Need some help on a pot resistor for small motor

[Slats]

Hi guys
I have an interesting problem.

I two very small (tiny) DC pager motors.

These are installed in parallel in a 1/72 helo, with the size fitting into the tail rotor, and easily in the main rotor too.
running off a single 1.2v AA or 1.5v AA, the speed is excessive.

What resistance pot can i use (i plan to link this to a servo so as to have a mini mechanical speed control)?

No the chopper won't fly, but having it with working blades is a nice effect.

Thanks

John

[oztruck]

Hi John,
Try using the PC in a micro servo. That will also give you speed control.

Cheers chris.

[Slats]

Hi John,
Try using the PC in a micro servo. That will also give you speed control.

Cheers chris.

Hi Chris -
thanks
am an electrical neanderthal - but was put off also by the fact I though I needed a minimum 4.8v for a servo.
If 1.2v is too fast I don't expect that type of servo pot to cut the revs down. In fact it may not work at all.
Any other ideas?

J

[oztruck]

Just put a normal diode or 3 in the circuit till you are happy with the speed. Each diode will reduce the voltage by about 1 volt

Cheers Chris

[Slats]

Just put a normal diode or 3 in the circuit till you are happy with the speed. Each diode will reduce the voltage by about 1 volt

Cheers Chris

Hi Chris
what is a "normal" diode?
I was hoping that someone could give me a pot value so that the speed could be variable.
The input voltage will be 1.5 v, so I am trying to find a pot that can pump out via variable resistance a range from say 0.5v through to 1.2v.

Any ideas?

J

[greenman407]

Here is a picture of a germanium diode available from Radio Shack. Its a 3 amp. one so its kind of big but they sell much smaller ones.

[Slats]

Thanks Mark.
I am just wondering how this will work.
Does it simply drop the voltage like a resistor?
Thanks
J

[Kazzer]

Aren't supposed to be using V/IxR here to work this out?

KMC! Where are you?

[Slats]

Am after a pot or way to vary the resistance.
Where I also get confused is understand ohms K and various other jargon.
If (I) is current how is it measured? Amps / ohms / what?

Kevin or Andy please jump in here.
J

[Albion]

V=IR

Volts = Amps x resistance (ohms)

[Slats]

V=IR

Volts = Amps x resistance (ohms)

Thanks

but getting back to my example what are we talking about.

Are the volts - the input volts or the desired volts?
As far as the current goes, do I measure this now with the input voltage? If so this will be wrong as I am yet to load up this helicopter with its main and tail rotor...is this a problem?
What about the variable speed sought?

Thanks

J

[Subculture]

Wiring in a diode as mentioned by Mark will give you a forward voltage drop (up to 0.7v), lowering the speed of the motor. That's one way, however the speed might not be right for your needs.

Another way is to wire in a potential divider set-up as a rheostat. It's a tiny motor, so a normal carbon based pot should suffice, I would try a 5k linear model and hook it up (if you have an old knackered servo kicking about, they usually have 5k pots in them). Twist the shaft of your pot until the motor reaches the desired speed and then measure that resistance using a multimeter. Replace the pot with the nearest discrete value fixed resistor 1 watt rating. Good explanation and diagrams here-

http://www.allaboutcircuits.com/vol_6/chpt_3/7.html

[Albion]

Thanks

but getting back to my example what are we talking about.

Are the volts - the input volts or the desired volts?
As far as the current goes, do I measure this now with the input voltage? If so this will be wrong as I am yet to load up this helicopter with its main and tail rotor...is this a problem?
What about the variable speed sought?

Thanks

J

the volts are fixed, unless you drop using diode etc. use resistor to reduce amps

[Slats]

Wiring in a diode as mentioned by Mark will give you a forward voltage drop (up to 0.7v), lowering the speed of the motor. That's one way, however the speed might not be right for your needs.

Another way is to wire in a potential divider set-up as a rheostat. It's a tiny motor, so a normal carbon based pot should suffice, I would try a 5k linear model and hook it up (if you have an old knackered servo kicking about, they usually have 5k pots in them). Twist the shaft of your pot until the motor reaches the desired speed and then measure that resistance using a multimeter. Replace the pot with the nearest discrete value fixed resistor 1 watt rating. Good explanation and diagrams here-

http://www.allaboutcircuits.com/vol_6/chpt_3/7.html

Thanks Andy I'll give that ago.
J

[Slats]

Well I have hit upon the idea that the best variable speed control is indeed an ESC....now here in lies a problem.

The two identical motors I have are not up to the job.
The tiny servo motor works fine in turning 2 tenths of nothing in a 1/72 scale tail rotor, but its twin in trying to spool up the main rotor blades is not up to the job. These motors have heaps of RPM but bugga all torque and as per design they normally work well through a gear box.

The solution is that I have a larger (still small) DC motor for the main rotor. But the further problem is the tail rotor still spins way too fast.
Via the ESC feeding both these motors I found in series the smaller motor tends to always spin faster, and in parallel it spins just as fast.
I am using a brilliant Electronize ESC (http://www.electronize.co.uk/model_e...ics_frames.htm ) FR15 series, where I can wind down a pot to limit both motors RPM, but this is still too fast for the tail rotor. I tried sticking in a parallel circuit setup the diode recommended (above) for the tail rotor mini motor but this takes no noticeable change in RPM. I have observed that the ESC using the minimum 6v input puts out voltage to the two motors between 0.3v and 2.4v using my mutli meter. Is there a way I could put a resistor in the circuit between the ESC and the mini motor to take this range down to 0.1 to 0.8v if so what would be the value of the resistor in K / ohms etc? Would doing so cause any long term damage to the ESC or tail rotor's motor?

BTW Andy what measure / setting to I put my multimeter on to measure resistance? The value displayed means what? (ohms, k, something else?)

Thanks

John