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# Electronics and Electrical Engineering Design Forum

## Calculate Inductance of a 12V Relay Coil

How to calculate inductance of 12V relay coil if only coil resistance is known?

###### Asked By: abhijeet Sathe
1 year ago

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The coil resistance cannot be used to calculate the inductance. In some inductors, such as a loudspeaker, the impedance is often approximately the same as the DC resistance.

The inductance of a coil can be calculated if you have the necessary information, there is a calculator on this site:
http://www.eeweb.com/toolbox/coil-inductance/

Otherwise, the inductance can be measured directly with an inductance meter, most commonly an LCR meter is used.

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The inductance of the relay coil can be found using a power supply (or battery), an oscilloscope, and a current viewing resistor. The resistance of the current-viewing resistor should be less than 1/10 the relay coil resistance.

Using a mechanical switch connect the coil to the power supply (have the power supply already ON). Observe the coil current on the oscilloscope and note at what time it reaches 63% of the final value.

Using the formula T=L/R and solving for L=T x R, where T is time in seconds and R is the resistance of the coil, you have found the coil inductance.

The inductance of a relay coil is a valuable parameter to know. With it one can calculate the RL contribution to relay drop-out time, the stored energy to be dissipated when the relay is de-energized, and the flyback voltage of a non-clamped coil (given the circuit capacitance).

Note that adding series resistance to a relay coil clamp circuit decreases the RL contribution to drop-out time. For example, a resistor is added in series with the commonly used clamp diode.

For AC relay coils the inductance is often the dominant current limiting impedance. So, the inductance is more important than the resistance. This is why operating an AC relay coil on DC can damage it.

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Thanks David,
I think method which you shared is the best & esy one.

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I have a similar question, I have a LCR meter and know the L and R values.
what is the importance of L and especially the L/R ratio for coil design.

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I think it affects the Q or damping-factor – between two coils of the same inductance but different DC resistances, the one with higher resistance will exhibit lesser damping (or greater Q) when used in a resonant circuit.

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