AVR-Chat

>  For some reason the original post hasn't shown up here yet.  The vagaries 
>  of Yahoo and e-mail  I expect.  In any case this circuit will not work.  It
>  also strikes me as too complex but that's a separate issue.
>  
>  - Problem 1:
>           Unless there is a couple of invertors you haven't drawn then what 
>  you have is a single 1/2 H with two FETs in parallel tied to both sides of 
>  the load (your motor).  The only way the motor gets current is if the FETs 
>  don't turn on at the same time  when commanded to.  Given the lack of gate 
>  resistors oscillation between the FETs  is a possibility.
>  
>  - Problem 2:
>           None of the Buz11's have sufficient drive voltage, unless your 
>  CMOS buffer is a quadruple high side FET driver.  That seems 
>  unlikely.  From an earlier post I believe you had a simple logic chip in
> there.
>  
>  I believe you are on a pretty tight schedule.  The easiest way to solve 
>  your problem maybe to use relays instead of FETs to provide the H-bridge 
>  part of your design.  I can do up a quick schematic to give you a direction
>  if you would like.   You would then still use the K2232 for speed 
>  control.  You could probably find automotive relays with sufficient 
>  capabilities.  They would be large than the FETs but a lot of the drive 
>  problems would be gone.  It would be a simple circuit to hook up.
>  
>  
>  Otherwise you need to do a complete re-design of the drive circuitry.  I'd 
>  use four high side drivers (IR2125s come to mind) and probably some charge 
>  pump circuitry to maintain the gate voltage.  Actually if I was doing that 
>  much work I'd eliminate the K2232 and use a pair of 1/2H driver like the 
>  2110 and as long as you don't require a full 100% in either direction no 
>  additional charge pump circuit would be required.  I'd really recommend you
>  go the relay route and then look at an all MOSFET version after you get a 
>  relay version working if you still want to.
>  
>  I suspect you also need a supply for you gate drive.  You want 10 - 15V for
>  that and if you are driving off of CMOS logic you probably only have 5V
>  
>  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
>  
>  
>  
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I actually was using relays originally but the ones we could find
couldn't deal with the current and welded quickly.
relooking at my drawing i agree that the gates are paired wrong, i do
have it built correctly, must have been caught as i soldered it
together.  i am going to prototype the circuit using higher voltage on
the gates, we both thought that the mosfets would work on 5 volts or
higher but looking at the datasheet again it makes sense although i
have also done circuity on another vehicle using only the k2232's
using 5 volts on the gate successfully but with out pwm and with out
an h bridge. using the oscilloscope i have noticed some noise coming
out of the buffer that may be turning on all four h bridge transistors
partially, about 1.5 volts at roughly my PWM frequency.

Well i am going to have dinner and try it out using some small npn
transistors suppling higer voltage to the gates,
by the way: the data sheets:
BUZ11's http://www.fairchildsemi.com/ds/BU/BUZ11.pdf
the buffer: http://focus.ti.com/lit/ds/symlink/cd74hct244.pdf
the odyssey of the mind website if any one is curious: www.odysseyofthemind.com

thanks for all the help so far
Ben Matthews