Well, ran a 2.5 lb minibot with the Tetrix PTC and 10 ft of extension wire… Time was about 2.5 seconds. It should have been around 1.25 seconds according to my simulation. So I can match 2.5 second time in the simulation with 10v battery voltage rather than 14v. I suspect the resistance of the PTCs , battery and the wire resistance is creating a substantial voltage drop. The PTCs have a resistance between .05 and .14 ohms with a typical of .1 ohms. This was confirmed by an ohm meter with the PTC cool. The wire is 18 or 20 gauge and has a measured resistance of .3 ohms round trip.

The motor resistance is 1.6 ohms and these are in parallel with an effective resistance of .8 ohms. So we would expect a voltage divider ratio of .8/(.8+.1 + .3) = .75. which means motor only gets 14*.75 = 10.5v driving it. When I run this in the simulation I get a 2.2 sec climb.

If we add the resistance of the battery (unknown) it might add another 50 to 100 mohms and then the results would match better.

So, I am confident that we will see the expected times when the final wiring is done and doesn’t use the ptc’s. I will do a test with the PTC’s under a 2.5 second current load for 2 seconds and see how much the resistance increases when the PTC has heated up a bit.

We may decide to run the competition with the PTC’s inserted just to be safe.

This entry was posted on Friday, March 4th, 2011 at 12:25 am and is filed under FRC. You can follow any responses to this entry through the RSS 2.0 feed.
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