Victor Burnouts

[Al Skierkiewicz]

Mark,
This is all very puzzling...Is there any chance that the controllers that failed were installed backwards or two were wired to one motor? I know I sound strange on this but I am grasping at straws. Everything seems in order except the unusually high rate of failure. In my regular job I repair and maintain electronic broadcast equipment. Finding a smoking gun (cause to the effect) is what I am trained to do.

[Mark McLeod]

Thanks Al, I really appreciate your help with ideas, maybe especially the "straws!" We too are grasping at straws. I'm only leery of straws when implementing potential solutions starts adversely affecting the operation of the robot. I had to fight to keep the Victor's on "brake" this year, because we used coast exclusively the last few years and that change was a natural suspect. Any issues with "brake" that you know of?

From your experience if one FET goes would the others naturally follow from the increased workload?

No miswiring that I'm aware of, all replacements were dropped in and wired the same. Backwards would be more likely than two to one motor which wasn't physically possible.

In the build season before we tightened up our construction practices and student (& engineer) training, metal filings could have gotten into the Victors, so that's what we blamed initally. We haven't uncovered any evidence to that effect and that shouldn't have been the cause on later replacements, but you never know for sure.

It's really a concern of course for next year when we won't want a repeat of the issue. I'll get together with the elec. students and other engineers to see if we can add to your straws, and get any stories of static discharge while handling any of the equipment.

[Al Skierkiewicz]

Mark,
Foreign objects leave little parts of themselves in and on components. An inspection under a bright light will usually reveal an arc mark (burned deposit) on a component leg or on the board.
Brake mode should not have any adverse effects unless a controller was in brake for a full match while other motors were back driving it while running. To my knowledge, brake mode turns on the lower half of the "H" bridge thus putting a short on the motor through the two bottom sets of FETs. As you suggested earlier, the FETs died in one line, I took that as one set of positive FETs and one set of negative FETs, the two of which are in series through the motor.
If the controller is trying to push a lot of current, then there might be a domino failure. The first FET just pops and then the second and third have to share the current. When one of those fails, the last one is suddenly running all the current and blows it's case apart as there is no place for the magic smoke to go. If the controller were wired backwards, it is possible but I think unlikely, that FETs could be turned on across the battery input. They might be able to fire up in reverse polarity under these conditions and the failure might be instantaneous vaporization of the internal wiring causing an open FET. I think that the power input is reverse polarity protected but the motor output cannot for obvious reasons. That would also explain why only two sets of FETs were failing and not all four.

[Kris Verdeyen]

Originally Posted by Mark McLeod

I believe the FETs opened, but I'll have to verify and get back on that.

Open FETS would lead you away from static as a cause of the failure. Static or other overvoltage conditions tend to result in a short.

[Mark McLeod]

We (one of our electrician mentors) reinspected the blown FETs and in our cases, the FETs shorted, which means it wasn't a case of one going then the others sharing current, etc. Something, and the backdriving or motor brush theory is a real possibility, causes all three in any one quadrant to blow simultaneously. We didn't notice any burn or flash marks, but that doesn't mean that it didn't happen.

[MikeDubreuil]

Now that we know they were shorted. Kris Verdeyen mentioned a closed FET as a sign of static electicity. Is it possible your wheels are generating static electicity, and the high voltage made it's way to the wheel's speed controller's FETs? Also, what were the speed controlers connected to when they failed? Just the wheels or an arm or roller?

[Mark McLeod]

Originally Posted by MikeDubreuil

Now that we know they were shorted. Kris Verdeyen mentioned a closed FET as a sign of static electicity. Is it possible your wheels are generating static electicity, and the high voltage made it's way to the wheel's speed controller's FETs? Also, what were the speed controlers connected to when they failed? Just the wheels or an arm or roller?

Could be. I never heard reports of static discharge from the drive teams until Nationals (when we didn't have problems), but I'll ask them. I never encountered static when the robot was driven on the carpet before shipping though. This is a theory that could easily be tested at home when the robot gets back.

The controllers that blew were on both the drive and winch motors, I don't believe the arm (van door) controller was one of the victims.

[Al Skierkiewicz]

Guys,
Static of sufficient energy to kill FETs would have been causing problems everywhere else, too. These FETs have internal protection to prevent damage (zener diodes) and are part of a circuit board where 12 FETs live. For only one quadrent to die from static is unlikely. It is still a big puzzlement though. For only one quadrant to fail (Not two opposite quads) then there had to be something wired incorrectly in the external circuitry. Something like one of the battery leads going to the motor and the other going through the controller. This could put full battery voltage across one quad set but not destroy anything else. It seems so unlikely though since the wiring error that would cause this would be easily diagnosed. It could happen, just not high on my list of possibles. It may remain a mystery for the rest of eternity.

[Mark McLeod]

If we discover anything new through testing during the off-season I'll post the results. I think I'll take one Victor (maybe an 883) and run controlled tortures. I might replace the FETs with lower amp models so they self destruct more easily (and are cheaper to replace).

I want to thank everyone for their thoughts and ideas!
FIRST people are just so VERY nice!

[Bob Ward]

I am trying to replace the FETs on a Victor. First, they seem a bit tough to unsolder intact, so I clipped them off. Then, when I took the remaining pins out, the one with the square pad on the top side of the board lifted that pad off. Is this a problem? It didn't seem to be connected to a trace, but it is hard to tell with that silicon goop on there. Are the holes plated through, that is, do the FETs need to be soldered on both top and bottom? Does anyone have any tricks or advice to share on replacing the FETs? Thanks!

[Al Skierkiewicz]

Bob,
The best way to handle the pins is this...
1. Use a solder sucker or solderwick to remove as much as solder as possible.
2. Heat just the pin with your iron and wiggle the pin with a small screw driver. While the pin is moving, remove the iron and the pin will not solder itself back in place.
3. Remove the device.

Any pad that lifts off is likely part of a plated through hole. When you install the new FET, just add solder to the bottom of the board and watch to see if the solder wicks up inside the hole. What is unknown is whether the board has more than two layers. Although some teams report that replacing FETs works, they can take out other devices. For the cost and reliability issues, I recommend a replacement. Anything that is stressed enough to take out FETs has other problems waiting to happen.

Edit...BTW a square pad is usually an indication of a particular pin (often pin 1) to assist with part insertion. Watch which way you put the FETs back in, they don't like to be reversed.
You also may need to clean out the holes if the new one doesn't go in easy. Forcing it may push out the plated through hole. If this is a multilayer board, pushing the hole out kills it in most cases.

[BrianBSL]

Did you replace all of the FETs or just the blown ones? What part # did you use to replace them (was it the same as the ones IFI uses)? I've had some trouble finding fets with the same part # as the ones in one of our blown speedcontrollers.

[Mark McLeod]

Originally Posted by BrianBSL

Did you replace all of the FETs or just the blown ones? What part # did you use to replace them (was it the same as the ones IFI uses)? I've had some trouble finding fets with the same part # as the ones in one of our blown speedcontrollers.

I believe our electronics group typically replace only the blown ones, then bench test the rebuilt unit.
The FETs used are IRL3103 (International Rectifier) or an equivalent like FDB6035AL (Fairchild Semiconductor) for our competition victors, but close matches have been used for ones on the practice or older robots.

Digi-Key
http://www.digikey.com/scripts/DkSea...321579&Site=US
$1.63 individually or $1.16 ea. for 10

http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid= 199954&e_categoryid=277&e_pcodeid=51229 Mouser has them for $1.19 each if ordered individually or .90 each for 25

[Bob Ward]

Al,
I've unsoldered parts from a lot of non-USF boards, including packages like these FETs, but the solder seems very difficult to melt out on these. My always reliable heated solder sucker can't melt it. I put a larger 1/4" chisel point iron on all three pins and it took a while to melt. I guess the traces must be huge since these just pull the heat away. Parts of the board and the screw terminals get pretty hot. I seemed to lift a pad on one side or the other on just about all of them. Then I started snipping the FETs off and taking out pins one at a time. Still lifted pads. I guess you need a very hot iron. Since my sucker wouldn't work, I ended up using a PC board drill to clean the holes out. Of course, if they are plated through, I may have partly or completely removed that. I tried soldering in a new FET, but there wasn't anything for the solder to stick to on some pins.

I am sure if I play around longer I can get the knack, but I don't think I will. You are right about the stress probably shortening the life of the rest of the board. I will either have them rebuilt (I see http://www.rotordesign.com/victor.html is doing them for $75 now plus shipping) or we will just get new ones. The rebuilts will be good for practice and we'll save the new ones for competitions.

I guess we have a motor problem that is blowing them. We lost one Vic and the replacement also went. I wish the breakers 'broke' fast enough to protect the Vics. As someone mentioned, the Vics 'sacrifice' themselves to save the breakers. Well, for my sake, they don't have to be so selfless. I wish they would let the breakers do their job.

Since I have trashed this board, I will probably rip it apart and see what I find in the way of layers.

BTW, I have now 24 brand new (and one slightly soldered) IRL3103 FETs from Mouser that I probably will not be needing any more. I don't know if its legal to offer things for sale here so I won't do that, but if anyone wants to discuss FETs, let me know.
Bob

[Bob Ward]

Mark,
When one of our Vics went, it just smoked. It was running a match at the time. The crowd loved it. But when its replacement went in the pits, it was spectacular. We and another team saw a huge spark. I was about 50 feet away up in the stands and it sounded like a firecracker. All three FETs were cracked, of course, but their damage was incremental. One just cracked, another had a blob of white stuff on it, and the third had a larger blob of white. Must be vaporised innards. It could be that they went one at a time, but very close together. I am guessing the destruction got worse as the load on the remaining ones increased. As I mentioned earlier, we likely had a motor problem that caused it. Haven't opened the motor yet.
Bob