>>> Do you like the brake circuit ? :-)
>>>     http://russell.servepics.com/temp/r100b.gif
>>> Too many parts, but it works well.

>>I can't figure out why it should, though. How is the basis of
>>Q44 ever 700 mV more negative than its emitter?

> When Q43 is conducting, C42 is charged via D43.  When Q43 is 
> switched off, R41 pulls the negative side of C42 up to the positive 
> supply rail, so the positive side of C42 (and also the emitter of 
> Q44) is at a potential of around double the supply voltage.
> It's a fairly classic voltage multiplier circuit, though used in a 
> "one shot" mode here.

__

The voltage step up is standard enough - but by itself it didn't do 
the job. The problem was that without Q44, diode D43 raises the gate 
to V+ while the motor is being driven and the FETs would turn on to a 
limited extent. I needed something which would completely remove the 
gate drive until it was boosted to ~~~2 x V+. Q44 does this. Its base 
is held at V+ so as long as C42 is less than V+ then Q44 is off. When 
the right hand end of C42 and therefore Q44 emitter is raised to ~~ 2 
x V+, Q44 is turned on and it's collector drives the FET gates high. 
They stay high unto EITHER a new input cycle starts (when C42 is 
pulled low and removes drive) or until C42 discharges via Q33 base and 
R46. This sets the maximum brake fully on time and also allows it to 
be instantly removed if a new cycle starts. R47 is optionally provided 
to give the gates a defined low value relative to their sources when 
Q44 is turned off but in practice  is hardly needed. I've never seen 
this combination of voltage booster and emitter switched output stage 
in use before but it's so useful that I'm sure it must have been 
"invented" many times before. Odds are it's a standard FET high side 
driver which I've failed to notice :-). I suspect it's switching speed 
may be limited by practical aspects, but it works well here. Unlike 
most voltage doublers it is not allowed to have an output "reservoir" 
capacitor as the voltage must be removed as soon as a new cycle 
begins.



            RM 

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