AVR-Chat

--- In AVR-Chat@yahoogroups.com, Robert Adsett <subscriptions@a...> 
wrote:
> 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 :) ).

This is Ben's father.  A little more info.

Ben's coach is the person in charge of the program and is about to 
lose it.  She is coaching 3 teams (balsa, technical, and vehicle) 
and all of them are having major problems.  I am trying to help the 
vehicle and technical teams.  I used to do analog hardware (Nike 
Herc missle) but have been a software guy for over 20 years now. 

The two types of MOSFETs are in use because I made a Digikey order 
for the first batch.  Between Ben's team and another with an 
impulsive teenager the supply went down in a hurry.  The local 
electronics surplus store has BUZ11s in stock and they looked 
adequate.  Thus the mix of types.

The buffer outputs are connected directly to the MOSFET gates.

The diodes are there for spike supprssion as called for in the book.

He has access to a 65MHz dual trace scope and knows the basics of 
using it.  I can get him through the rest of it.

Logic power is supplied by a 4 cell nicad pack.  Drill power is the 
15 cell pack that came with the unit.  Negative is common to both.

The drill is the Harbor Freight 18v unit.  About $17 US complete 
(cheap junk).  There are no noise supprssion caps on the motor.

He had blown a transistor earlier in the day from a backwards 
diode.  That one was found with low volts testing.

The 7.2 v pack was a nicad "9v" battery, about 120 mah.  I handed 
him that because it can't source much over a couple of amps. 

Oscillation is an interesting guess.  I hadn't thought of that one. 
The only way that I could see getting enough gate leakage to wipe 
the buffer was tranistor failure and since they were not getting 
warm it did not make a lot of sense to me.  I am guessing that when 
the failure occurs he is dumping one battery pack into the other.

Plan for today:
Check the transistors.
Put a 300-500 ohm resistor between the buffer and the MOSFET gate to 
limit current when the failure occurs.
Put an auto light bulb in place of the motor in the circuit (dummy 
load).  That will get rid of the motor electrical noise while 
testing.
Fuse it all at 5 amps
Test with 12 volts (or less).
I am going to look through the junk box for a low ohm resistor to 
toss in series with the battery as a limiter for testing.

Odyessey is a great program.  The vehicle and technical problems are 
really difficult this year.  We are in a small public school (100 
graduates/year) in upstate NY.  Lots of community involement for 
coaching and skills training.

Dave Matthews




> 
> 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