Motor Wires

I was working on my winch bot…and the vex motors are too slow! so now i’m trying to use a high speed motor from a toy car. i tried hooking it up to the microcontroller. i know the red and the black wires are the power, but what is the white wire for? i only put two wires in the power ports, but when i turn my robot on, it runs automatically. i want to have full control over it. what is the white wire for, and how do i wire it? it’s a DC motor with two terminals, like the ones you’d find in a toy.

I don’t know much about circuits, but i do know you can buy the vex motor controller to convert dc to pwm

yeah, the vex microcontroller outputs a PWM signal, which needs a speed controller to convert the pwm signal into varying voltage levels on a dc motor.

DO NOT connect the DC motor to the microcontroller directly, because i believe that the DC motor will pull too many amps and damage the microcontroller.

This topic has been mentioned in several posts. All you need is a speed controller to control the DC motor. Parallax sells the HB-25 motor controller which works perfectly with Vex and can handle very high amerage motors. All you do is connect the motor ports to the HB-25 and the HB-25 to the Vex controller motor inputs. Here is the HB-25 controller. Its a bit expensive but it works great.


The Vex motor ports are protected to prevent more than 4 amps being drawn. So you shouldn’t be able to damage the controller by hooking a small motor up to the red/black wires. As octanetripledax noticed, though, the red/black wires are always powered. The white wire is the control signal, but it is not able to directly drive a motor.

Since this is a toy motor, the HB-25 may be overkill. Perhaps a better starting point would be the Vex motor controller which is good for an amp or two. Scroll down to the bottom of this page to find it.


  • Dean

for the vex motor controller, what does it mean when they say “The Motor Controller is thermal limited to about 1A”? does this mean it will only allow the motor to draw 1A?

Yes - They use a kind of fuse that allows current to flow until the load passes a certain level. It then heats up and (mostly) stops conducting current. When the load or short-circuit is removed, the current stops flowing and the fuse cools down, allowing normal operation.

The more current you draw, the quicker the fuse heats up and shuts off the power. I’ve seen these called “PPTC fuses”, “thermal fuses”, “resettable fuses”, auto-resetting fuses". Here is a typical example.


  • Dean

I’m pretty sure that the microcontroller trips at an overall current 4A, but each individual port trips at 1A. Correct me if I’m wrong.

We just discussed this here, and John V-Neun cleared things up: There is no per-port current limiting, just the 4A overall current limit.

The confusing information that lead folks to think there is a per-port current limit was on the “Current Draw” line of the Micro Controller specification page, but that is referring to how much a Vex motor draws, not how much the controller can supply. [EDIT]It looks like the text there has been improved to make it more clear (thanks John!).[/EDIT]


  • Dean

It’s the other way around. The white wire is the varying voltage which the speed controller converts to PWM. It isn’t good to vary the voltage to control the speed of a motor, instead you turn it on and off really fast.


Actually, it is both - the white wire on the Vex motor port is a low-power PWM signal. A motor controller uses this signal to decode the intended speed and drives switching electronics to run a motor. This is usually PWM too, but it can be variable voltage or current, pulses (for stepper motors), etc…

See my post here that outlines the two flavors of PWM that are commonly used when talking about DC motors.


  • Dean

if you are good at wiring, and are ok risking it, you can just use the controller inside one of the vex motors. To do this follow these steps:
You Will need a soldering iron and a ph1 screwdriver.

1: Take the 4 screws out of the top of the motor and remoce cover.
2: remove all gears to reveal motor head.
3: remove the 2 small screws which hold the motor in place
4: turn the motor over and remove the 4 screws holding the rest of the motor together
5: remove the green cover to reveal the circut board.
6: put gently to remove the board and motor
7: on the opposite side of the motor, there are two big solder spots, use the soldering iron the heat up the solder, and pull the motor away from the board.
8: strip the ends of the wires on the new motor and twist together.
9: stick the wires through board and fold over.
10: use the soldering iron and solder the wires to the board
11: either put the board back in the motor encasement, or use electrical tape to cover the circut board.
12: test to see if it works

this has worked for me, and it costs only $20 for a vex motor rather then $50 to $150 for a speed controller

Happy Building ^.^

  • i assume no responsibilty for damage to your system if this is done incorrectly
    it worked for me
    if you are not that experienced with a soldering iron or electronics, i would recomend that you do not do this, or have someone do it for you.

IFI has already done this for us, and sells it for $10. Scroll down to the bottom of this page to find it.


  • Dean

ohhh, i see… thats new
very nice =D
the this is thou that this can only draw up to 1A before triping, and chances are that the motor he is talking bout probably draws on the high end of 1A, maybe even much higher, with my rig the motor can draw upto and including 4A


Have you actually measured that? I suspect you won’t be able to draw more than 1A for more than a few seconds.

This $10 adapter is actually just the motor board from a Vex motor with wires and shrink-wrap added (pics here). Both it and the Vex motors are limited via a resettable fuse to about 1A. The little part in the upper right corner of the 2nd pic with what looks like a “7XL” marking is the current limiter. Unless you’ve bypassed that part, you are probably current limited too.


  • Dean