Relay


This chapter is under construction


Wiring

Pins names are according to the diagram below.

The GPIO pins can only deliver a few mA, and cannot drive directly a relay. Therefore a few electronics components are required :

  • The NPN transistor BC54xC acts as a valve to control the current through the relay coil (switching mode). It is either passing when GPIO is HIGH, or blocked when GPIO is LOW. Its maximum collector current is 100 mA. The final "C" ensures than its high gain (>400) will minimize the current drawn from the GPIO.
  • The 5,6 kOhm resistor limits the current drawn from the GPIO pin below 0,5 mA ;
  • The diode, 1N4007 prevents damaging the transistor (and/or GPIO pins !) when the relay is commuted.
  • The relay is a 5VDC relay, with an internal resistance of 60 Ohm. It requires less than 90mA, compatible with the transistor. (e.g. printed circuit relay Omron G5RL-1-E-HR 5 VDC 5 V/DC 16 A ). It is available from Conrad on line shops.

Relays used in automobile industry should be connected to + 12 VDC. They will draw a higher current (4 to 500 mA), and call for a more elaborated design (cascading two transistors).

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