Driving Heavy Electrical Loads

The Vex controller is not designed to directly switch heavy electrical loads, such as relays, motors, lamps, lasers, etc.

To do that you need some kind of switch between the Vex controller and the load. Here is one way to do it:


You should choose the FET to handle the amount of current required by the load, and it should be turned on completely with a gate voltage of 4-5 volts.
For relays, I use a VN3310 MosFet.


You might want to add a 100 Ohm resistor in series and a 100K Ohm in parallel with the gate lead. Most MOS-FET’s (especially logic-level) like to remain latched-up (stay on) unless they are driven by a dedicated driver, not just the output from a PIC. The resistors force the issue - the series resistor protects the PIC output from having to deal the the somewhat large gate capacitance (large instantaneous current spike to supply the gate voltage required) that the PIC has to address driving the gate fully on. The parallel resistor acts as a pull-down resistor, making sure the gate is really off, by bleeding down the residual charge, in addition to the PIC sinking it in the off (logic low) state.



The Vex controller already has a 1K resistor in series with the output, so I don’t think the 100 ohms would do any good. A large value pulldown on the gate would keep the load from being switched on and off from picked-up noise whenever the gate is unplugged from the controller, and therefore would probably be a good idea. The controller also has a 470K pullup on all the analog/digital lines, so a pulldown wouldn’t do any good whenever the FET is plugged into the controller.

By the way, my username is lower case LTK, not ITK.


Thanks for the info on the internals for the Vex controller.