My lead
> acid chargers stay in equalize mode (7.5 Vdc on a 6V gell cell) until they
> drop out of current limit, whereupon they drop down to float mode (6.75
> Vdc).  They switch to equalize mode both on powerup and anytime the charger
> goes back into current limit.

I'm not sure what equalize mode is. Please tell me more.

> I also see that the circuit appears to be set up as battery backup with
> automatic change over - the battery powers Vsupply thru the intrinsic back
> diode in the FET when Vsupply drops more than 1 diode drop below the
> battery.  I assume this is intentional.

Yes. This is also the reason for the undercurrent sensing. As soon as the currentdirection changes,
the PIC sees a undercurrent and keeps the FET switched on
hard.

I hit upon the idea of using the FET's channel as a 'shunt' to measure the
current flow when I was paging through a datasheet. I won't claim being the
first to try it, because I am sure someone else has thought about it before me.
This, and the fact that you can charge the battery to supply AND use the
battery without a diode drop makes it quite handy (I think so, anyway!)

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za

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