Thread: Interesting Jaguar Death
View Single Post
  #14   Spotlight this post!  
Unread 22-01-2013, 18:39
dyanoshak dyanoshak is offline
Registered User
AKA: David Yanoshak
FRC #2158 (ausTIN CANs)
Team Role: Mentor
  
Join Date: Dec 2007
Rookie Year: 2007
Location: Austin, TX
Posts: 191
dyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond reputedyanoshak has a reputation beyond repute
Re: Interesting Jaguar Death
It took me a while to remember, but I have seen these symptoms before.

When there is a short on the 5V rail inside the Jaguar the 5V DC/DC converter can't come up. It tries to turn on, but it quickly shuts off due to its over-current shutoff. This shutoff is inside the DC/DC converter and has nothing to do with the software current limit in the Jaguar firmware. After the shutoff time is over, the converter tries to power on again, repeating the whole process.

The clicking you hear is the inductor changing shape due to magnetostriction* every time the converter tries to turn on.

The fan is powered by the 5V rail so it twitches at the same period as the shutdown time.

Many things can cause a short on the 5V rail, but one in particular stays even after you disconnect all the wires and clean out the debris from your Jaguar.

There are several ESD suppression diodes connected to the sensor inputs on the front of the Jaguar (added after Grey Jag and Lunacy ). If one of the pins experiences a static shock, it shunts the discharge to ground and hopefully protects the microcontroller. The diodes can handle high voltage (several kV) for a short amount of time, but if they experience a much smaller voltage for a longer amount of time (6V for these diodes in particular), they can fail in the worst way possible; short-circuit. There is one diode in particular that is connected to the 5V rail and if it fails, you now have a short from 5V to ground.

So, what can cause this diode to see more than 6V? Debris is one way. It is definitely possible that a piece of debris bridged 5V and some higher voltage rail.

I'm not sure where the melted motor terminal plastic fits in to the story. It may not even be related to the failure.

You mention this Jaguar was from the 2012 KOP. You seem to have debris under control this year, but can you guarantee that debris wasn't introduced last year? Was this Jaguar tested and guaranteed working right before adding it to your 2013 chassis and noticing the failure?

-David

*Thanks EricVanWyk for my new word of the day!
Last edited by dyanoshak : 22-01-2013 at 18:42. Reason: props to EVW