5v Brownout Issue

At our first regional last week, our robot appeared to brown out several times during matches, but the battery voltage never dropped below 10V. After looking at the data log in the driver station, we discovered that we were having anywhere from 20 to 40 brownouts on the 5V rail per match. We’ve checked the wiring and breakers, but can’t seem to find the issue. We have 3 encoders, a nav-x MXP, a USB webcam, and several PWM’s connected to the roboRIO. What might be the issue? Could there be a problem with a motor controller or an encoder, or do we need to replace the roboRIO?

Thanks for your help.

> the battery voltage never dropped below 10V

I’m curious how you are sure the voltage never dropped below 10V. My belief is that it is dropping below 10V (and that appears to be proven by the data in the log, since a brownout is triggered by a voltage level below a fixed threshold). In my experience this happens most often when the drive system transmission ratio is such that the robot is driving like a bike in a high gear - which requires lots of current to (a) get started moving, (b) change directions or (c) stop/slow down (if “brake mode” is enabled in the motor controller).

The voltage drop can occur so quickly that a human can’t see it at the driver station.

Our team has experienced this before, and learned that the drive system transmission ratio must be matched well to the expected driving behavior - or very rapid dips of multiple volts can occur especially in case (b) above. If it’s too late to change the transmission ratio, the workarounds are (a) train the drivers to avoid quick direction changes and (b) ensure you have a fresh battery (>13V) before a match starts.

Do you happen to have a good picture of your drivetrain? Also, do you remember which matches you were browning out?


Sounds like an intermittent short somewhere.

I doubt you’d exhibit any brown-out symptoms for main system voltages above 7v. At least we don’t!

Things to check:
-All 5V connections (no tinned ends in the spring terminals!)
-Frayed wires that could short around the spring terminals
-Loose mini bussman fuse (they need a lot of force to seat)
-a 5v device with a grounded body (not uncommon) might be shorting somewhere

Also, just doing a tug test on all connections to make sure they’re all properly seated/crimped every now and again is good.

Brake mode does not draw any current from the battery.

If the condition is occurring in the RoboRio power supply, then a list of possible causes is in no particular order:

  1. Not wired to the dedicated PDP output for the controller
  2. The fuse on the PDP is not fully inserted in the terminals
  3. Wire size is too large for the push terminals, should AWG 16-18
  4. RoboRio power connector is not tightened on the wires and not locked into the RoboRio.
  5. Drawing too much current from the servo supply (6v) buss.
  6. SWARF inside the RoboRio


The motor’s speed generates current in the motor coils, which creates torque to oppose that speed (Lenz’s Law).

There are other work-arounds, such as slew-rate-limiting the joystick commands in software, voltage ramping, and other more exotic schemes.

Does the RSL light stopped blinking, then start back up? that is a sign of processing stoppage.

Last year our team did not properly crimp the battery cables. The cables were also under tension. It was my first year working with FRC teams. The senior student programmer told us it was a software glitch that caused the program to stop, (RSL light stopped blinking). No, it was a brown out.

This is the most likely culprit. We had the same problem at the Dallas regional, which turned into a full blown short. Had to pull the Rio out, take it apart, and clean it out. There were a few bits of Al rattling inside, and it sorted the problem out.

A little additional technical info regarding the other suggestions though:
The only 5V rail that you can monitor is the one that the Rio generates internally from the wide Vin input. As such, if the other rails are fine, then the power going into the Rio is likely fine as well. All of the PWM outputs are 6V as Al alluded to, so there isn’t an issue with the motor drivers. Your GPIO pins and MXP port are the most accessible 5V output, but the USB port and other comms ports are open to shorts as well. It’s worth using something to cover all the gaps to keep scrap metal shavings where they belong (outside the Rio).

Ouch. Did you get Al the first aid kit after? I’m glad he went the extra mile and inserted himself into the situation though.