Dr. Iguanas Vivarium

Posted on Feb 4, 2014

A good thing too since I initially hooked the p-channel mosfets up backwards. No smoke fortunately. I`m using Pololu gear headmotors (although continuous rotation servos will also be supported). In order to drive the motors in either direction I`m using H-bridges. This circuit gets it`s name from its distinctive shape. It consists of four switches arranged in an H as seen below.

Dr. Iguanas Vivarium
Click here to download the full size of the above Circuit.

The load occupies the bridge of the H. When opposing upper and lower switches as closed, current flows through the load. The direction of the current, and the direction the motor rotates, can be controlled by which pair of switches you close. I`ve built a prototype of the H-bridge in order to ensure that it works as planned. In my case, the upper switches are p-channel mosfets and the lower switches are n-channel mosfets. These mosfets are rated in excess of 4A and 30V. The Pololu motors have a stall current of 1. 6A and should pose no challenge. Mosfets have three terminals; gate, source and drain. For n-channel mosfets, when the gate and source are at the same voltage, the resistance between the drain and the source will be very high; effectively the open switch. As the gate voltage (Vgs) rises there comes a point where the drain-source resistance (Rds) begins to drop very rapidly. When the gate voltage is sufficiently high, the drain-source resistance will be very small, often in the milli-ohms. For many mosfets the gate voltage needs to be quite high, perhaps 10 to 12V above the the source voltage to reach this very low resistance state. There are, however, logic level mosfets that can achieve this state when Vgs is only 2. 5V (I am using 3. 3V). A similar situation exists for p-channel mosfets. When Vgs is zero (the gate voltage is high), the source-drain resistance is high. To get the mosfet to conduct, you lower the...

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