"Y" Cables?

I was looking into the VEX hardware. i noticed the “Y” cables which say you could power 2 motors with one output, which would be handy. but would the cables split the power to each motor in half, or is it wired so that it will pull double the current needed to run both motors 100%?

Always remember to search before posting a question.

Here are similar threads found by searching for Y-Cables Power:

Specificly this post should answer your question:

The latter. Using a Y cable will not reduce the power going to each motor. However, the Vex microcontroller is limited to 4A total for all motor outputs. You have to avoid driving too many motors at the same time, but it doesn’t matter if they are on Y cables or not.


  • Dean

You can usually take what Dean tells you “to the bank”; but this time I think he overlooked the current limit each port has.

On the Vex Microcontroller we are talking about, each PWM port can only supply 2 Amps, if two motors/servos on a Y-Cable attempt (when added together) to pull more than a total of 2 Amps, they will be starved for current and will not be able to produce as much mechanical output as they could have if they were attached to two separate PWM ports.

PS: Dean is 100% right about the 4 Amp limit across all of the Microcontroller ports.
PPS: Corporalchee is right - Search first - The message he pointed out has this described a little more completely.

I’m happy to be corrected, but in this case I don’t think there is a 2A limit per port. From taking my controller apart and studying it, all the power/ground pins on the PWM ports are bussed together and fed via a 16V/4A resettable fuse. There is no per-port current limit built into the microcontroller as far as I can tell.

2A may be all that the connectors/wires are rated for, but the controller will happily provide up to the 4A limit. For some experimental verification, I checked and was able to draw 12A through a single PWM port for about 10 seconds before the thermal fuse shut it down.

Looking over some of the references you sited, I see the 2A number being asserted (and even a 1A number in one post), but all the official FAQs and spec sheets only mention the 4A number. Where did the 2A number come from?


  • Dean

On this page in the row that says “Current Draw” the following is located: “5mA to 2A per Motor”.

I’m fairly sure that is the draw range of the Vex motor, not the supply range of a motor port: 5mA while stopped, and 2A while stalled at full power.

That same line gives a different range for servos (20mA-1.5A). If that is a per-port limit, that would imply the controller knows if a motor or servo is plugged in and limits the current differently.

Perhaps somebody from IFI can clarify,

  • Dean

Yup I see what you mean.

However I could not find anything else that said anything about limiting current so I think you may be right. The only other limit that I could find on that page is the 4A.

The VEX Microcontroller has an overall limit for the motor ports which will trip at approximately 4-Amps.

Each VEX Continuous Rotation Motor & Servo has a limit which will trip at approximately 1-Amp.

Both of these limits will reset once they are unloaded and given time to “cool down”. Under continued load they will NOT reset.

After tripping once it will be easier for them to trip again, until they have time to FULLY cool down. Thermal factors will affect these limits.

Note: Each motor is capable of drawing 1-2 Amps when stalled, but not for long. :wink: Good thing that 1-Amp limit is in there.


Mmmmm - Yummy Crow:)

On that page http://www.vexrobotics.com/vex-robotics-design-system.shtml
Under the heading of Vex Robotics Design System - Microcontroller Specifications
Under the subheading of General Features
On the row you and Corporalchee are discussing (Current Draw)
I presume I too was lead astray by interpreting that “5mA to 2A per Motor” statement to mean that the Microcontroller could supply a max of 2A to any motor through Microcontroller PWM output port.

Because crow really isn’t very tasty, I think that web page could use some editing, as could (at least) these two old misleading posts, to incorporate JVN’s info into it.
Post 39 in this thread https://vexforum.com/t/tomahawk-help/13498/1&highlight=Y-cables+power
Posts 3 and 5 in this thread [https://vexforum.com/t/tomahawk-help/13498/1&highlight=Y-cables+power


There is no crow to be had here. You were posting the best information you had available, and could very easily have been right. Your post was gracious and professional, and the end result of the thread is that the community gained new information.

Please don’t hesitate to correct me (or anybody else) if you see questionable info being posted. Everybody on this forum has the opportunity to be both a student and a teacher, and we can sometimes be surprised to discover which role we are playing :slight_smile:


  • Dean

and just another question, as i don’t want to start another post, if one motor is being stalled, will the electricity find too much resistance in the stalled motor and give all the power to the second motor which is running with no load? or is it not just as simple as soldering two cables to one, is there a circuit board that always gives the same amount of current to each motor?

Unfortunately, it is just the opposite. A stalled DC motor represents a very low electrical resistance.

If you have two motors hooked to a Vex microcontroller, both set to full speed, but one is stalled: The stalled motor will try to draw a large current (several amps). The increased load on the battery will cause its voltage to drop some. The lower voltage will cause both motors to draw proportionally less current; the stalled motor will still be stalled, but the free motor will slow down as a result.

After a second or so, the current limiter in the stalled motor should trigger and significantly limit the flow to that motor. The reduced current demand will cause the battery voltage to go back up and the free motor will speed back up. Once the stalled motor becomes free, its current demand will return to normal and the current limiter will reset, allowing the motor to operate again.

In simple circuits, E=IR (or Volts = Amps x Ohms). In this case, the resistance (ohms) of the motor is determined by the mechanical load placed on it. Volts is whatever the battery voltage is. Therefore Amps will automatically vary as the load on the motor changes or as the battery runs down.

You can’t directly control the amps that you feed to a motor, and the motor itself sets its resistance. All that is left in the formula is volts. So, to control the current that a motor draws, you monitor its current draw, and adjust the voltage you are feeding it accordingly.

If you wanted to play around with a current regulator, this page shows a fairly simple design (see “LM317 / LM338 / LM350 Current Regulator Calculator”). You would need one of these per motor that you wanted to regulate.

If you used a 1.25 ohm resistor for R1, that would limit the current to about an amp. If the motor tries to draw less power than 1A, the regulator will try to increase voltage to compensate - up to the positive battery supply. If the motor tries to draw more than 1A, it’ll lower voltage to compensate - down to zero volts if necessary.

The advantage of this design is that the stalled motor never “gives up”. It will constantly try to free itself. The disadvantage is that you are burning 1A on a motor that isn’t doing any useful work. Also, Vex motors can (and do) draw more than 1A for brief bursts without triggering the current limiters. The current regulator circuit would react much more quickly and would therefore limit the motor’s peak power.

Also, if the current regulator has to drop the voltage too low (because a motor is stalled), it may cause the microcontroller in the motor to reset, which will cause the motor to pulse on and off. This may be handy to free a stuck wheel, but it is also probably hard on the motors.

The current limiters (PTCs) that Vex uses are a good compromise between fuses and current regulators. They allow brief bursts of high power, but they don’t allow continuous draw that would damage the motors. And they don’t need replacing or manually resetting like fuses and circuit breakers.

Whew - that was a long-winded reply. I hope it was of use to you…


  • Dean

thanks! don’t worry, it was of use. i know i don’t have enough Y cables (or motors for that matter), but could you run 32 motors with one microcontroller? or would that starve each motor of so much current that it won’t be able to even spin? motor out to Y cable, and two Y cables onto those outputs…repeat for all other ports… that would be pure awesomeness!

and if you’re going to write that on that huge post on a forum, would you mind writing my english essay? lol

Hmm. Lets do the math… A Vex motor running full speed (no load) draws about 150ma. So 0.15x32 = 4.8A. This is just over the 4A limit, so it would work for a while until the Vex microcontroller PTC tripped.

Also, motors tend to draw a spike when you first start them. I don’t have a good way to measure peak current, but my power supply meter touched at least 1A when I reversed a Vex motor (full CW -> full CCW) even with no load.

So starting all 32 motors at the same time would draw up to 32A, which would brown-out the controller and make it reboot. You might be able to ramp them up to speed slowly, but that is still at no load.

As soon as you apply a load to the motors, the current quickly goes up.

Heh - Writing about electronics is easy; english is a whole 'nuther story. :rolleyes:


  • Dean

If you’re ever going to run that many motors at a time, consider wiring them to get power from a secondary battery.