As Ken originally wanted to add the feature already in alpha 18 I asked him about the feature:

*snip*
Zaphod wrote:
Hi Ken, at the moment I am building my MS3X and before I have to build the somewhat complicated alternator control circuit for my Miata, I'd like to know how the software solution will be working - which outputs will be available for the function or better which outputs will be usable.
Could you eventually give me a short explanation.

Thanks a lot
Sven from Germany

*reply*
The outputs available should be the same as any of the PWM features.

Right now I plan on it being an "on/off" implementation with a low voltage threshold and a hysteresis, and a high threshold also with hysteresis, so when voltage goes above a certain point it turns off the high output on the alternator and when it goes low it turns on tha output. When it is on it will feed forward its on status to idle control to allow response to any extra load it causes.

Ken
*snip off*

As I am not too much electronically inclined -
Would you think that this is sufficient for the 99- cars? Or won't this work on our cars (and I will have to build the circuit....)

Thanks
Greets

 

This doesn't sound like a good way to control a Miata alternator- I would think that this would cause the system voltage to oscillate.

In particular, I'm confused by "when voltage goes above a certain point it turns off the high output on the alternator," which sounds to me like a description of some alternator which is internally regulated but is externally switchable between two output levels, rather than one whose field coil has no internal controls at all. Does such a thing exist?

 

A simple hysteretic controller would work so long as the hysteresis band is like 0.05 V or so, and the delay in the decision is < something like 50 us. Or the polling is at 50 us. My hardware controller circuit works that way. It's a hysteretic PWM controller with a minimum on and off time, with a variable maximum.

If the delays are too long then you *will* get oscillation. Early versions of the hydra software was like this.

 

Y8s tried this with the existing outputs, just a simple if > 14v then turn off; something simple like that. It did oscillate--tripped too slow.

But y8s was the one working with Ken to get the MS to control it.

 

yeah the standard outputs are way too slow to control alt based on system voltage.

I'll work with ken on it since he has been responsive about implementing it for 1.2. their priority is finishing alphas for 1.1 and then betas for 1.1 and releasing 1.1 firmware.

 

why not just build it in hardware? It's just 2 TO92 parts a transistor and a couple of R's and C's.

 

Saboteur here on the forum even made a pcb for it. He might still have some left.
Found more info here and pics here: http://mx5cartalk.com/forum/viewtopic.php?f=55&t=48722

 

I already built the circuit... (apart from 1 cap) ;-)))