Sup fellas,

I'm currently working on a little project for the Maxima.

I have two identical rotary motors and need to slow them down as they spin way too fast. They are wired together and will be powered by the same source.

Here's the specs on the motor(s)

• 12vdc
• Stall Torque 171.445(mN.m)
• Max Efficiency 59.23%
• Torque at peak efficiancy 24.002(mN.m)
• Current at load 2.146(A)
• Power 15.91(w)
• No load current=0.377(A)
• 400RPM
• High torque, (using a 3mm pitch and 12mm dia screw can provide 40lbs linear force)
• All metal gears
• Current draw 3amps

I would like to use a variable Rheostat (variable resistor) to drop the voltage way down to about 0.5v to 1.0v. When I hooked up the motors to a AAA 1.5v battery, the speed was about right, but still too fast. I want total control of the speed of the motors by using the dial on the Rheostat. I don't want to (and don't know how) to size the correct size resistor to get an EXACT RPM speed.

Anyway, I went to RadioShack and bought their one and only Rheostat as well as several Potentiometers (guy at Radioshack said it's the same thing)

Specs on Rheostat are as follows

• 25-ohm +/-20% tolerance
• 3 watts
• Wirewound variable resistor

I hooked up the Rheostat and it got fairly warm right away. Once I hooked up the ground, nothing happened (motors didn't spin) After I turned the dial, the Rheostat caught on fire and smoked.

I hooked up all three variable potentiometers and they all blew too.

Questions:

1) Is a Rheostat the same thing as a Potentiometer? I looked them up in my dictionary just now and a Rheostat is an adjustable resistor, and a potentiometer is a voltage divider. Same thing?

2) How do I size up the correct Rheostat for my needs and what I'm trying to do??

3) Do I really need to hook up the ground on Rheostat? I mean, if I wired in just an ordinary resistor, I would wire it in the positive wire to accomplish the same thing. What's the purpose of grounding the Rheostat?

Thanks in advance.

 

you need a PWM to control it... they are like 20 bucks... or u can make ur own.. i would just buy one... there is a site where people get them from but i forget what it is...

 

Thanks for the reply. I know this is a noob question, but what's a PWM??



Anyone else??

 

Are you sure your wiring was appropriate? Sounds like mis - wiring to be blowing those out like that. A potentiometer should work fine, IIRC it does a similar thing and is a variable resistor sort of.

 

PWM = pulse width modulator, Basically you vary the duty cycle of the signal going to a device to change the average voltage you're giving to the device. Lower duty cycle means lower average voltage, means your motor will turn slower (also works well with LEDs).

I don't think that it was a wiring problem. I think it was a component selection problem. The power capabilities of the rheostat that was purchased was only 3W, whereas the motor is a 15W motor. If you were putting 15W through the rheostat, there's no wonder it fried.

To answer the OP's original questions:
1) All squares are rectangles but not all rectangles are squares. You could think of a rheostat as a square and a potentiometer as a rectangle. The difference for the pot is that it has a third terminal on it so that you can easily use it as a voltage divider. The rheostat is just a two terminal device, so you'd have to add another resistor to get the divider.

2) To be safe, I would get at least a 15W rheostat because that is the capability of the motor.

3) It depends upon how you're using it. If you're just setting up a voltage divider you would wire it up as:
12V ---/\/\/\----|----/\/\/\--- Gnd (where the | denotes the node where you tap into to go to the motor; --- denotes wire; /\/\/\ denotes resistor)

 

Come on Wizard, this is child's play. You connect the negative polar solenoid lead up to the positive terminal of the fluxuator and set the ohm voltage to Avogadro’s number squared so that the drop across the terminals has a maximum field deflection of 20 degrees at T = 50 degrees Kelvin and so that the resonator measures +/- 4.0 volts at both nodes.

Oh, and don't let the smoke out of the variable resistor - the smoke is what makes it work!

 

You forgot the flux capacitor!

Thanks for the informative response D Love. I have a few follow up questions if you don't mind.

1. Both the rheostat and potentiometer I purchased had three terminals. There were no instructions on the package, so I merely went off what I remember seeing in the store on a another potentiometer's package. Therefore, my wiring could be mixed up, I guess.

2. Since I'm using two motors (15w each, and wired together) do I need a 30w rheostat, or will 15w suffice?

3. Ok. Now what's the difference between a PWM and a rheostat? Different ways to accomplish the same thing I take it?? Being, to lower voltage to slow down the speed of the motors...

4. Where's a good source to a get a PWM or rheostat? (I bought the one and only 3w rheostat at Radioshack)

Any help would be much appreciated.

 

Straight from Wikepedia


"Any three-terminal potentiometer can be used as a two-terminal variable resistor, by not connecting to the 3rd terminal. It is common practice to connect the wiper terminal to the unused end of the resistance track to reduce the amount of resistance variation caused by dirt on the track."


Here's the Radioshack Rheostat..
http://www.radioshack.com/product/in...ductId=2062299

Maybe I should get this one???
http://www.skycraftsurplus.com/index...OD&ProdID=1518
or maybe this one..
http://www.alliedelec.com/Search/Pro...4&DESC=RJS1K0E

 

Another question.

Let's say I get a 50w rheostat. What resistance in Ohms do I need???

see here..
http://search.digikey.com/scripts/DkSearch/dksus.dll

 

i'm giving this my best shot just by using math (don't know anything about rheostats and how they work/are rated). my (semi-educated) geuss is a 6ohm 50w model is the best fit.

how i came to this:

(13.6v/6ohms)*13.6v=30.83w

this is using 13.6v (standard running car voltage), but i think a voltage regulator would be a good idea to prevent spiking. i also only used the 50w models available from digikey to choose from.

if any of you math/electrical guys coulld confirm or eplain why it is wrong i'd like to know just for curiosities sake.

 

You're welcome.
1) On a 3 terminal device, two of the terminals will always give you the rated resistance (within its tolerances). The third terminal will change with respect to the other two terminals as you rotate the ****. To properly wire this as a voltage divider, you would connect the two "static" terminals to power and ground, and connect the "moving" terminal to your motors.

2) I like your later idea of a 50W device.

3) That's pretty simplified, but you can think of it that way.

4) The rheostat is the simpler solution, so I would probably reccomend going that route. A few of the online electronics distributors are:
digikey
mouser
allied
newark

 

I agree with BLACKonBLACK98, 6 Ohms would be your best bet for the 50W rheostats. If you wanted to save some money, you could go with a 5 Ohm, 50W Adjustable Power Resitor. It would be a little harder to tune, but you should be able to get the same result out of it.