AVR-Chat

At 07:33 PM 2/19/05 +0000, Doug Locke wrote:
>I am sure that one of the hardware gurus will be able to clarify things 
>more and correct me if wrong, but on first looks the circuit in the pdf 
>looks incorrect to me if all the mosfets are the same. If I remember 
>correctly, the idea of the H configuration is to have one of the "top" 
>mosfets on one side of the motor and one of the "bottom" mosfets on the 
>other side on with the other two off and vice versa for the opposite 
>direction of rotation. As things stand, my interpretation of the circuit 
>would be that the top control line just switches the two "top" mosfets and 
>the bottom line just switches the "bottom" mosfets and there is no 
>complementary action with regards to driving the motor.
>
>Doug.
>
>
>-----Original Message-----
>From: Ben Matthews []
>Sent: 19 February 2005 14:51
>To: AVR-Chat@yahoogroups.com
>Subject: Re: [AVR-Chat] Re: Motor Control Problems - more info
>
>Thanks for all the help so far. i am going to add the resistors in a
>couple minutes, all my electronics stuff is over in our Odyssey of the
>mind room. i have a hand drawn scematic which i scanned.
><http://benlinuxisbest1.tripod.com/hbridge.pdf>http://benlinuxisbest1.tripod.com/hbridge.pdf 
>if any one wants to take
>a look.

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