Automotive – Why Use Relays Instead of Transistors in Cars


Recently I did some work on my car electric circuits. I see many relays are used in car circuits. These relays are used for simple switching, and I wonder why these circuits are based on relays and not on transistors or other electronic components usable for switching purposes. I thought that transistors were cheaper, smaller and more reliable than classic el-mech relays for switching.

Note: In car applications a 12 V car battery is used to power the coil of a relay and the same 12 V power is what's switched by the relay. Sometimes a relay switches just another signal line, i.e. without any high power load on it. And still, I can see no transistors in there. So I believe there is a solid reason why it is done this way, and I must be missing something here. 🙂

Best Answer

Relays are much more stable temperature-wise: a sealed relay has essentially the same characteristics at -30°C and +70°C, both temperatures being common for cars. A transistor works quite differently at -30°C and +70°C, so the schematic has to be designed to account for those variations.

I once worked on a product with temperature range starting at -55°C, which used both relays and semiconductor devices. The funny part about the design was that below -20°C only the relay part was powered, which activated air heaters and would only switch on the semiconductor part once the temperature reached 0°C.

Relays also offer galvanic isolation, which effectively confines faults. Common failures like short circuits usually damage only one relay, whereas in transistor-based circuits several devices along the way would be affected. I bet people still want their car's motor running even when the air conditioner or a window lifter dies.