Why is a reverse (freewheeling) diode essential on coil loads?
When the current through a coil is cut abruptly, the coil produces a very high reverse voltage across its terminals in order to keep the current going. This pulse punctures the switching element or wears the relay contact with arcing. A diode connected in reverse in parallel with the coil offers the current a closed circulation path at the moment of interruption and limits the voltage to the order of the diode's forward voltage.
The basic property of a coil is that it wants to maintain its current. When the switch opens there is nowhere for the current to go, and the coil creates a very high voltage across its terminals in order to keep the current flowing. This voltage can reach several times the supply voltage.
The consequences:
- Sudden puncture of semiconductor switches, or fatigue and failure over time.
- Arcing on relay contacts, vaporisation of contact material, welding over time.
- Interference propagating along the cables; false readings on nearby signal and sensor lines.
The solution: a diode connected in parallel with the coil, reversed so that its conducting direction does not pass the supply current. When the switch opens, the coil current circulates through this diode and decays, and the voltage across the coil terminals stays limited to a value equal to the forward voltage of the diode.
Application details:
- Mount the diode as close to the coil as possible; long connecting wires do not stop the interference from spreading.
- The reverse voltage rating of the diode is chosen above the supply voltage and its current rating to handle the coil current.
- A plain diode slows the freewheeling down and lengthens the release time of the relay; if fast release is needed, versions with a resistor or zener in series with the diode are used.
- On DC loads the polarity is critical; a diode fitted the wrong way round creates a direct short circuit and blows the fuse.
The full topic: Installation, redundancy and field continuity