DIODES & RESISTORS IN RELAYS

Some automotive relays are designed with built-in diodes or resistors. 

These devices help to suppress coil voltage spikes and protect electronic circuits.


Diode relay protection:

When the voltage drops and the relay is de energized, the magnetic field collapses. 

This causes voltage surges in the opposite direction. These low stray surges can have significant high voltages, usually up to 100 volts. 

To prevent damage to sensitive upstream electronic circuits, a diode can be installed on the coil. Diodes absorb and dissipate transient voltage spikes to protect upstream damage.



Resistance relay protection:

The resistor can be used to provide protection similar to the diode in the previous example. They can also absorb voltage spikes caused by sudden magnetic field collapse. 

However, resistors allow small currents to flow and are not as effective as diodes in absorbing voltage surges. 

The advantage of the resistor over the diode is that it is polarity insensitive and can be connected to terminals 85 or 86 (according to DIN 72552).

MAA-S-112-C-R