I’m trying to create a manual speed control for a small DC motor. I have at my disposal a very small (0.25") potentiometer, and want to wire it up in series with the motor. The components I use have to be very small and very light.
I’ve figured out that I need to vary the resistance from 0 to about 100 ohms to make the voltage across the motor vary from 6V to 1V. At 1V, the motor is running at 16%, which is close enough to stopped for my purposes.
But the potentiometer I have is 1 kilohm, and doesn’t have enough sensitivity in the 0-100 range for good control. I checked the local Radio Shack, and couldn’t find anything better for my uses. Is there anything I can do to this particular pot to decrease its effective resistance by a factor of 10, but keep it relatively linear?
I tried doing some circuit math (not my strong point), and tried putting various values of resistors in parallel with the potentiometer. But nothing let me go from 0-100 with any linearity. Are there any creative alternatives?
Well, I just did the math on that, and it doesn’t act much differently. It seems that the part of the pot that ends up in parallel with the motor has very little effect on the total resistance. The total just ends up being a hair more than the other half of the pot.
This is probably because the motor has an effective resistance of only 20 ohms over most of its range. (0.3 Amps at 6V).
Try connecting the two ends of the pot together and tie them to the supply voltage. Then tie the wiper to the motor input. The maximum resistance in series with the motor will be 250 Ohms which is when the wiper is in the middle of the pot. Be sure the pot can take the motor current when the wiper is near one end.
Here’s two that could work. One is a 100 ohm trim pot, and the other is a 200 ohm pot, but it also takes 25 turns, which could be a pain.
I have a question though, have you tested the motor at one volt to see if it runs? DC motors tend to make a horrible whining noise when they are not given enough voltage.
With the setup that you have now, your motor would draw 300 milliamps with no resistor in series. With a 100 ohm resistor, it would only draw around 50 milliamps which may not be enough to make it rotate.
What would probably work best would be a small circuit that could vary the duty cycle of a six volt signal sent to the motor. But I’m not sure how small or easy that would be. I’ll see what I can think of.
With a little tinkering, this should work. The main differences that I can see is that the example runs on 12 volts, and you are running on six. The only change that should need to be made is possibly powering the op amps with a higher voltage that you use to run the motor.
Depending on your level of experience, it may be a little complex, but the good news is that it should be fairly cheap. The 555 timer costs about 50 cents, the op amp is probalby under a dollar, and all the other parts are basic resistors, capacitors, and diodes. The most expensive thing would likely be the breadboard the circuit is built on.
Here isthe circuit. Sorry the graph isn’t very clear. The vertical axis is resistance in series with the motor. The maximum is 250 Ohms. The horizontal scale is proportional to the position of the wipe from all the way to either end to the center of the pot.
It isn’t all that linear over the whole range but is quite linear up to 100 Ohms.
For resistors arranged in parallel, 1/R[sub]tot[/sub] = 1/R[sub]1[/sub] + 1/R[sub]2[/sub] … +1/R[sub]x[/sub]
So you could connect a 120 Ohm resistor in parallel with your pot and across the two together, you’d get a maximum resistance of just over 100 Ohms.
The drawback is that the system will not be linear any more; at the low end of the resistances, a small angle of turn will have a bigger effect than at high end.
1K pot and 120 Ohm resistor in parallel:
Pot Res. System Res.
1000 107
900 106
800 104
700 102
600 100
500 97
400 92
300 86
200 75
100 55
1 1
A couple of thoughts. First of all, are you sure you have a linear pot? If it’s an audio potentiometer, then it’s not going to have a linear response.
Second thought is that if I wanted a small, light motor control, I’d go to a RC hobbyist store. The RC blimp of SDMB notoriety, for example, had a very light motor with variable speed control. RC stuff is so cheap nowadays that you could probably cannibalize something for cheaper than you can buy the parts separately.
I feel like I should jump in here and mention that it’s generally considered a bad idea to try to regulate motor speed using a potentiometer alone. It’s much more effective to connect the motor to a pulse width modulation circuity and vary the duty cycle of the pulses to achieve speed control. here’s an example of what I’m talking about. A little more searching through Google will probably turn up a more thorough explanation of why it’s better to do speed control this way.
And to top it all off, the pot you described sounds like a candidate for an early death . Those little pots aren’t made to carry a lot of current. You’ll probably burn the lower end (lower resistance, higher speed) range of the pot out in relatively short order. Try this.
You can use any comparable type for the BC369. I just happen to know that the BC369 will handle the current you can expect.
You’ll probably have to change the 100 Ohm. It might be too small - or too large. It controls the current that flows through the base of the transistor, and thus the current that flows through the motor. You cannot leave it out! If you do, then the first time you turn the pot all the way down you will fry the transistor.
I’d also have to discourage the straight-pot approach. Rather, go with a simple op amp variable voltage power supply circuit. If you take care when you put it together, it will work for all sorts of circuits and projects.
While I appreciate the suggestions for doing this more elegantly, I should reiterate that I need this part of the circuit to be SMALL and LIGHT. And whatever you think small is, divide it by 5. It sounds like some these ideas might be too large for my requirements.
I basically need this to fit entirely within a 0.5" tube, and weigh 1 to 2 grams (not including the battery or motor).
The motor itself measures 8mm x 10mm x 15mm, and weighs 3.8 grams. At 6V, and the loads that I will be subjecting it to, it will consume 1.8W of power.
Is 0.3A max a lot of current? Sounds kind of small to me. This pot is also rated to 50V DC.
But I do agree that the transistor idea doesn’t seem too bad. Too bad I don’t know enough about transistor operation to navigate my way through that circuit.
I am by now insanely curious to know what this project actually is - my best guesses so far are a miniature model submarine or a lightweight, variable speed sex toy. Am I even close?
Transistor or three-terminal voltage regulator (e.g. low-power 317 variants) circuits can satisfy these requirements, if you use the TO-39 (metal can, 8mm diameter) or TO-92 (plastic, even smaller) packages. The TO-39 weighs about 0.9g; the TO-92 weighs about 0.2g. For a complete circuit, add one 1/8W resistor, the pot, a capacitor, and maybe a diode; this should still keep you under 2g. A PWM controller might be harder to build in under 2g unless you buy a one-chip solution. I think the main advantage of PWM is reduced power consumption, and it doesn’t sound like that’s one of your major constraints here (is this right?).
If you are comfortable with soldering, the circuit in my link, as well as the circuits in other links, could be made that size. All you would need to do is trim down a cheap PCB to the size you need. The weight restrictions could be a problem though.
For a circuit as simple as this with such weight restrictions, I don’t think I’d bother with a PCB; unless the accelerations are pretty severe, good solder joints (and the tube) will hold it together without any extra board weight.
The submarine isn’t too far off…but you have to think air and not water. It’s for a Micro Air Vehicle (MAV). 6" wingspan, approximately 1 oz total weight. It will eventually be capable of video surveillance, but I’m not to the point of integrating a camera quite yet.
It’s also not going to have any R/C capability until I can prove the concept with the flight of a dumb prototype. That’s why I’m only trying for a simple manual throttle for the motor. I want to be able to trim it to level flight by trial and error, just to show myself that it works. Then I’ll worry about adding control.
That is, until this bores me. I already see the next project looming on the horizon: Flügtag
