FET DRIVER

>> Why is 'a true totem pole MOSFET driver' necessary?
>> Could not a small logic level FET driven directly from a PIC pin drive the power FET?

> Imagine that the MOSFET gate is one end of a capacitor and the other end goes to ground ...
   and
> BTW you need a totem pole (or push pull) driver for a MOSFET because you
> need to be able to turn it on and off as quickly as possible, and that
> means having a strong current source and sink capability.

I see now. I was ignoring the drain requirement for turning it OFF quickly.
Thanks for your patience.


> Also look at the FET drive chips that Microchip make, such as MCP1401 and MCP1402.

$0.62 from Microchip in ones. I can live with that.



PWM

> When you are doing PWM and switching the FET on and off thousands of times per second, ...

Mine operates at 500hz right now. 
It looks to me that the switching loss is directly (linearly) related to the PWM frequency.
I don't think the possible hum will be an issue in a go-cart, so 
I'm planning to keep the PWM frequency low unless there is a good reason to increase it.



MOTOR LOADING

> It does not have to be a special "alternator" - it can simply by a
> second motor with a shorted output or a resistor across its output.

That was actually my first thought, but I didn't have any comparable motor to use.
(I do have an old alternator, but physically linking the motor 
and alternator together would be a little sketchy.)
But we are getting another motor, with the same gear, so I will be able to do this soon.


>> Is there some trick to get a more accurate reading for low resistance values?

> Do you have an accurate current limited power supply? 
> If you put a controlled current through the motor (say, 1 Amp), 
> and then use your meter in voltage mode to read the voltage drop across the motor ...

I don't, but I think I can make one from an LM317.



CURRENT SENSING

> I have used the Allegro current sensors and they are nice. 
> Digikey does sell some which work for much higher currents. 
> Take a look at the ACS756 series. 

Ok, here is one, ACS758 100A:
   http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=620-1237-ND
$7.00  Ouch, but worth it.



MOTOR CONTROL DESIGN

> First off, I'd recommend you download a copy (free) of LTSpice
> ...
> thermal analysis that you should do
> ...
> 2.6W is perfectly reasonable for a TO-220 or similar package on a good heatsink. 
> ...
> maximum sustained current you could have,
> ...
> compute the power dissipation during each time. 
> ...

This will take quite some time to digest.
It really helps and I appreciate you taking the time to go through all this.



NEW STRAWMAN CIRCUIT

Through BAJ's link to ecomodder, I ran into this circuit, designed and used 
to replace the old controller in a golf cart type 'electric trolley'.
24V 500W motor - same as mine.
This is the first circuit I've seen that I think I can actually understand and build.
I would of course substitute a PIC for the ATMEL and probably an MCP140x for the IXDD414PI mosfet driver.
   http://www.zeva.com.au/speedy/

Opinions on this design would be most welcome.
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