PWM operation of DC relay coils in order to save power while holding is now common. This is at a frequency that is high enough to avoid chattering or audible noise and low enough to not have to worry about switching losses or exotic circuitry.

The in circuit coil inductance can be measured by looking at coil current and analyzing the slope. In this case if the PWM frequency is well known like say microprocessor 50% duty cycle at 8KHz and the supply is regulated 24V then you can easily get the coil inductance.

What I actually did was a self osc hysteric scheme which gave a change in frequency for a change in relay coil inductance. The Idea is that one can tell the position of the relay coil by its inductance. What I did Vs the orthodox ripple slope approach really comes down to implementation. It is all the same in that you are detecting inductance change to tell if relay armature is in or out.

I was expecting big changes like more than 3:1 but that was not the case. Why? Doesn't closing the magnetic circuit raise inductance markedly?

Sure the change was totally adequate to make a reliable decision regime for the firmware guy to do. The real odd thing was that the pulled in relay exhibited less inductance. Why would it appear to work back to front? I didn't put this into production because the big relays were not needed after all but I have concerns about the scheme performing differently on different relays. Is the concept flawed due to relay peculiarities? Can it only be used in a specific case?

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    \$\begingroup\$ I am going to edit our peculiar punctuation to make this easier to read. In future I suggest you place the spaces after your periods and question marks. \$\endgroup\$ Commented Dec 20, 2015 at 3:39
  • \$\begingroup\$ Michael Karas an edit would help ,Maybe it could have more tags .My screen name has an element of truth in it ! \$\endgroup\$
    – Autistic
    Commented Dec 20, 2015 at 3:46
  • \$\begingroup\$ Think of it this way: the punctuation marks the end of a phrase and not the start. No spaces before punctuation, and always one after. ;^) except in smileys. \$\endgroup\$
    – Transistor
    Commented Dec 20, 2015 at 8:43
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    \$\begingroup\$ Have you considered that the relay armature is probably a lump of iron and that when operating with an element of AC, induction and eddy currents are going to lower the perceived inductance of the coil? \$\endgroup\$
    – Andy aka
    Commented Dec 20, 2015 at 10:39
  • \$\begingroup\$ Yes I have and was worried that with some relays one effect could try to cancel the other .This would mean that my approach or other related approaches would have to be relay specific .I want my circuits to be as general as possible so they work for everyone without hassles. \$\endgroup\$
    – Autistic
    Commented Dec 20, 2015 at 10:49

3 Answers 3


This suggests to me that the relay coil may be saturating its core.

Consider this : if you are measuring the inductance in-circuit, while operating the relay, there are two important states : open and closed.

In the open state, the inductor is gapped and therefore its reluctance is relatively low. You can apply all the power you like and it won't saturate, therefore your inductance measurements (using the AC component of the PWM signal, right?) reflect the true inductance of the (gapped) coil.

However, it will pull in, closing the magnetic circuit. And here I'm speculating. The closed circuit saturates, so the inductance you can measure is a small-signal value on the relatively flat saturated portion of the B-H curve.

Then you reduce the PWM duty cycle, but presumably not quite far enough to bring the core out of saturation (or you'd see an increase in inductance).

If this is the case, then inductance measurements made unpowered, on an opened relay, operating it with a finger or screwdriver, would show the expected increase in inductance when closed.

  • \$\begingroup\$ Interesting +1 core saturation from you and eddy currents from Andy can mean that the inductance change is less than expected.PWM frequency would also affect the eddy currents and saturution would raise frequency on the self osc scheme. \$\endgroup\$
    – Autistic
    Commented Dec 20, 2015 at 19:25

I was unable to reproduce this behavior.

I found a relay with a removable cover (stolen from my heating system!) and connected an LCR meter to the coil.

I tested the inductance of the coil with the relay open and found it to be 360 uH.

Then I manually closed the relay arm by pushing it with my finger and found the inductance to go up to 585uH.

This is what I'd expect to see since moving the arm closer to the core of the relay's coil should increase the flux though it, thus increasing the total inductance of the coil.

So, possibly...

  1. The inductance change you are seeing is really an artifact of your PWM strategy.

  2. Your relay is very different than mine.

I'd recommend you try try testing the inductance with the relay open and manually closed (not energized in either test). This will give hints as to where to look next. If you again see the inductance unexpectedly go down, then we have something interesting here!

  • \$\begingroup\$ Good Stuff big josh .We measured the inductance on our relays with the cheap Dick Smith L C R meter .Big relative differences were noted which implies that its feasible to detect if the relay is pulled in or not in a reliable way .My LR osc gave frequency changes that are easy for the software people to deal with.My waveforms of coil current and volts were fine on the scope .Maybe the DC relay coil when pulled in thinks its got a shorted turn due to its unlaminated construction .+1 for giving an answer .I find that asking questions attracts downvotes like snails to lettuces. \$\endgroup\$
    – Autistic
    Commented Dec 20, 2015 at 2:49
  • \$\begingroup\$ Interesting question. My first though was that there was maybe some current path created from the top of the coil core to the bottom when the arm closed. After looking at a few relays, I realized that this was very unlikely since (1) the core it typically ferite which is a bad conductor, and (2) if the arm did create a current path it would dissipate power which would make for very bad relay performance. \$\endgroup\$
    – bigjosh
    Commented Dec 20, 2015 at 2:54

Agreed. The inductance should go up but maybe not as much as expected. Many relay armatures have a bump on the moving part to restrict the contact area when the relay is energised. This is to prevent the relay latching with residual magnetism in the armature core.

Incidentally, the increase in inductance is an energy saver in AC contactors as the impedance of the coil increases dramatically when the coil pulls in. I've seen several cases where the contactor jammed and the coils burned out as they aren't rated for continuous energisation in the "open" position. I don't think the "bumps" are required as residual magnetism isn't going to be a problem with AC and weakening the magnetic circuit would increase the current in the coil, increasing the heating, shortening the life.

What's the 'ripple slope approach'?

  • \$\begingroup\$ Ripple slope was something I was told about that others have done but I did not do.Imagine the relay coil current flowing continiously either via the transistor or the freewheel diode.The coil current is never pure DC because of the finite coil inductance.The slope of the ripple was used by others .I know this because I initially asked if they sensed pk to pk ripple. I thought pk to pk would be easy to do in analog components.I was told politely that they were doing things digital.Then I did the LR ocsillator because the frequency change would be easy to count . \$\endgroup\$
    – Autistic
    Commented Dec 20, 2015 at 10:03

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