Given a low power signal relay, 3V 140mW coil, controlled by a 2n3904 npn transistor, is a flyback diode required? At what point is a flyback diode needed for any given inductor? Is it total power, voltage, current, henries?
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Best Answer
The appropriate rating would be the total energy required to damage a transistor. This is similar to ESD discharge, where typical rated energy is in microjoule range. Low current reed relays (with currents in single-digit or lower tens of mA) will actually qualify: a transistor designed to survive ESD shocks will survive the shocks from such a relay for some time.
It's still a pretty bad idea for a product subject to any kind of certification. Semiconductor devices are rarely certified to withstand repeated ESD abuse, and transistors often lack upper rating for breakdown voltage, which means that some devices will fail early, and others may experience voltage spikes in hundreds of V, giving you EMC problems or unexpected creepage issues.