# Tyco Schrack 12V relay #RE030012 question about inductance

#### Tonyr1084

Joined Sep 24, 2015
6,258
Sometimes I get a bug up my bonnet and just have to ask what may amount to a silly - or even useless question. Well, guess what I've got:

The above relay (data sheet) is a 12 volt relay with internal ohmage of 720Ω ±10% (measured 742Ω) and I didn't see anything in the data sheet about inductance of the coil. Is that because the inductance changes when the relay pulls in? Yes - or No - that's only part of my question. The other part is if I take a coil and measure its resistance is there a way to calculate the inductance?

Someone's going to ask what I'm doing with it. It's no secret. I'm not inventing free gold. Just looking at a possible solution to a question someone asked on this site before about momentarily closing a relay when power is applied and again when power is removed. You may remember the thread; and I'm not trying to help anyone solve anything. I'm just wondering if a particular circuit might work. The answer is "Probably not" but it's an answer I'd like to come to on my own.

So, this relay - what is its inductance, given a resistance of 742Ω?

#### Tonyr1084

Joined Sep 24, 2015
6,258
Data Sheet link not functioning the way I expected. Working on rectifying that now.

OK, after clicking on the link choose "If You Want to View Data Sheet, Click To Here!!".

#### nsaspook

Joined Aug 27, 2009
8,662

#### Tonyr1084

Joined Sep 24, 2015
6,258
Article says:
"If a wattmeter isn't available, inductance may be determined by use of a dual-trace oscilloscope, one input of which is fed by a current probe. In this method, rated voltage at the proper frequency is impressed on the coil, and the time displacement, t, of applied voltage and coil current is measured by the oscilloscope. Inductance is calculated as above, where:
θ = 360°t 16.7 ms
t = time in ms by which coil current lags coil voltage."

I have a dual-trace oscilloscope but I don't have a current probe, nor do I have a wattmeter. Any workarounds?

#### crutschow

Joined Mar 14, 2008
27,714
Any workarounds?
You can measure the voltage across a small resistor in series with the ground side of the coil to determine the current.
(Use a 1X probe for max sensitivity.)
10 ohms will give a sensitivity of 10mV/ma without significantly affecting the measurement (just add 10 ohms to the coil resistance in your calculations).

#### nsaspook

Joined Aug 27, 2009
8,662
Article says:
"If a wattmeter isn't available, inductance may be determined by use of a dual-trace oscilloscope, one input of which is fed by a current probe. In this method, rated voltage at the proper frequency is impressed on the coil, and the time displacement, t, of applied voltage and coil current is measured by the oscilloscope. Inductance is calculated as above, where:
θ = 360°t 16.7 ms
t = time in ms by which coil current lags coil voltage."

I have a dual-trace oscilloscope but I don't have a current probe, nor do I have a wattmeter. Any workarounds?
That's for measuring AC relays. The datasheet you provided was for DC relays.

The inductance of DC coils should be measured by the L = tR method by use of an oscilloscope. This method requires the application of rated DC voltage to the coil while physically holding the armature seated. The value, t, is the time for coil current to increase to .623 of its steady state value, and R is the coil DC resistance in ohms as measured by an ohmmeter.
The small resistance current shunt described above should work.

#### Tonyr1084

Joined Sep 24, 2015
6,258
Two things: I don't think I mentioned it's 12VDC; and second, my scope isn't a memory scope. I don't think I'll be able to measure the time quick enough (visually); and my Hantek won't run on my Windows 10 computer.

#### Tonyr1084

Joined Sep 24, 2015
6,258
That's for measuring AC relays. The datasheet you provided was for DC relays.

"This method requires the application of rated DC voltage to the coil while physically holding the armature seated."

Perhaps I should also mention this is a sealed relay. So I'm not going to be holding the armature down.

#### nsaspook

Joined Aug 27, 2009
8,662
Perhaps I should also mention this is a sealed relay. So I'm not going to be holding the armature down.
Then you won't get an accurate measurement using these procedures.
Coil inductance with armature seated is greater than that when unseated. This is because inductance varies directly with incremental permeability ( µ ) and inversely with the length ( l ) of the magnetic circuit path. The air gap in the magnetic circuit of an unseated armature both decreases µ and increases l. Of course, the greater the inductance, the greater the energy released into the coil circuit upon deenergization

#### Tonyr1084

Joined Sep 24, 2015
6,258
Yes @nsaspook I gathered that from the article.

#### nsaspook

Joined Aug 27, 2009
8,662
Yes @nsaspook I gathered that from the article.
Bug up your bonnet then sacrifice (open it up to access the armature) one in the name of knowing.

#### Tonyr1084

Joined Sep 24, 2015
6,258
Here's what I'm trying to understand: In the below circuit when SW1 is closed current won't immediately flow through L1. C1 is conducting the current until charged, thus toggling K1 to the NO position and momentarily lighting the test LED. Once C1 is charged K1 drops out and the test LED goes out. L1 now builds a magnetic field, thus effectively becoming charged.

When SW1 is opened L1 should discharge its current in a reverse direction driving C1 to charge in the opposite direction and momentarily toggle K1 again, thus flashing the test LED once again. As soon as L1 and C1 are neutrally charged (zero volts) the relay drops out again and extinguishes the LED.

Like I said, I'm not building anything particular, I'm just trying to get a handle on understanding coils, chokes and inductors on a more personal basis. This circuit (below) is my own creation and I have no idea what value L1 should be in order to make this thing theoretically work. Since I have some relays in hand and the afore mentioned relay (K1) is on my bench, why not breadboard something and test to see if it works?! But knowing the inductance of K1 will help me choose an inductor with a higher inductance, which, in theory (in my mind), should drive the current towards C1 and K1.

#### Tonyr1084

Joined Sep 24, 2015
6,258
Bug up your bonnet then sacrifice (open it up to access the armature) one in the name of knowing.
Have been considering that.

#### nsaspook

Joined Aug 27, 2009
8,662
Have been considering that.
The truly committed would have jumped already.

#### Tonyr1084

Joined Sep 24, 2015
6,258
Well, that's the end of that relay. Was carefully grinding off the corners and the darn coil goes across the top. I ground through several windings. I guess I now can go all the way and completely demolish it. I'm now suspecting it's going to have a reed switch inside the coil.

#### Tonyr1084

Joined Sep 24, 2015
6,258
The truly committed would have jumped already.
Did. Landed on my arse! Ruined the coil.

#### Tonyr1084

Joined Sep 24, 2015
6,258
In conclusion - there's no way I could have pushed the armature down. And now there's one less relay in the world. A cheap sacrifice, but one without learning anything of value.

Thanks y'all.

Have a wonderful day.

#### nsaspook

Joined Aug 27, 2009
8,662
In conclusion - there's no way I could have pushed the armature down. And now there's one less relay in the world. A cheap sacrifice, but one without learning anything of value.

Thanks y'all.

Have a wonderful day.
Sure, there was value. I enjoyed it and you learned how not to open a sealed relay.

#### Tonyr1084

Joined Sep 24, 2015
6,258
Sure, there was value. I enjoyed it and you learned how not to open a sealed relay.
So - you enjoyed my expense. I had respect for you before. Now I'm not so sure.

It was my decision to go ahead and cut it open. Didn't take much pressure from anyone else. I own the failure. But what is dismaying is your enjoyment of it all.

Have a good day.

#### nsaspook

Joined Aug 27, 2009
8,662
So - you enjoyed my expense. I had respect for you before. Now I'm not so sure.

It was my decision to go ahead and cut it open. Didn't take much pressure from anyone else. I own the failure. But what is dismaying is your enjoyment of it all.

Have a good day.
No Humor today. Pretty thin skinned. I didn't enjoy your failure, I enjoyed your willingness to try.

Dude we all learn by failure. Those are the one's that we remember.

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