Why Every Relay Needs This Diode

Switch off a relay driven by a transistor, and the transistor can be destroyed. Here's what happens inside the coil, and what a flyback diode does about it. Current through the relay coil builds a magnetic field, and that field stores energy. When the transistor turns off and blocks the path, the coil's voltage flips polarity and a spike far above the supply voltage can appear across the transistor. If it's more than the transistor can handle, it can be damaged. A diode across the coil, with its band (cathode) toward the positive supply, gives that current a closed loop. The current keeps its direction, fades, and the stored energy ends up as heat in the coil's resistance and the diode. Install it backwards and it shorts the supply when the transistor turns on. The slower decay also means the relay contacts can take longer to release. The voltage waveforms are conceptual; the actual peak depends on the coil and the circuit. #flybackdiode #relay #electronics #transistor #insideembedded