convert 12V down to 9V DC

I like the buck converter idea, but if I was doing it, I think I would try to put a control pot in place of the on/off switch, and rig up a current mode buck controller, so that the torque was proportional to the pot voltage. That would allow a wide speed range and smooth acceleration with controlled motor current, instead of controlled voltage.

You could use a PNP transistor as the switch if you insist on switching the positive rail.

Even if you go with a voltage controlled buck and the on/off control, it should still have a current limit to protect everything in the event of a stall and not strip the gears on the hard starts.

Just put some large diodes in series, each will give you a ~ 0.6 volt drop./ The simplest method I can think of.

Controlling large currents like that is always a PITA. For cars and scooters I usually make a "shifter" by tapping batteries at 3,

Tom

Of course it is. A PWM speed controller won't waste all the energy, though.

Big power resistor!

John

Supply):

mosfets.http://www.daycounter.com/LabBook/BuckConverter/Buck-Converter-Equati...

Update - surge current is around 15-20 A actually with the 12V battery. A linear regulator might not be such a great idea after all...

Michael

Supply):

mosfets.http://www.daycounter.com/LabBook/BuckConverter/Buck-Converter-Equati...

Big power resistor!

John

I'm not sure what the exact function of the '3904 is, I've seen similar circuits without it. There are better ways (see LM723(?) linear regulator controller).

That would certainly work.

But before you go there, consider a few things:

(a) the battery may be sagging under load. You may get better performance just by putting in a bigger, newer, or higher current capacity (_not_ amp-hour capacity, but higher surge capacity) 6V battery.

(a2) you can check this by measuring the motor's terminal voltage under load; if it's much less than 6V start chasing around for the lossy parts of the circuit.

(b) A high-power series resistor from a 12V battery will give you a better top speed, and some measure of short-circuit protection. It'll also get HOT. I'd design for a 15-20 amp stall current from 12V, which would give you the same starting characteristics, plenty of zip, and lots of heat.

(b2) and you'll need a physically bigger battery if you want to go as far, because of all that heat you're generating in the resistor.

(c) A buck converter is the 'right' way to go. There are controller/driver chips that will do this with NMOS parts switching the positive rail, and you can always switch the negative rail to the motor. Either way you can control the current in the circuit.

(c2) But if you're at the stage of looking up schematics on the web you'll have to learn a lot before you could apply a buck converter.

Interesting; I'll look into that...

Thanks; I'll look into that. I have a slightly (physically) bigger 6V battery that I'll try. (None of them spec their surge current capacity, only A-hr capacity... more research needed...)

Yep, heat... I guess I could be safer by using 20 3V, 1A light bulbs to allow 20 A to pass, but then that wouldn't be exactly cheap either...

Yep, especially for 20A...

Thanks,

Michael

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I should have thought of that...

Look into 6V car (or motorcycle) headlamps. They should be cheap, and you can hook as many as you need in parallel to drop down to 6V at the motor. Even better, the resistance will go down as the current goes down and they cool, so they'll give you an even higher top speed (I see spun bearings in your future).

The only downside to putting them in parallel is that if one goes the other(s) will see higher currents, so they may go pop-pop-pop.

...zap...

That did it! A fatter 6V battery does the trick.

Thanks,

Michael