Re: Designing a PWM DC motor controller for under a car hood...


they call me frenchy! wrote:
Hello all,
I am designing an open loop PWM DC motor speed controller for a 12vdc
pump. It will reside in the "harsh" environment of under a car hood.
The motor speed/PWM output will be controlled by a sensor CV (0-5v).
Motor requirement is 12vdc / 3amps continuous (6amps@start). My
question is whether I should lean in any of these following
directions...

1) 556 (Dual timer circuit) driving a MOSFET
2) LM324 quad opamp circuit driving a MOSFET
3) Some type of dedicated motor control IC

There will not be a microcontroller in the system.

****
So, I know that 1 & 2 above will theoretically work, but will the
control voltages be susceptible to the "harsh" noisy environment? I am
a PCB designer, so designing a PCB with proper noise rejection is not a
problem. I am leaning toward #2 above...but I wonder if it would be
smarter to find a dedicated motor control IC that has protection
circuitry built into it (over temp, over current, etc). Lastly, the
LM324 is pretty wimpy on the temperature range, so I would probably
need to use the military LM124 which is waaaaay more expensive. Target
price for the electrical system is <US$10 (not including my sensor).

Any advice would be appreciated.
thx,
frenchy

1) temperatures under the hood vary quite a bit, depending on location
and operating condition. Worst case is obviously anywhere near exhaust
components and away from airflow, and worst case for electronics is
usually when starting up after a hot shutdown of the engine.
Temperatures continue to rise for a while after shutting off the
engine, since there may be no airflow or coolant flow.
2) Commercial temp range ICs may not be much different than industrial
or military these days, except with respect to how much testing they've
undergone before shipment. Lots of products use commercial temp range
ICs at extended temp ranges. (Oh, horror!)
3) Regardless of which type circuitry you choose, your main challenges
are likely to be in packaging the electronics for shock, vibration,
temperature changes, ESD, EMI. Find out about auto electronics
operating conditions like 'load dump transients'.
4) Brush-type motors generate lots of conducted EMI. Learn about
bypassing and otherwise filtering this.
Paul Mathews

.

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