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?


Another question occurred to me.  Where are you getting the supply for the 
logic portion of your setup (The AVR, HC244 etc..)?  Also where does the 
supply for the MOSFET gate drivers come from?

If they are being fed off the same 7.2V NiCd as the H-bridge are they diode 
protected against droop?  IE a diode from battery positive to the positive 
side of a large filter cap on the input of your power regulators.

If not there is a very good chance those supplies are being cannibalized to 
drive the H-bridge.  The resulting voltage drop could cause almost anything 
to happen on the H-bridge.  One of the more likely scenarios is the whole 
thing would turn into a high power oscillator at some frequency depending 
on pack resistance stall current, lead inductance and SW response times.

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