Controlling high-current circuits

Posted on Feb 5, 2014

Digital output from a microcontroller is typically a low-amperage signal. For example, when you put a pin HIGH on the microcontroller (in Wiring/Arduino, it`sG‚ digitalWrite(somePin, HIGH); ), the voltage coming from that pin is typically +3. 3V or +5V, and the amperage coming from it is typically 10 milliamps or less. This is fine if you`re contro

Controlling high-current circuits
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lling an LED, whose required amperage is tiny. Most devices you`d want to control require more current than that. In order to control them, you need something in between your microcontroller and the device that can be controlled with this small voltage and amperage. Relays and switching transistors are most often used for this purpose. A relay is a switch that`s controlled by a small electric current. Relays take advantage of the fact that when you pass an electric current through a wire, a magnetic field is generated surrounding the wire as well. When you place an iron shaft inside a coil of wire and pass current through the wire, the magnetic field moves the iron shaft. If that iron shaft is part of a switch, the switch can be turned on and off by putting current through the coil, which moves the shaft, closing the contact (see diagram below). Note the diode in parallel with the load here. This is used only when the load is a motor, solenoid, or some other inductive load, and when it is switched off, will generate a blowback voltage. The diode protects against this. See my motes on motors for more detail. The current needed to move the shaft in the coil is very low (less than 10 milliamps) so the coil can be energized by an output pin of our microcontroller. The current that can flow through the switch, however, is much higher. The lamp circuit is mostly separate from the microcontroller. A separate power source, with the...

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