Low cost repair of Dolphin

by Hans MEKEL
(Toulouse France)

Hello. I am a Dutch electronics engineer, and live in the south of France. I have a Dolphin which worked well during eight years, but when I put it in the water this spring, it did what most of you describe, it stopped after one minute. As I analysed a bit what happened with other Dolphins, it must be a mechanical problem. I took it apart and changed a couple of ballbearings, but still it stopped after one minute. Then I decided to "fool" the microprocessor, by changing the value of the resistors which measure the current in the two motors. Those are the 1 Watt resistors next to the mosfets, the biggest on the PCB. I don't know the numbers of them. They are aproxx. 1 Ohm, and I soldered a 10 Ohms 1/4 Watt resistor parallel to them, to give them the value 0,9 Ohms. Now the microprocessor "sees" a lower current than before. Only 10% less, but that's enough to make it work properly again. During the conception of the PCB, they calculated the measurement to narrow, maybe as a programmed obsolescence. I hope that you can repair your Dolphin, with these very low costs (2 times 1 cent) and only an hour of work. Greetings Hans.

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Aug 13, 2015
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Resistor Value is 18 Ohms
by: Dan

Hans,
I took apart my new motor unit and the burnt resistor is Brown, Gray, Black, Gold, Brown = 18 ohms 1% tolerance 1/4 watt.
Puzzle solved!

Aug 11, 2015
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Circuit Board Resistor Value
by: Dan

Hans,
Thank you for the updated information.
I sure would like to repair the circuit board with the correct resistance. It would be great if Maytronics offered replacement control boards.
Dan

Aug 11, 2015
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Resistor value
by: Hans

Hello anonymous,
I wrote before that the value of the shunt resistors are approximately 1 Ohm, but I was wrong. They must be around 0,1 Ohm, otherwise there cannot flow 7 amps through the motors. But then as I soldered a 10 Ohm resistor parrallel to them,which is only 1 %, it cannot be the fact that I added a resistor that changed the measurement, as it is in the same range as the precision. So probably it was the fact that I changed the ballbearings that repaired the robot. Mine is still working fine. Maybe there's a flag in the software, that's lifted after a couple of error detections, and which is dropped after a couple of on/off cycles without errors. I used this once in a system I developped.

I hope I didn't lead anyone in the false direction.

The fact that the resistor burned means that there was too much current flowing due to a blockage, probably a bad ballbearing. But still the current limits are very small, and soldering a resistor of approximately 1 Ohm parallel to them will prevent from failing with the slightest mechanical resistance.

Maybe someone else can tell you the exact value of the shunt resistors, as I'm not ready to open it to measure them.

Hans.

Aug 10, 2015
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Resistance value of burned resistor
by: Anonymous

Hans, my control board has a burned resistor probably due to the motors having higher resistance due to the unit being about five years old. I have tested the motors and they are working fine.
Can you tell what the value of the original resistor was before you added the others in parallel?
Thank you!

May 15, 2015
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by: ADMIN

Thank you fort the terrific information!

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