# Inductance of a relay coil

I'm using a magnetic latching dpdt relay from Teledyne company. The value of inductance for the coil in the relay is needed and not given in the data sheet. The part number of relay is 422d from Teledyne.

How can I know it? Is it possible to know the value without actually buying the product and testing?

• part number of relay is 422d from teledyne – Naveen May 20 '15 at 8:50
• How about a hyperlink? – Andy aka May 20 '15 at 9:53
• www.teledynerelays.com/pdf/electromechanical/420422.pdf – Naveen May 20 '15 at 10:01
• The inductance is not specified it seems so why do you need it? – Andy aka May 20 '15 at 10:09
• i need it because the driver that i'm using for the relay is a simple n-mosfet and it is to be designed based on the inductance value of the coil. – Naveen May 20 '15 at 10:15

A simple N channel MOSFET driving the relay will not care about the inductance - the final and full value of current the FET sees will be determined solely by the supply voltage and the coil resistance. All that the inductance can do is slow the current down a few milliseconds before it reaches the final value.

This applies whether it is a logic gate, a BJT or a switch; it's simple ohms law: -

I = V/R where V is the supply voltage and R is the coil resistance. The data sheet even tells you what this current will be so no need to do the calculation.

Make sure you use a back-emf diode though or when you open circuit the coil there will likely be a big spike of voltage (due to the inductance) that can easily damage a FET or BJT.

• i'll improve my question..actually we don't want current to flow all the time as we have power constraints.So for that we just want current to flow for some 20ms for which we are putting a capacitor at emitter. henceforth we need the inductance value. – Naveen May 20 '15 at 12:50
• I don't think you will need to the inductance value because the resistor will still dominate. Have you tried simulating it? – Andy aka May 20 '15 at 13:07
• yeah! it's almost independent of inductance value.i've tried simulating the circuit.Thank you very much – Naveen May 20 '15 at 13:30
• @student I don't know if you are aware of the conventions on EE - an answer deemed correct by the questioner (you in this case) can be marked as "accepted" but there is no hurry of course. Usually waiting a day will give a chance for others to answer if there's a more valid answer to be given. – Andy aka May 20 '15 at 13:34

1.Electromagnetic Ralay coil has two values: a. when contancts not activated (Low value). b, contacts activated (high value) because the magnetic circuits are different for ON & OFF states. vtingole