Let's Make Robots!

More fun with Wild Thumper


  My adventure with getting started with my Wild Thumper continues. In this episode, calibrating the motor current readings! As instructed I installed the diagnostic pde, removed the fuses and watched the serial output. Instead of seeing high currents measured for both motors I saw 0's. On a regular basis the fuse LEDs would blink and for just a moment there would be a reading of ~35. So that's the first strange thing.

Next I pushed on and got a multi meter and 10 ohm (I used one rated at 10 watts rather than 5) resistor hooked up the controller and drawing power off my 7.2v (rated) battery pack. However the readings I was getting did not match the manual. When the multimeter was set to measure mA range it was reading ~350 mA and would occasionally flicker to -350 mA. When it was set to measure in the A range it measured .55 amps. The multimeter is very new and accurately measures other loads, and voltages, so I don't suspect it. These measurements were repeated with both motor terminals and were consistent. The manual calls for a measurement of 940 mA at this point. I don't know how this number is arrived at, but that's not anywhere close to what I measured.

During this time the voltage, as measured by the board and sent to serial out had decreased to the 400 range, so perhaps my battery is getting low. That said if a 6.3 V battery (409 * 65) attached to a 10 ohm resister is only measuring 350 mA I'm not sure how a 7.5 V battery attached to the same 10 ohm resister is expected to measure 940mA.

As you can probably tell I do not yet have a strong intuition about these relationships in a circuit and would appreciate any insight or explanation anyone has into what I may be doing wrong. 

Many thanks and I appreciate everyones time and attention!


P.S. I was able to get python serial communication to the Wild Thumper working so if you want code it turned out to be much simpler than I was making it.

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If you want to get accurate readings then the first thing you need to do is solder the fuses in. The reason is that the resistance across a fuse is very small and can be swamped by a less than perfect connection of a clip in fuse.

Hopefully you will never need to change the fuses as they are "slowblow" fuses and the code will attempt to shut the motor down before they can blow.

If you do need to change a fuse then it is fairly easy to desolder a fuse.

The only way I could see that you could get 940mA draw based on the readings you got would be if the motor was stalled.