Saturday, May 14, 2011

Safer Motor Control

This is the schematic for a safer motor control than the original engine room board in Arisa. Two changes have been made. The major one is that the optocoupler is now used in pull-down mode. When done like this, the transistor Q1 is turned off whenever there is no signal at OK1A pin 1. This is opposite of the old design, which would turn the switch on when there was no signal. This was because in the old design, the diode in the optocoupler was off when the signal was high, and running the optocopuler output as pull-up did a double negative and turned Q1 off when the input signal was high.

So, I just took out this double negative. Now, the diode is on when pin 1 is high, which turns on Q1 and runs the motor. When there is no signal from the optocoupler input, Q1 is off and the motor is off. This is safe.

BAT is the high-current battery for the motors, usually a 7.2V rechargable or something like that. The exact voltage isn't important as long as it is within the power capability of the other components. Most of the parts on the power-side of the circuit are rated for 100V or more. M1 is the motor terminal for the high current motor. Q1 is an IRF540, an n-channel MOSFET. This means that the part is on when the gate (left terminal) is more positive than the source (bottom terminal). Q1 claims to be able to handle 28A. D1 needs to be able to handle this at least momentarily when the motor switches off. A 1N4007 is just barely able to handle this, it has a peak current of 30A for about 8ms.