Talon SRX Passing Power to Motor without code?

Hi all,

I am having a problem with two of my Talon SRX motor controllers. Each controller is plugged into a 40A terminal in my PDP. One of them has an encoder wire plugged into a vex integrated encoder module on a versaplanetary gearbox. Both motors are double-sided taped to 8020 aluminum. When I connect them to their respective motors, they power said motors even though there is no code currently on my roboRIO because I just re-imaged it. I took a multimeter to both controllers and read 11.7V coming from both. When I replaced one of the controllers with a spare I had, the problem was fixed. However, the project I’m working on has a tight schedule, and I do not have any other spares. I have tried recalibrating the motors by pressing the B/C Cal button and following the instructions on the Talon SRX user guide. How can I fix this problem? Is it possible that VexPro installed a normally open relay onto these controllers?

We had an issue where the Talon SRXs would power the motors even when the robot was disabled (although the robot was on). We found that we plugged in the Talon SRX backwards (motor side to the PDP and vice versa).

Fixing this also solved the issue.


Fixed the issue.

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I’m assuming your situation and solution was similar to what @Prateek_M described?

If this is the case, that Talon SRX unit is damaged and we highly recommend not using it/replacing it as soon as possible.

When you apply power directly to the motor controller output, internal components are irreversibly damaged. The damage may not be apparent immediately - it’s possible the motor controller will appear to be working as expected. At some point, however, continued use with the existing damage will cause the motor controller to fail completely and stop working.


This has happened a couple of times on our team. Our expirence has been that if the motor is never commanded when the Talon is powered by the motor leads then the Talon is not damaged. I’m nearly certain we’ve used them for the rest of the season with out any problem…So does powering through the output instantly damage it or does it have to be commanded (throttle not 0) while wired incorrectly in order for the components to be destroyed?

I was in this school of thought as well, noticed it on our 2017 practice bot, and I even labeled it to see what would happen.

3 practice bots later and it seems to be running fine.

Damage can occur when voltage is applied to the motor outputs regardless of whether or not you attempt to drive the Talon.

The only time damage may not occur is if the Talon is both not commanding output and in coast mode.

If you must keep using the Talon, I highly recommend only using it in a situation where failure isn’t catastrophic, like a test bench setup.

Out of curiosity, what components are damaged? All the usual suspects (the many ICs required for the talon to function normally) seem to work fine.

Is a Talon damaged whenever it is used as an electric brake? If not, how is the effect of voltage applied by a battery so different from voltage applied by a DC generator?

Jacob, often when we push our robot down the hall the motion is enough to power up components. Is that safe, since it also powers the motor side of the connection? I’ve always assumed it was ok since so many people do it.

Specifically, the components in question that fail are select transistors in the H-bridge.

Whenever the transistors are active while battery voltage is applied to the motor controller output, a very large amount of current is drawn through the transistor, causing it to be damaged. In some cases this causes immediate failure while in others it causes damage that will manifest as a failure sometime in the future.

There is one specific condition where the transistors aren’t active - when the motor controller is both (a) Not commanding output and (b) in coast mode. In this case there is no current draw and no damage to the transistors. My earlier statement about damage regardless of output applies to brake mode.

In the case of backdriving a motor, the scenario is the same but the current being generated is usually not enough to cause damage to the transistors.
In my earlier statement I should have specified battery power, not just voltage, as it’s the excessive current available from the battery that ultimately causes the damage.

@Tom_Line this is the reason that you can push your robot down the hallway and your motor controllers will be OK, as the motors can’t generate enough current in that case to cause damage.


Hi Tom,
Pushing a robot across the floor is not ideal for the electronics. CIM motors and NEO motors are excellent generators and that resultant voltage does rocket across the wiring plant. I have demonstrated this very issue to our students using an Oscilloscope on a spare PDP channel.

We coach our students and mentors to limit pushing a robot on the floor to the greatest extent possible. This is not always easy to do ,but it’s in the best interest of the long term health of the electronic devices.


What peak to peak voltages did you observe?

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Those peaks vary with the push force applied. Mostly between 2 and 8 volts. Though with a jolly good push over distance, I imagine the electric plant could see some peaks approaching 12 V.

It’s not uncommon to see LED rings illuminate this way and that is evidence of voltage surges approaching 12 V.

The point of my short duration demonstration is to illustrate to students what happens when you push a robot across the floor so they understand why this is not a good idea.

One other notion to keep in mind is when the main breaker is open that battery is not available to even out these things.


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While it is possible to blow out the motor controllers by pushing a robot across the floor, you would probably have to push it much faster to approach the maximum voltage ratings of the various motor controllers (24 V for SPARK MAX, 28 V for the Talon SRX, 16 V for the Victor SPX).

I agree with you for the most part.

With that said, using our 2019 bot as an example (16 motor controller with a mix of Talon SRX’s, Victor SPX’s and SparkMax’s). At $90, $60 and $70 each.

Due to the potential cost of the back EMF experiment we didn’t want to push the envelope too hard on a fully functioning bot.

I don’t think this is actually a serious concern - as noted by the manufacturer who commented to that effect in this thread. Seeing voltage back fed into the speed controller isn’t a huge issue - you are not generating a ton of current which is what does the damage. This is not a problem, you can push a robot around on the floor and backdrive the wheels short distances. Teams constantly do this.


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