Power expander the answer?

My team recently went to a Vex Competition in Greenville… It was well done I must say. Anyways, we ran 2 393 motors (1 for each wheel) for our power. Omni wheels in the front. When practicing at school we have a slick tile floor and never experience the robot cutting out. Well when competing on the foam floor we cut out after about 45 sec- 1 min… We had them in ports 2 and 9 so I thought we spread out the power evenly. Will a power expander help the situation? Does it help prevent motors from over heating? Also, we noticed a lot of teams had LED Displays. From what I read on Vex they diagnose problems with the robot or power? Is that worth buying? We are getting about $100 (Which for us is a lot) and we need to either invest in two of the following: Power expander, LED Display, or turn table bearing kit. Which two would help us most in the long run? Thank you very much.

well, i had a similar problem when running 4 393 and other motors at once, and when i got a power expander, it seemed to stop …

hope this helps !

It sounds like you just need to dedicate more motors to your drive train. 2 motors might work for a very light robot (under 8 lbs maybe) but for a full robot pushing the size limits, 4 motors on drive is the typical distribution. As for what to purchase, I dont think a power expander will help you much in this specific situation, but in my opinion it is the best investment if you want to maximize your power usage and really push the limits when you choose your gear ratios.


I assumed everyone used 2 motors to push their bot along? I guess I was mistaken. This is only our second event to go to. We are new in the game. So do you suggest to put omni wheels in the front but with motors as well?

No worries, it would be nice to have a picture of your bot to get a better look, but yes I would highly suggest 4 motors to move your robot around the field. By 2 wheels do you mean only 2 wheels total or do you mean only 2 wheels being powered by motors? If the former, bump it up to 4 wheels for better stability and power all 4 of them.

The most common motor distribution for robots is 4 on drive, 4 on lift and 2 on an intake mechanism although there is always deviation to fit a design’s specific needs.

Sweet Mochi,

If you look up North Lamar or 4607 in gallery you can see pictures of our robot… ALTHOUGH we have already taken it apart and did away with the front in scoop, was giving us the most problems…We are going with a “combine” intake… The ones that spin in the front and sucks them up into a plexiglass contraption :slight_smile: Ours worked really really well except for the scoop and cutting in and out… We will deff put four motors total for moving… I appreciate the help.

No, you were tripping the PTC in the motors most likely.


It’s an LCD, anyway, it’s under control of your program so it displays whatever you program it to.

The LCD Display also needs a serial Y cable (an extra $8), if you only have $100 then I would skip that. The power expander is useful if you are using all 10 motors, it needs its own 7.2V battery and slightly increases wiring complexity, a smaller, lighter robot can manage without but it does increase the overall power available for your motors by 50%.


You are right LCD, not LED… My fault…So no LED I guess… We have 3-4 batteries so I have enough if we get an expander… I only have 2- 393 motors… Should I invest in those? I noticed not just for the wheels/drive but a lot of them use 393 motors for everything… How do you prevent tripping the PTC? Thanks for all your insight guys…

Well, that’s the $64000 question.

If you search the forums there has been lots of discussion on this subject as well as (literally) hundreds of hours of tests and theoretical analysis by several forum members.

The short answer is that you need to keep the motors running at 65rpm or above when sending them full power, if the motors are running slower than this (again when sending full power) then the current being drawn will trip the PTC, not straight away but it will happen. The worst case condition with a 393 motor is when they are stalled and full power applied, the PTC will trip in under 2 seconds.

How do you control the rpm of these motors? I use Easy C… I know you have the -127 and 127… But I really haven’t messed with those settings much.

Sorry, you misunderstood what I was saying. When you send the motor a control value of 127, the motor will run at an unloaded speed of approximately 100rpm. As the load on the motor increases, that is you use the motor to do something useful like power the robots drive system, the speed will decrease. As the load on the motor is increased the motor will eventually stall, ie. stop moving. The graph showing this relationship has been posted here.


Many things will determine how much power is required from the motor, the weight of the robot, the gearing, friction (as you have found). All of this is difficult to calculate and is often unknown until the robot is finished. So start by measuring the speed of your drive motors with the design you have, then based on the graph calculate the theoretical torque they are generating and how much current is being drawn from the battery. With these numbers you can determine a course of action.

Thank you so much… So instead of hitting 127 full speed all the time, change it to a lower speed so it doesn’t trip or over heat as fast?

Just dropping the maximum control value from 127 to some lower value may help but is not the real solution. What you will most likely have to do is either;

Use a different gear ratio so the motor works at higher rpm but still (via the gearing) produces the same torque at the wheels, if you do this you drive will be slower.
Add more motors.

Back to your question of whether to get LCD screen, turntable bearings, or power expander. I would say to get the turntables, our club has found that they slip a lot less than just using gears (maybe others have seen differently).

As far as the 393s go, it would be wise to stay around 4 393s per power source (microcontroller or power expander). If you use too many 393s, you’ll find that you need to recharge a lot more. Our team is using 4 393s on our lift and 4 269s for chassis. Since the sacks weigh a lot (compared to other years), the extra power is nice to have on your lift, rather than your drivetrain.

Anyway, I would get the LCD screen if you’re doing lots of programming, particularly if you have more than one autonomous. You will not see a performance gain from your robot. The Power Expander would become useful if your team had a lot of power drain (more than 4 393s, etc.). So unless you purchased more 393s, the power expander would be useful. However, you have a $100 budget, and I personally believe you won’t find as much of a performance gain with the 393s as you will with the turntables.

By the way, if you do decide to get the turntables, be sure to get two of them. You can put the larger ones for your lift and the smaller ones on the tray.

You can put motors on the omnis. But another alternative is to “double up” or even “triple up” motors like this, then couple your drive wheels to the omni wheels using gears or HS chain (low strength chain is not recommended, as it breaks easily). This allows the motors to be in one place, so the front motors don’t gobble up space that could be used for something else (like a sack collector).

P.S. We eventually pushed back/trimmed the short axles in the picture, because they were “in the way.”
P.P.S. We also have 2 HS motors and are waiting for $60+ to buy 3 more (2 for a 4-motor drive or lift, one for a spare, as we once had 3, but 1 broke a pin at a critical time). Hence, our coupling of “old” motors.

We have found that we need to charge our batteries often. A lot more than with regular 269 motors. We have 10 393’s on our robot, and it kills the battery like no body’s business.

Off-Topic: Will I see you at Border Wars?

We will not be at Border Wars: it’s the weekend before our finals.

Ah. That stinks.

Were you at Zionsville? I don’t remember formally meeting you.

Describing your current robot might help us help you.
Of the things to purchase, Probably more motors would help you the most, if you don’t already have the maximum. My view is that adding more motors to the same load (eg drive wheels) actually reduces the current used for that load.