The 500W inductive load question got me thinking. Cars have large radiator fans that often draw more than 15A and are pretty inductive. Do automotive relays have any sort of protection from inductive kick back? Also, in a situation where a physical switch does the switching of power does the switch take the grunt of damaging kick back or does the damaging effect propagate to the source as well? The physical act of breaking the circuit does generate a spark when a simple switch is used.

Thanks. Andrey

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    \$\begingroup\$ I think things like fans, lights, etc. are the sources of load dumps and thus no protection is mandated for them. High voltage dumps can cause relays to arc, which shortens their life. SOme relays can weld themselves shut - the arcing causes the contacts to stick together. Relays operating in the engine cabinet have to meet strict requirements including operating temperature, vibration resistance, life span, failure mode (if any) and electrical durability. \$\endgroup\$ – Thomas O Nov 13 '10 at 1:18


I checked google images for "Automotive Relay" and found picture of what is inside. There is no diode there. The reason can be that everything made for auto is massively produced, designed for low cost. It evolved over multiple generations into perfectly cheap, reliable and very low part count things.

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    \$\begingroup\$ some have diode's some don't, look through the bosch automotive relay catalog. \$\endgroup\$ – Mark Nov 13 '10 at 16:00

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