Quote:
Originally Posted by Al Skierkiewicz
Tech,
The chip is called a shunt current monitor in that it was designed to be used in that application. The resistor in the Jag is merely in series with the output and as such is not really a shunt since it is the only path through which current flows. It is a convenient chip to use in this application since the gain is fixed, it is designed for single power supply and can swing to within 0.2 volts of the positive power rail. The output current of the battery is speced at around 600 amps for a few seconds and is primarily limited by the internal resistance of the battery which is .011 ohms. While it is easy to say currents in the Jag are normally around 50 amps, that is not correct. Many teams design mechanical systems that draw significantly higher currents. There has been at least one post in the past few days where the team started out draining their battery in less than two minutes. For the black Jags, the manufacturer states that the current monitor will fault the device based on current over specified time. I believe that the black Jag is more aggressive at current monitor due to the reduction of MOSFETs in the output stages. The Victors do not use a current monitor for a variety of reasons. I believe this is due to the fact that the sense resistor limits current delivered to the load, adds to the parts count, and is not needed with three 40 amp FETs in parallel in each leg of the output circuit. While the FETs have a higher series resistance than the Jag, in most of our applications, the added resistance is minimal as it amounts to about the resistance of two feet of #10 AWG wire. Please remember that the power rating on a resistor is based on it's temperature rise and sustained temperature over time. A 1/2 watt resistor can handle 10 watts for a few seconds while it can never handle 1 watt over a several days.
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You are absolutely correct. How long these currents are drawn is a vital factor with regard to the resistor (hence why I noted that using this power calculation is cheating to some degree). One of the problems being that outside the case of Jaguar you can't see what's going on with that resistor. There is to my knowledge no measurement of it's temperature and you can't make contact while it's assembled or even measure that temperature with infrared. You can't really trust the digital measurement of it either, because once it's too far you might not have time from the CAN bus to back off...unless you back off early. Now I'm not clear on exactly when the Jaguars will current limit and I haven't looked yet in the software. They state openly they'll let you drag 100 Amps through them at motor start...but that implies that there's a loose limit there somewhere...probably based on time (as you say they note the limit as being a function of how long it's over the limit). The Jaguar microcontroller itself is obviously quite fast and I'd think that when it does finally decide things are out of hand it would react pretty fast. In short it's much more practical for the Jaguar software to stop this situation than the software in the cRIO.
The real problem using time to determine the longevity of this resistor is that you might not be accounting for heat that is already baking the resistor from past overloads or ambient air temperature. I don't think they have any mechanism to force you to back off until things cool off. Even if they did...it would probably depend on the MOSFETs working correctly and if they turn into shorts (or fully saturate with no relief) and complete the circuit...the situation will not be possible to salvage.
Things is, there's also the issue with the Victor that they don't have a current mode. With the Jaguars they implemented that mode and therefore needed some way to measure current as well (hard to know which decision came first...did they start off wanting a current set point or current limit...and then need the resistor...or did they put the resistor in and then decide they could monitor it for more than over current).
Personally I find the empty slots in the ring that surrounds the Jaguar's MOSFETs under the fan to be sort of a dead giveaway that they thought they might add more MOSFET like the Victor.