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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?

1 year ago

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Why do you want to know the inductance?

A relay-coil does not have a specification of inductance. In other words, no-one needs to know the value of inductance for a relay to operate in a circuit.

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Hello Colin
Where this guy comes from copper wire are rare.
This guy wants to build an AM radio. He had found that relay coil on the street. In order to know +/-
range of the var. Capacitor he wants to find the L. Freq. resonance=1/ 2 x Pi x root LC .
He knows the uper freq of the AM band and the lower and with them he can calculate the upper range of the RADIAL CAPACITOR AND THE LOWER RANGE.

From you i have learnd how op AMP’s do work – those yellow GIF animations are beautifull
In 2 second i understood everything. The whole concept of an op amp Output goes up rail or down rail(earth) when one of the inputs is passing the value of the other. And then feedbacking and determining the Amplification.
With this Animations you have a uniqe product . I have seen this kind of animations for the 555 Ic also top work. what still needed is such an animation for that COIL PARALLEL with CAPACITOR describing what is happaning in Resonace (circuit gets to its highest Z).

Wish you heakth and happiness.

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Hello Collin,
I know to operate relay we don’t require inductance value.
I want to calculate what value of back EMF relay coil can generate & formula for that is e= – L* di/dt for this L value is necessory.

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The back EMF has nothing to do with the inductance. It depends on the “Q” of the inductor.

Why do you need to know the back-EMF? It is always quenched via a diode.

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Because he wants to use it as to deliver an electrical shock Most relays will do that anyway from the back emf

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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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We used to strip the old transformers and chokes for the copper wire then to make the radios

That’s how I started as a youngster building flash lights with hand held torch light bulbs(1v2) No longer around anymore and learning how to wire in parallel and series using wood and screws to connect the components A slight touch of Morse then Now its all PC Orientated or micro chip and of course led’s, Oled, strip film inductors embedded into the PCB etc

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Impossible to beat