If your motor takes .3 A. the square of that is 0.09 and it the pot resistance at that current is 10 Ohms there is .9 W. being dissipated in it. I’m not sure such a little pot will take that for very long.
If you are interested in building a UAV, check this out.
http://www.seattlerobotics.org/encoder/200311/weimer/bxflyer.html
It may be a lot bigger that what you are thinking of, but it could give you some good pointers.
I’ve been away and came back to see how the thing turned out. No answer … but …
I don’t think that the values Mangetout gave are the effective impedance that will appear in series with the motor.
I think you need to draw the circuit and compute the output resistance using Thevenin’s theorem or some equivalent approach.
The output resistance of any circuit is the open circuit voltage divided by the short circuit current. That is, you compute the voltage across the output terminals with the output driving an open circuit. Then you compute the current through a short circuit across the output terminals and divide that into the voltage.
When I do this I get approximately the same resistance curve as when you tie the ends of the pot together and drive the motor from the wiper. And that is nearly the same resistance curve you get just using the pot.
I’m curious as to the results of trials with the suggested motor controls.
On looking into this a little further I’m not sure that any method using the pot is going to improve on what audilover just using the pot the way he first started out.
The method I suggested results in practically no improvement and neither does Mangetout’s I think.
Sorry about that but that’s the way it is.
You’re probably right; inductance was the stage of electronics where I started to hear the whooshing sound.
I would imagine that pulse coding is the way to go here, even if it is really crudely implemented (i.e. a 555 timer and a few discrete components including a transistor capable of driving the motor). This should end up as a smaller and lighter solution than just a potentiometer, since you can use micro trim pots to control the cycle of the 555.
For best efficiency you will want to use a PWM controller.
Here’s a schematic of a 555-based controller:
http://www.nomad.ee/micros/pwm555.html
Allegro has a nice selection of motor drivers:
http://www.allegromicro.com/ic/motor.asp
The 3948 looks pretty neat:
http://www.allegromicro.com/sf/3948/
http://www.allegromicro.com/datafile/3948.pdf
Okay, after taking a while to learn how transistors work, I’ve come up with a schematic for my circuit, based on Mort Furd’s design. I took the time to size the resistor off the transistor base, and 470 gave me the best, most linear curve on my graph of motor current vs. pot setting.
Once again, while I appreciate the elegant suggestions, it seems that this one will do the job for me. I don’t care at all about efficiency for now…this thing only needs to last about 5 minutes, on a 300 mAh battery that ought to run the motor at full throttle for an hour.
Just an update…
Today, I got a chance to wireup this circuit on a breadboard. I gave the poteniometer a try, and it adjusted the motor very linearly over nearly its entire range. There was a lag before the transistor switched on, and a platuea after the transistor saturated. But that was exactly what I expected based on my calulations, and about 80% of the pot’s range remained useable and linear.
The transistor is a PN2907. The super-bright LED is there as an indicator light, primarily to add visibility when this craft is flying at altitude. I forgot to include a switch on my diagram. But that’s okay, because the breadboard circuit didn’t have a switch either.
If this is for an aircraft, then I’m not sure that you’re taking a sensible design approach. Let’s say you get the right speed with 3 V across the motor. You’re using a 6 V battery. That means that half of the battery’s power is being used to run the motor, and the other half is being used to heat the transistor. Another way of looking at that situation is that you’ve:
a) used a battery twice as big and heavy as you needed to, and
b) used a motor bigger and heavier than you needed to.
The optimum solution would’ve been to use a 3 V battery and a 3 V motor.
Let’s stick with the 6 V motor, though. What sort of battery are you using, and is there any particular reason why you couldn’t replace it with a variable number of 1.5 V cells?
You can eliminate the “lag” by putting a 130 Ohm resistor between the 6V and the high end of the pot.
If you are hitting saturation, then the 470 is too small. The data I’ve got for the PN2907 say it has a gain of 100 to 300. To get your max of 300ma you would need a base current of 3 ma. At gain of 100, you would need to change the 470 to about 1.5KOhm. Depending on the gain of the actual transistor, you would maybe have to go up to 4.5KOhm. I’d try the 1.5 KOhm first.
Mort Furd, the lag is really small. Maybe the first 10% of the pot’s motion on either side. I’d rather hit saturation a little early than take a chance at cutting off my max current. In short, it works satisfactorily.
And Desmostylus, I don’t know if I need only 3V across the motor. I might need 7. I might only need 2. But at this point, I need to have up to the maximum rated thrust of this motor/prop combination available to me. I’ll likely end up using less, but this is truly a case of too much is better than too little. Let me explain my reasoning:
The problem with designing an MAV is the way that conventional design equations spectacularly break down at the very low aspect ratios (about 1) and Reynolds numbers (about 80,000) that I’m dealing with. Lift curve slope, form drag, skin friction drag (to name a few) get fiendishly hard to predict. Couple all of that with the fact that this aircraft uses a flexible membrane wing, and in flight will assume an airfoil shape that may vary anywhere from flat plate to optimally cambered to horribly wrong.
Since all these factors are huge drivers for finding power required, I can’t say with any certainty how much I’ll need. My worst case assumptions have led me to a number for thrust, and the motor I’ve selected can generate that thrust. I might need a lot less thrust if my worst case turns out to be much worse than reality. But that’s what the pot is for.
Also remember that this is a prototype. I’m pretty sure I said this before, but it doesn’t have to be efficient. It doesn’t have to be optimized. And it definitely doens’t have to be pretty (aesthetically or electronically). It’s a flight demonstration test article, meant as a proof-of-concept, and crude by nature. It’s made with cheap off-the-shelf parts, and when I find out what kind of battery I really need, rest assured that it will be the smallest and lightest that will do the job. Ditto for the motor. But the final iteration of this craft will have a full set of radio control features, which will undoubtedly have a far more sophisticated throttle for the motor built right in (one that I didn’t have to design).
As it is, I have a power subsystem that weighs a total of 12 grams, can run for a long long time, and cost a grand total of $25. Good enough for the first phase is what is, admittedly, a spiral development process.
My original question was simply “how can I make a 1 Kohm pot act like a 100 ohm pot?” And that’s all I ever needed to do. I think I’ve got my answer, even if it is ugly.
And while it may seem like child’s play to the electronics wizards out there, but you can’t really understand how happy I am to actually make a transistor work in a circuit. That, in itself, is a major accomplishment for me.
OK.
Good luck and open skies.