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Old 04-30-2009, 06:51 PM   #1107 (permalink)
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Originally Posted by MPaulHolmes View Post
I spoke to the EV Tech list about the mechanical chopper, and I guess it's been around for quite a long time. Here's Lee Hart's comments:

This idea is the original PWM "chopper" of Tom Edison's time. Before
transistors, before SCRs, even before vacuum tubes there were still

Yes, you can build a PWM controller with mechanical switches. But this
crude version implies that the builder has no idea how a PWM works, and
never studied the old ones that really *did* work every day in the 1900s.

The fundamental problem is arcing. You can't just switch an inductive
load on and off; the inductance insists that the current *must* flow
somewhere, so you get hideous amounts of arcing that will rapidly
destroy the switch.

Nowdays, we put a freewheel diode across the motor to carry the current
while the switch is off. This at least keeps the switch voltage from
rising past the battery pack voltage.
I snipped the rest. This is nowadays, so putting a freewheeling diode bank (or a FRED or two) across the motor isn't a problem

Same issue with the caps. I'm not trying to build a 1890's replica. If it needs caps, then we can put them on.

As for the servo motors Paul, it would be absolutely no problem for you. Servo motors are driven by a PWM signal with a duration of between 1 and 2 ms in a 20 ms frame. So the duty cycle is between 5 and 10 percent.

Here's a brief tutorial:

Whats a servo: A quick tutorial

I'm thinking that my initial prototype won't need the servos. I plan to keep the bushes stationary and make the rotating shaft move on a sliding rack. This will keep the heavy power wires from having to move.

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