Can you break it this way?

Submitted by dcuish on Wed, 04/01/2015 - 11:46

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I had 4 motors driving a small rover with a picoborg using 4AA batteries and a model B and 4 x 3V DC motors.

I wanted the thing to go in both directions so a switched the polarity on 2 of the motors. This way the front two wheels move it forward and the back two wheels can be used for reverse. It worked very well and was able to do nice turns and drive in both directions.

The problem was when I turned on all motors to see who would win in a brief motor tug-o-war. With all 4 motors on, It lasted about 1 second before all motors stopped and my picoborg seemed to be dead. No pops or smell of burning PCB, just suddenly not working. I had a look over the picoborg board with a magnifying glass and can't see anything that looks frizzled.

I tried the same picoborg with my model B+ and no joy so it seems to be the picoborg itself that has died.

Any ideas on what caused this or how it can be fixed?

piborg's picture

piborg

Wed, 04/01/2015 - 14:25

It is possible that the current used by having the motors work against each other exceeded the 2A current limit.
If this is the case then the fuse may have blown to prevent the board damaging itself.

If you have a multimeter you can test the fuse fairly easily, simply measure the resistance by probing either side of the fuse, if it is okay it should be about 0 Ohms.
If it reads open circuit then the fuse is definitely dead.
I have attached an image circling where the fuse is on PicoBorg.

If the fuse has gone you can replace it with a new one and everything should start working again.
You will need to use a soldering iron to remove the old one and solder in the new one, but it should not be too difficult.

The type of fuse you want is a Fuse 2A SMD 1206, and should be rated for at least 20V.
There are plenty on Ebay, for example the following would all work nicely:
http://www.ebay.co.uk/itm/Fuse-2a-SMD-1206-Quick-blo-Marked-S-63v-3-1mmx...
http://www.ebay.co.uk/itm/10pcs-New-SMD-1206-2A-63V-4mmx1-5mmx1-5mm-Fuse...
http://www.ebay.co.uk/itm/10-X-FUSE-SMD-1206-2A-Part-MULTICOMP-MC12CT-00...

We would not recommend using resetable fuses (aka: PPTC, polyswitch, polyfuse), they work slower then standard fuses and may not react quickly enough to prevent damage.

Images: 

dcuish

Fri, 04/03/2015 - 15:57

Wow that's super helpful of you guys! Many thanks for the reply, I'll get right on this.

dcuish

Fri, 04/03/2015 - 16:28

Well done! Problem solved. I touched a wire across the fuse briefly to confirm and the motors spun back to life! Will order some new fuses from ebay now. You're a lifesaver!

(Unrelated: Just saw the address of PiBorg in Earith! Am living in Ireland now but I used to teach science in Cottenham and I always wondered how those boards got to the village college so fast! Only down the road sure!)

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