Can one backfeed power to the uCtlr through a motor cable?

Good morning, folks!

I’m trying to save some wiring weight and power loss in my current project and am wondering if one can power the uCtlr by backfeeding power to it through a motor cable.

My understanding is that the motor/servo ports provide raw battery voltage on their power pins. If this is a simple conductive path to the incoming power bus (where the battery cable connects), then one should be able to feed power to that bus (and, thereby, everything else) from the motor end of a cable.

Here’s why I’m interested in doing this:
My design has two subassemblies (feet), each with two motors. For balance reasons, it will be best to place the batteries near those motors. The controller and receiver, similarly, will work best if placed on a third subassembly (pelvis). (Yes, I do realize that I’ve just located the “brain” in an idiomatically popular piece of anatomy. ;))

I could run power cables up the legs to the uCtlr, then have the uCtlr send power (along with motor control signals) back down the legs. However, that will require installing a set of power wiring in each leg and will require that all the current to run the motors travel twice the leg length.

What I’m hoping to be able to do is connect the motors to the batteries inside the feet, so that the motor current doesn’t have to go up to the pelvis and back. I’m confident, from various things I’ve read, that that should work reasonably well. Furthermore, if I tie the two batteries (one from each foot) together, that should balance the differences in discharge curves.

What I’m asking is: If I tie a battery’s voltage lead to a motor cable leading both to a motor module and back to the uCtlr, will that have the effect of providing power to the uCtlr? The total load I foresee on the uCtlr is much less than the 1 A that a motor can draw, as the power going through the uCtlr will be feeding only that box and a few passive sensors (bumpers & limit switches).

I’d appreciate any thoughts you can provide or experiences you can relate, preferably before I liberate an "vital smoke’.


Yes this will work, but there are a few caveats:
*]Wiring it this way will bypass the power switch on the Vex microcontroller. You’ll have to add a switch inline with your battery, or just unplug the battery to turn things off.
*]You will lose the over-current protection that limits battery draw to 4A. The vex motors have internal 1A protection, so they should be fine. But, be aware that a sharp edge cutting into the servo cable that carries power to the Vex can create a direct short across the battery and allow much more than 4A to flow. (sparks, melted wire, fire, etc).
*]Never ever plug your back-feed servo cable into an I/O or interrupt port. Doing so will probably end your microcontroller.


  • Dean


Thanks for the prompt reply & important reminders!

I hadn’t thought about that, but I tend to disconnect batteries when I’m shelving this sort of thing.

What, you’d deny this old man one of the few chances for excitement he has left? :wink: Seriously, though, it’s an important point. If, however, I were to use a more traditional wiring scheme, I’d have a similar exposure on the long leads from the batteries to the uCtlr. What I should do is drag myself to an auto parts store and pick up a handful of in-line fuse holders and 5A slow-blow fuses and put them in the battery extension cables near the battery ends.

Thanks for the reminder, but I figured the power pins on the I/O and interrupt ports were on the load side of the regulator for the uCtlr and wouldn’t tolerate raw battery voltage at all well.

Thanks again,

Quazar, not sure what happened but the numbers 1., 2., and 3. are appearing outside the margins of the site. I’m using Google Chrome.


Same thing with Firefox.

This doesn’t sound worth it to me.

Why not just get or make a long Y cable, put the batteries in your legs like you plan and then use the Y cable to merge the two batteries up at the pelvis.

Those servo wires are very small guage and a 6 or 7 cell NiCD/NiMH battery can supply a huge amount of current. It just doesn’t sound safe for your electronics or wiring at the least and at worst you could end up with plastic on fire catching your cat on fire which runs around setting everything else on fire.

Saving the electrical trip up and back down the leg of your robot just doesn’t sound worth it.

I agree - this is what I would do in this situation. If anybody is in need of cheap high-quality pre-wired battery connectors, I recommend these and these.

Regarding the numbers outside the margin, I saw that in the preview window before I posted.
I generated those using the “Ordered List” button, so I didn’t embed anything weird.
Must be a CSS bug…


  • Dean