Monday, 20 May 2013

Earth Rover 

Part III

Providing Power, Direction and Speed Control

 Wiring the Motors, DPDT Relays and PWM to the MOSFET

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The diagram above shows how I wired four of the DPDT relays (on my eight relay board) to control the direction and speed of the motors for the Rover's wheels. NB the circuit for controlling the relays using an Arduino pin is ready made on the eight relay board - here.

A logic level, power N-MOSFET switches high current to provide motive power at 12vdc vdc. The gate is controlled by an Arduino PWM pin so speed can be adjusted accordingly. However each individual relay is only capable of switching 2A at 12vdc, but that can give me 24watts, per wheel in theory. There is also a 24v version.

When the Arduino PIN V1 is set to logic 1, the motor goes in one direction, and when set to logic 0 the  the DPDT relay switches it to go in the opposite direction. Fly-back diodes  (D1 and D2) are used to reduce emf spikes from the coils in the relay and motors. D1 is an integral diode in the MOSFET package.

 I decided to make a perf-board version of the Mosfet PWN driver circuit x4 using this free design app.

Mosfet PWN driver circuit x4

 I could have made it more compact, but I wanted to keep the symmetry of the design:

Perf-board Mosfet PWM driver circuit x4 with heat sinks

I glued aluminum heat sinks to the Mosfets using epoxy-based thermo-paste, and used Schottky power diodes for fly-back [those are the big black monoliths]

1 comment:

  1. NB Snubber circuits will be needed with high powered motors. eg