Flyback Diode
This is the diode connected in parallel with the winding in reverse polarity, to safely discharge the reverse voltage generated by the inductance when coil current is interrupted. Without it, the voltage spike reaching hundreds of volts punctures the switching device or creates arcing on relay contacts, wearing them out in short order.
An inductance wants to keep current flowing; if the circuit is suddenly interrupted, the voltage spikes because you have left the current no path. The diode offers that current a closed loop around the coil, and the energy is dissipated as heat in the winding resistance.
- The diode must be connected as close to the coil as possible; long wiring increases the loop inductance and weakens the protection.
- Polarity is critical: in normal operation the diode must be reverse biased, that is, non-conducting. If connected backwards it is a direct short circuit.
- Using the diode alone slows the current decay and lengthens the valve's closing time; when speed is required, the diode is used together with a zener or a resistor.
On modern proportional driver boards this protection is built into the output stage. But if you are driving a coil with a relay, a switch, or a circuit of your own making, adding the protection is your responsibility; skipping it is a typical mistake that also causes other faults in the cabinet.
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