Calculate Inductance of a 12V Relay Coil

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

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.

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.

http://mynerdstuff.blogspot.co.uk/2011/12/inductance-meter-based-on-555.html

http://reibot.org/2011/07/19/measuring-inductance/

If you are going to make an AM radio, there are two things you need to know about the relay. Firstly, the iron core of the relay has magnetic prorerties which make it far too sluggish to work at tadio frequencies. The second thing is that the self capacity of the coil windings (because of the large number of turns) would render the tuning range too low by a long way to work on the AM radio band. You will need to carefully unwind some of the relay and teansfer enough wire to make about 100 turns to a cardboard or plastic tube of about 2 to 3 inches in diameter, The tuens should be close spaced in a single layer to get the highest tuning range.If the wire is very thin, scramble winding should suffice.This becomes the main inductor for tuning. A second coil may need to be wound about 1/8 of an inch away from the main coil for matching into the radio receiver electronics. The number of turns wll depend on the circuit but start wit 10 turns as this may be satisfactory for most circuits. There is much more I can tell you but this may be sufficient. (I am assuming that you want to build a radio receiver)..

http://homepage.ntlworld.com/henry01/ladybird_radio/ladybird_radio.htm Right back to 1972 about 8 or 9 years old then

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

Or these days its beat the chinese National Supercomputer Centre in Guangzhou, based in the country's south-eastern Guandong province, where it will be offered as a "research and education" resource to southern China.

The machine uses a total of 3.12 million cores ( Incredible power ) How the hell do you outsmart that !! Theres me thinking 8 core blimey super fast they ll be firing ray guns at each other next Star wars stuff the stuff we thought would be make believe and fantasy 3 d projection stuff first you see it then you dont , " Oooh I thnk well declare war on syria " , zzzzzzzzzzzzip Whoops wheres my troops gone One micro second to impact , we have problem's houston , Uh I dont know how to explain this but our six satelights orbiting earth just vanished into thin air Not Russia you worry about anymore is it !!


Now all they do is write the software to play against the US Defence force and bang cock or put your head inbetween your legs and say "Goodbye cruel world" Launch the first war heads Who get the massge out first wins (The Communications war ) , ( The Gat wars ) Wonder what it will be next

Impossible to beat

If you want more pointers go to my home page


http://www.crystalradio.us/crystalradios/2005-2.htm

http://www.harrington.force9.co.uk/Projects/ElectronicProjects.htm

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

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.

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.