Simple Motor Control from AC
By Stan Leech

If you need to control a DC motor, but only have an AC power supply, the simple circuit shown in figure 1 might be just right.  It uses only two diodes and an Single Pole Double Throw (SPDT) centre-off switch and will allow you to switch the motor on and off, and change its direction.

The AC supply from your power supply transformer needs to be 1.5 to 2 times the voltage required by the DC motor that you use.  This is because the diodes, which each form what is known as a 'half-wave rectifier' when they are active, waste half of the power in the process.

Further Information

Rectification - Includes a description of a half-wave rectifier

You will need to use diodes that can pass the current that the motor will consume, at the voltage you will be using.  1N4001 diodes can pass up to 1A at 50V, whilst 1N5401 diodes can pass up to 3A at 100V.  In figure 1, 1N4001 diodes are used because the power supply voltage is 12V AC and the motor runs at 6V DC.

Article Information
Source: Electronics in Meccano - www.eleinmec.com | First published in EiM: Issue 5 (September 1999)
Topic: Controlling Motors | Created: 30/12/2002 | Last modified: 02/11/2007

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