Re: Cheapest and easiest way to slow down a motor
 

another easy way to drop voltage: silicon diodes have a fixed 0.7V drop. If you have diodes that can handle the current, you can put them in series till you get down to the voltage you want.

BTW, the easiest and cheapest way to slow down a motor is to wack it with a sledge hammer,

and the 'right' way to slow down a DC motor is to increase the strength of the magnets. The stronger the field, the more torque it will have and the slower it will run (due to the EMF voltage).

Re: Cheapest and easiest way to slow down a motor
 

The wipers have resistors for the different speeds. If equipped with intermitent wipers (almost all vehicles these days) then there's also a timer, this is what greencactus was mentioning.

Depending on the vehicle the resistors and timer can be part of the switch, a separate unit somewhere, or part of the wiper motor. My 1987 Chevy Caprice has a circuit board mounted to the wiper motor where all this is.

The blower motor (for heat, defrost, a/c) for a vehicle also uses resistors to get the different speeds. These resistors would be attached to a single board in the heater box. They are in the heater box to help keep them cool with the moving air.

Re: Cheapest and easiest way to slow down a motor
 

Many, many, many moons ago I did a science project in high school where I needed to dim a car head light like you would a light in your house. I went to the local auto parts store and asked if I could look a some car dimmer switches. This was a long time ago but back a then a lot of GM cars had a single knob that controlled all of the lights in your car. You pulled the knob out half way and your park lights came on. You pulled it out all the way and your headlights came on also. You turned the knob to the left and your interior dome light came on. Now the important part: You turned it to the right and it dimmed the dash lights. If you wanted to hit your high beams you either pulled the turn indicator or mashed a switch on your floor board under your floor mat. This was long before anything like daytime running lights. So anyway, I roamed up and down some of the shelves behind the counter at the auto parts store until I found this type switch. I put it in a little aluminum box and wired the headlight to the interior dimmer terminals and a 12V battery and it worked great. The little aluminum box got a little warm but I had a 12VDC infinitely variable dimmer. I don't see why this wouldn't work just as well for a small 12VDC motor. Now a dash dimmer switch for a 20+ year old GM car may or may not be a stock item at your local AutoZone or Pep Boys I don't know. If not I bet a GM dealer will know what you need or head to an auto salvage yard. The dimmer switch shouldn't be expensive. Remember K.I.S.S. Sometimes you need to think "Old School".

Edit: The technical term for the dimmer switch is a rheostat and it looks like this.

Re: Cheapest and easiest way to slow down a motor
 

A two-speed wiper motor actually has two separate power inputs for the two speeds. Most also have additional connections to support "intermittent" operation and stopping at a park position. If your motor doesn't have more than two wires or terminals, it probably isn't a wiper motor.

Re: Cheapest and easiest way to slow down a motor
 

do you know whatthose 2 inputs are? like 12v and a lower voltage? which is through like a resistor or sumthing?

Re: Cheapest and easiest way to slow down a motor
 

If I was to just put a resistor in series with it (provided I could get a resistor rated for that much power), how would I calculate what resistance (in ohms) I would need to take the 12V down to oh, say, 6V? I should know this but I don't remember.

Re: Cheapest and easiest way to slow down a motor
 

Kirchoff's Voltage Law: sum of the voltage drops around a loop is equal to the sources

You have a 12V source and two drops. You can get the voltage across the resistor with Ohm's Law: V=IR. Whatever's left is the voltage across the motor.

12 = IR + V (V = motor voltage)

Notice that the voltage will change with the current.

Re: Cheapest and easiest way to slow down a motor
 

So if I wanted the motor to run at 6 Volts and it draws 5 amps (estimate), I'd need a 1.2 Ohm resistor in series with it?

Re: Cheapest and easiest way to slow down a motor
 

Yup.

Edit: By the way, that would be 30W through the resistor. That'd be a very big, very hot resistor. Keep in mind that lower powered soldering irons are 25W.

Re: Cheapest and easiest way to slow down a motor
 

I don't know what makes up a dimmer switch for lights in a house or office but would that work for 12VDC? (forgive me, I'm not really an electrical guy).

Re: Cheapest and easiest way to slow down a motor
 

Apparently, they won't work. I just read an interesting article on Howstuffworks. They take advantage of the AC to turn the bulb off for a little while at the beginning of each half cycle.

I might get my head bitten off for suggesting this dreaded device again, but maybe you can use a 555. I'm pretty sure it will run on 12V. To make a 50% duty cycle oscillator, it would only take the 555, two resistors, and two capacitors. Then, you could just feed that into a MOSFET. The motor only has to go one direction right? If so, you'd just the single transistor, not an entire h-bridge. You won't have to deal with trying to dissipate as much heat as the resistor will be putting out.

Re: Cheapest and easiest way to slow down a motor
 

You mean like this http://www.allelectronics.com/cgi-bi...cgi?item=MSC-6 Seems like a neat little thing that would actually be kind of fun to try out.

Not exactly cheap or easy, but it is kind of neat.

Re: Cheapest and easiest way to slow down a motor
 

Actually, that's exactly what I was talking about, but fancier. The motor is still driven by the single MOSFET, so if it's not beefy enough, you can easily swap it out for one of appropriate beefiness.

Also, the parts you actually need are probably <$3. If you don't want to spend $15 on that thing, I'd be happy to draw up a nice little schematic for you.

Re: Cheapest and easiest way to slow down a motor
 

Cool! I sent you a PM.

Re: Cheapest and easiest way to slow down a motor
 

Sanddrag,
So many questions, so little time. You can go to a junk yard and get a couple of wiper controls. They are big resistors (low ohms but high power dissipation) and they should be easy to wire up for what you need.
The router control device you saw is like a lamp dimmer for AC motors only. It contains a Triac that switches on for only a portion of the AC sine wave cycle.
Did you need to run all motors at the same low speed at the same time? If so you could buy a variable voltage supply. If the motors don't need direction control, a pot and a power transistor will likely do what you need. Make sure that the transistor is attached to a heatsink of some type. 2n3055 should do but anything Radio Shack has in NPN 5-10 amp range should work. Try a 10K pot to start and see what results you get. I think the junk yard is the best bet.