How do I hook up my potentiometer?

Inside the pot is a resistor with a connection to each end (the outer terminals) and a sliding contact (the center terminal>

• ,--/\/\/\/\--,

• | ^ |

• | | |

• 0 0 0

If you want to insert a variable resistor in series with the motor, use one end terminal and the center terminal to complete the motor circuit.

• [POT}

• | | |

• | |

• *---[DC]---* *--------[MOTOR]---*

• | |

• | |

• *-------------------------------*

Unfortunately, a 500k ohm pot has so much resistance that the motor will stop the moment you slide in even the tiniest part of the rotation, and may burn a hole in the resistive element at the end. You might try something like a 1k pot or a 100 ohm pot. If you have some fixed resistors you can try a few of them and get an idea what resistance is close to doing what you want.

Thanks, John. I'll pick myself up a 1k ohm or a 100 ohm pot today and give it a try!

Connecting a pot directly to a motor is not usually advisable. As a general rule, assume that if you do it, you will burn out the pot. If the motor draws less current than the pot can handle, that's ok, but you need to know that up front.

If you use a pot to drive a transistor, you can control a lot more current:

V+ ---+----[Motor]----+ | | P /c O

Oh, that seems pretty clear. Thanks for the explanation. I'll put an NPN and see what happens.

Unless you turn the wiper all the way to the top terminal and destroy the transistor. A resistor to limits the base current to a safe level should be added between that terminal and V+. (For once I got to make the correction before John caught it) ;-)

Yes! :-)

But you've ended my plan to gain world domination by the acquisition of all of the 220 ohm resistor factories. :-)

Ed

John has ended my plan for world domination (see my reply to him) with his better plan:

R_Motor*(1 + Hfe)

:-)

Ed

Thanks for all the help. But what purpose does the diode serve?

The motor has back EMF, and can damage the transistor. The diode is used to shunt it.

It is intended to pass motor current, limiting the voltage the motor's inductance generates, if the pot wiper happens to slide over a bad spot, opening up the base, while the motor is running. With it there, the most negative that inductive current can drive the emitter voltage is one diode drop more negative than the ground rail. If the to could be guaranteed to never go over a speck of dust, etc., you could do without it.

he better get busy. It lookks like I have over 10,000 on hand.

Curses ... foiled again! :-)

Ed

Do you want a really good deal on them? :)

Thanks, but I expect we share a problem: inventory > space That's why my plan was to acquire factories. :-)

Ed

You haven't heard my offer, yet! :)

You haven't seen my "available" space yet! :-)

Maybe you could make a resistor sculpture of a horse, a la Ocala Horse Fever? I still enjoy those pictures you pointed out.

Ed

I'll put up my lack of free space against yours, any day. :(

I had to put five printers in the garden shed today, so it wouldn't rain on them while I was gone. I have stopped refurbishing computers for a while to attack the problem from the other end. I'm doing triage, and separating the rebuildable from the hopeless.

I put the hopeless units in the dimbulb pile so they can be ripped apart for salvage. I can only give away so many win 95 computers to preschoolers. :(

The ones that work, or almost work will soon be in use at their new home in a daycare center.

WHAT???????????????? Use up my good 63/37 solder to build model horses? NEVER!!!!!!!!!!!!!!! They will have to pry my lead based solder out of my cold, dead hands! :(

;-)

BTW, I think they may do another batch of those horses. If they do, I'll try top remember to post a link. the Ocala Star banner did a recent series called Barchitecture. where the whole series was built around dogs. I can't find it online right now, but I'll see if I can locate a working link. It was published as a section of the newspaper.