I decided to pull out the old 10 ohm rheostat to put a variable load on a Tetrix ptc and see what the trip time was vs current. The times were measured with my Iphone stop watch and the current was measured with a clamp-on UNI-T 40amp digital meter. The times probably all have about .1 to .2 second reaction time and maybe a .1 sec meter settling time. I measured the ptc resistance with my fluke and found it to be around .1 ohm cold and after tripping it recovered very fast and returned to .1 ohm. If the .1 ohm is accurate, a current draw of 4 amps causes an I^2*R loss of 1.6 watts or about 9% of the available power. This is a big price for protection.
Here were my current/Trip Times
3.8/<1 Reaction time too slow for accurate timing.
It appears that the minibot could sustain near peak power current of 3.75 amps provided that it didn’t trip during the acceleration phase where the currents are higher. My simulation shows our super mini would require 4.3 amps to climb. We’ll see if it can make it up in real life tomorrow since we finally got the shut off switch, brake and wiring done today.
Update: Well, the PTC’s held up for the majority of the runs. As time went on the minibot developed a little higher friction and indeed the PTC’s shut the bot down about half way up. I believe that the acceleration phase is critical. If the bot doesn’t get out of that phase quick enough the run is toast. At this time, I am not willing to extract them from the circuit until the first meet is under our belt.