If an external mechanical doorbell (12VDC) is being powered, either through the AUX relay on a Single io or through another channel on an eight io or eight io+, a diode must be used in just the same way as is done for a strike since a mechanical doorbell is effectively the same as a strike.
A diode is a standard semiconductor device integral to the safe and proper function of a door controller. It acts as a grounding tool, allowing the flow of current in one direction only. When a mechanical doorbell (or strike) is called / requested / sent, the coil sends a spike (also called “kickback voltage”) down the line with as much as 50,000 volts. Without a diode, this kickback voltage would damage the control equipment. When properly installed, the diode keeps kickback voltage localized.
To protect equipment against electrical kickback, the diode must be installed across the mechanical doorbell, between the positive (+) and ground (-). (DC voltage is polarized.) The diode must be installed in the direction shown in the illustrations: Gray stripe to positive (+), black to negative or ground (-).
The diode MUST be installed when using an external mechanical doorbell. Install at the doorbell with the stripe of the diode on positive (+) and the black on negative (-).
IMPORTANT: The diode should be installed as close to the doorbell as possible. The best scenario (if possible) is directly across the doorbell terminals. The diode can also be spliced in parallel (similar to what is shown in image two), connecting it to the positive (+) and negative (-) doorbell wires using dolphin connectors, and then crimp the wires.