AVR-Chat

At 09:11 PM 2/18/05 -0500, Ben Matthews wrote:
>I am a 15 year old high school student using a tiny 26 on an odyssey
>of the mind project (we have a competition next week :) ). we are
>building a vehicle based on an 18 volt drill motor. i have built an H
>bridge based on the design on page 263 of  "The Robot Builder's
>Bonanza". I am using BUZ11 n channel mosfets. I am also using a k2232
>(also a n channel mosfet) to drive the h bridge between ground and the
>rest of the H bridge. i am using diodes from source to drain. I am
>using PWM on the avr to control speed at just under 4 kHz and about a
>40% duty cycle. everything works well in testing using a 7.2 V nicad
>and a small radio shack motor. when i try to connect to the drill
>motor and the 18V motor the cmos buffer i am using between the avr and
>the h bridge ( 74hc244e i think) smokes and has visible melting after
>the circuit works for a couple seconds. The drill motor draws just
>under 10 amps under load.  i am going to try adding a capacitor
>tomorrow to reduce electrical noise but i was wondering if any one
>else has any suggestions for me?


Do you have a link to the schematic and layout we can look at?  It will be 
hard to offer much more than generalities without that.

The first thing that occurs to me though is to ask what you are using to 
drive the FETs.  IE what is between the MOSFET gates and the hc244?  I'd be 
particularly concerned about the high sides.  I suspect the capacitor is 
unlikely to help. Depending on the problem and where you place it, it may 
make things worse.

A couple of other questions
         Why are you using different FETs for the high and low sides?  They 
have to deal with the same voltage and currents in most H bridges.
         Why are you adding diodes?  The FETs integral body diode is rated 
as high as the FETs forward capacity (normally for power MOSFETs 
anyway).  The diodes will only take up room and as a result may give you 
inductance problems.  It's a minor point in some respects and you won't 
want to try to change the layout at this stage anyway.
         Have you tried the circuit at different voltages without the motor 
in place?  Use something like a 1 K resistor and measure the current 
draw.  You may be getting shoot through.
         Do you have access to an oscilloscope (and know how to use 
it)?  It sounds like it may be a switching problem and an oscilloscope will 
make a big difference in trying to diagnose something like that.

Hmm, reading back through your note I realize I'm not clear as to what 
voltage you tested the 18V motor at.  Was that with the 7.2V NiCd as well?

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III