[quote]Ian Hooper wrote:
> Hi all,
> I built a little speed controller for an electric trolley this week, and
> since there's often discussion on home-brew speed controllers on here I
> thought it might be useful if I documented the design for others to
> So far it seems to work a treat! Hopefully might be of some use to
> others out there working on their own controllers, and I'm always open
> to any constructive criticism on the design from the experts
This is an excellent first controller, Ian. Your MOSFETs, diodes,
capacitors, and gate driver IC are good choices.
As you noted, it lacks current limiting. Even that little Baldor 0.7hp
motor will draw 100's of amps at stall, so it would be a good idea to
add a fuse, sized to blow if the motor remains stalled for more than a
Your circuit also didn't show any main contactor to turn it all off.
This is necessary because sooner or later the controller will fail, and
you don't want a runaway vehicle when it does!
I'd use even thicker wire to interconnect the MOSFETs, diodes, and
capacitors. Again, these parts could easily be asked to carry 100's of
amps under some conditions. Copper sheet metal strips are convenient.
For low voltage controllers like this (24v), schottky diodes are a
better choice, as they have about half the voltage drop.
15v zener diodes across the MOSFET gates are a good idea. They protect
against transients over 20v or so that could blow the MOSFET gates.
Likewise, a high value resistor from gate to source insures that the
gate can't float high if the gate driver IC is off or becomes disconnected.
I'd add a series resistor and separate filter capacitor to power the
voltage regulator ICs. When the motor switches off, you can get huge
voltage spikes on the battery leads due to their inductance. These
spikes can exceed the regulator's peak input voltage spec.
Hopefully, your microcomputer's software has some fail-safes in it to
detect a broken throttle pot, out of range battery voltage, etc. There
should also be a watchdog timer, enabled so it will shut things off if
the software crashes.
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net