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I was looking at the data sheet of a common 12v relay and noticed it mentioned using a resistor instead of a flyback diode for circuit protection.

On the last page for 12V models it reads;

When connecting a coil surge protection circuit to these relays, we recommend a zener diode with a zener voltage of 24 V or higher, or a resistor (680Ω to 1,000Ω). When a diode is connected to the coil in parallel, the release time will slow down and working life may shorten. Before use, please check the circuit and verify that the diode is not connected in parallel to the coil drive circuit.

My question is how does a resistor in parallel with a coil prevent induced voltage from reaching the rest of the circuit? If we considered the coil to be a high voltage source for a moment, won't that voltage potential reach anything in parallel with it?

Or, is the resistor connected from the coil drive circuit to ground? In either case - how is the math calculated for the resistor value required?

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The coil is not a "high voltage source", it's a current source, with the initial current being equal to the steady-state "on" current.

This only creates high voltages when the circuit impedance is high, such as when the driving transistor cuts off. If a resistor is placed in parallel with the coil, then the peak voltage across the coil will be limited to Icoil multiplied by the resistor value. Note that the voltage across the switching device will reach a value that is the power supply voltage plus the peak coil voltage.

The only reason that this method isn't more commonly used (than, say, the diode) is that the resistor also dissipates power for the entire time that the coil is on. Adding a diode in series with the resistor fixes this problem. And having the resistor (rather than just a diode) provides a faster turn-off time than the diode alone would give you.

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  • \$\begingroup\$ If the coil current is 117mA and a 680Ω resistor is used for protection then, the expected voltage spike is 79.5V? Is this not still an issue for some circuits? \$\endgroup\$ – JoshNZ Apr 9 '15 at 0:17
  • \$\begingroup\$ What do you mean by "some circuits"? \$\endgroup\$ – Dave Tweed Apr 9 '15 at 1:09
  • \$\begingroup\$ Say I'm using a 2N2222A transistor to switch the relay. It has an absolute maximum Vce of 40V. Will this cook it even with a 680Ω resistor in parallel with the coil? \$\endgroup\$ – JoshNZ Apr 9 '15 at 1:59
  • \$\begingroup\$ Yes, of course it will. In that case, you need a lower-value resistor or a higher-value transistor. It's only common sense, after all. Which part of this are you not understanding? \$\endgroup\$ – Dave Tweed Apr 9 '15 at 4:44

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