Quote:
Originally Posted by Al Skierkiewicz
I think that the color of the FET heatsink tab tells all. It indicates that a lot of current was passing through that device i.e. lot's of heat. Since there are three FETs in parallel to share the current, it is obvious that that side was not sharing. Two things are possible: 1) either the device was defective and started to produce a lot of internal heat or 2) the other transistors were not pulling their weight. They may have gone open or there is another failure on the board.
There is always an outside chance that a stray wire came in contact with the tab on that FET. It happens more often that you think. Examine the tab closely for signs of a high current entry point. It will be rough in texture and exhibit more discoloration than the rest of the tab.
I wanted to speak to the no load voltage measurement for batteries. One must pay homage to Ohm' Law when making measurements with modern equipment. A typical Fluke meter has an internal impedance in the Megohm range. Ohm's Law solved for voltage drop with that kind of load, will produce an imperceptible voltage drop across the internal impedance of the battery. (what is the current supplied by the battery into a 1 Megohm load?) Hence, even a depleted battery when cool can measure nearly full voltage with that type of meter. Add to that, the varying resistance of the connection of the meter probes to battery introduce significant error even in fully charged batteries. I have checked batteries from various teams that show good when fully charged but have one or more cells that have a reduced capacity after delivering current for a few minutes. This indicates to me that a plate(s) internal to that cell have become disconnected and therefore no longer supply current under load. The CBA II battery analyzer from West Mountain Radio is a simple device to check batteries under constant load (about 7.5 amps) and performs a nice plot of time versus terminal voltage. The data can be stored for each battery, recalled and compared with the subsequent tests of the same battery over a period of years. Data can be overlayed for several batteries as well. West Mountain also has a higher current addition to their line of products but we have not purchased one yet. http://www.westmountainradio.com/CBA_ham.htm |
You must remember that the robot was off when it started releasing smoke. I believe it was probably a defective FET (the only one that is not burnt to a crisp) that caused the problems, and while pushing the robot back to the classroom, the two FETs that were being back-driven and power the robot failed catastrophically releasing the all to familiar "Magic Smoke." It was one of the new Victors (with the blue Victor 884 label) I feel that the build quality of the new ones aren't up to par with the older ones that actually used screws. I still like the old 883s, and for just a little less tolerance, I would still like to see the 883s back in the robots. They are rated for more, and I think that's the best thing about them. It doesn't make sense to me to have an 884 Victor that can only handle 40A continuously with a breaker that can handle 40A continuously. I feel that the defective victors have the weak point and will fail before a breaker trips. The 883s would have a little leeway before they burn up. Personally, I've never seen an 883 fry, but I've seen plenty of 884s fry. I'd go for an 884 that can handle 60A any day, and maybe we'll see a speed controller in 2009 that does exactly this.
Also, I'm not sure where the stuff about the battery and the FLUKE meter comes into play in this thread... Just to let you know in case you wanted to put this in some other thread.