Cheapest and easiest way to slow down a motor

[sciguy125]

Originally Posted by Al Skierkiewicz

If the motors don't need direction control, a pot and a power transistor will likely do what you need.

Wow, after I was toying around with a nearly identical idea for this thread, you'd think I would have figured that out. You'd also think that after having done exactly that to control a motor a few weeks ago, I'd figure it out...

[sanddrag]

Al,
That attached diagram looks much simpler than what I was going to do (pwm control with a 555 timer) but not being an electrical guy, I'm going to need some pictures that look like the real thing (not just symbols) to help me understand how to hook it up. Also, does your setup get much hotter than the other more complicated way?

[Al Skierkiewicz]

There are only three parts, the battery on the left, a pot in the center and a transistor on the right in series with the motor. The transistor will need to be mounted to a heatsink because it will get hot. As you move the pot, a varying amount of current will be supplied to the transistor base thereby controlling the current in the collector/motor. With the wiper all the way up at 12 volts, the motor will be at max speed. Start with a 10K pot but you may need a 100k pot depending on the range you need and the gain of the transistor.
As an alternative, you could use the speed control from a 12 or 14 volt drill too or the motor control from a car blower which is usually three resistors and a switch.

[Gdeaver]

You already have a great DC 12 volt speed control in your possession. A cordless drill has a pretty good electronic speed controller. However, the problem is you want to run the motor continuously for many hour. It may be better to let the motor run at full speed and use belts to speed reduce. A 1.5" pulley and a 3.5" adjustable pulley would give a good range. A couple of shaft inserts would get you from a .5 shaft down to the van door motor shaft.
To bad you don't live in the frigid parts of the country. Heat build up wouldn't be a problem.

[sanddrag]

I've decided to try this out http://www.dprg.org/tutorials/2005-11a/

I have one question though, where is exactly is pin 3 on the 555 supposed to connect to?

[DonRotolo]

Wow, that's quite a thread.

1. To reduce the speed of the motor, cut the voltage. R=V/I, if you want 6 volts to the resistor and I = 4 Amps (You measured that?), 1.5 Ohms. A dozen feet of 18 gauge wire will cover that pretty well, just coil it loosely so it can dissipate the heat. BUT all this is OK only if you can accept significantly reduced torque and power.

2. To control the speed to any value, and not lose torque or power, use a PWM motor controller in "Locked Anti-phase" mode. Victors (at leats 2005 version) don't do this, but something like that should be easily findable, try a google search or visit IFI and other robotics sited. PM me if not.
(FYI: Locked Anti-Phase control switches voltage polarity quickly, so the motor actually has full current at all times (max torque) but at, say, 50% duty cycle the net current flow over time is zero, so the motor stands still (hums, actually). At 75% duty cycle, it moves forward around half speed, but again with full torque.)

3. To control the speed with a Victor, just build the circuit I posted near the end of this post and get a victor, or similar. You do lose some torque though, but it seems to work for most FIRST teams...

4. Modern cars use a special motor for the wipers, with two windings, one for low and the other for high speed. Both windings take 12 volts. Resistors haven't been used for decades.

5. The cordless drill idea from Gdeaver is pretty good, though.

[Alan Anderson]

Originally Posted by sanddrag

I've decided to try this out http://www.dprg.org/tutorials/2005-11a/

I have one question though, where is exactly is pin 3 on the 555 supposed to connect to?

The schematic near the bottom of the page you linked to gives the answer.
Pin 3 connects to the wiper of the 100K potentiometer. The commentary after the schematic explains why.

[sanddrag]

In Al's drawing of the simple pot and transistor setup, how do I know which way to oreint the transistor so the pns go to the right places. I know where the middle one goes, but how do I know which way the others go, or does it not matter? Does that trapezoidal shape represent the shape of the actual transistor? Does the arrow signify something?

[Tom Bottiglieri]

Buy a dimmer switch from Home Depot.

[greencactus3]

Originally Posted by Tom Bottiglieri

Buy a dimmer switch from Home Depot.

do they have DC ones at homedepot? they tend not to have anything for 12V....at least from what i can see.
car dimmer switches are pretty cheap on ebay too..

[sciguy125]

Originally Posted by sanddrag

In Al's drawing of the simple pot and transistor setup, how do I know which way to oreint the transistor so the pns go to the right places. I know where the middle one goes, but how do I know which way the others go, or does it not matter? Does that trapezoidal shape represent the shape of the actual transistor? Does the arrow signify something?

I've attached a labeled transistor symbol.

If you've got the 2N3055 that Al suggested, then hold it such that the pins are facing you, then rotate it so that the pins are closer to the left side (see P.4 of the linked datasheet). The lower pin is the base, the upper pin is the emitter, the case is the collector.

If you have the TIP3055 that I mentioned to you, hold it such that you can read the writing on its face and have the pins pointing down. From left to right: base, collector, emitter. The tab is also connected to the collector.

There's also an issue that someone, maybe Al, can resolve. I noticed that the gain with Ic = 4A is only 20-70. That would require a base current of 200-57mA to get up to the 4A. With 12V, your pot would have up to 2.4W going through it. That much power seems a little disconcerning to me. The readily available transistors with higher gains can't handle enough current. Any ideas?

[Al Skierkiewicz]

Originally Posted by Don Rotolo

2. To control the speed to any value, and not lose torque or power, use a PWM motor controller in "Locked Anti-phase" mode. Victors (at leats 2005 version) don't do this, but something like that should be easily findable, try a google search or visit IFI and other robotics sited.

Don,
Victors are in "locked antiphase" operation. They are a classic "H" bridge with one input signal representing both magnitude and direction. You could set the input signal to switch direction as rapidly as the PWM allows but the current draw is significant. Since that is not needed in most of our applications, you won't see it very often.

Sandrag,
your 555 circuit is an excellent method, just be sure that the MOSFET can handle the current for you motor. The spec sheet for the device shown says it is good for 50 amps if it is at 25 degrees C. It might need heatsink but any chunk of aluminum should work if it is insulated.