Why is the Relay Coil Ohms Changing When Measuring

Thread Starter

dw85745

Joined Oct 5, 2015
32
I have a 5-2-1 normally closed relay. Pin5 to Pin2 are the Coil. Pin2 to Pin1 are the contacts. Pin2 is Common. A BackFlow EMF is what causes the contacts to Open when incircuit. (Still don't get how the BackFlow EMF "actually works", but that is a question for another post.
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I just put an Ohm meter on the Relay Coil. It initially registered about 14000 ohm (EDIT: s/b kOhms NOT Ohms), but as the leads were held the value keep dropping 1 or 2 ohms at a time. I assumed the meter was trying to find a value. I got tired of waiting so removed the leads and tried again. The Ohm value picked up about the same Ohm value when I removed the leads and then continued to drop further. I repeated this one more time and the same thing happened -- started at the prior value and continued to drop. Can someone explain this?
Thanks
 
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WBahn

Joined Mar 31, 2012
27,880
Your ohmmeter works by applying a voltage and measuring the current that results. It then essentially divides the voltage by the current to get the resistance. When you put your meter across it, the inductance of the coil prevents the current from changing immediately, so it ramps up over time. Your meter is displaying a value based on the average current over some sampling interval (or the current at a sampling point -- depends on the meter). So it will display a higher resistance value. As the inductance effects go away, the current will stabilize at the DC resistance of the coil. How long this takes depends on the inductance of the coil and the output resistance of the meter combined with the coil resistance.
 

dcbingaman

Joined Jun 30, 2021
798
Possibly the cause and something you need to consider:
14Kohms with a 1 to 2 ohm drop is not significant. That is an error change of 0.014%. I don't know what meter your are using but I doubt the accuracy is better than that at that small of a change. In other words the reliability of the meter in that range should not be looked upon or considered real.
Another thing I have recently ran across: I am dealing with a solenoid that has a resistance of around 4.5 ohms, I have found the actual resistance for this solenoid happens to be temperature dependent. I have found at higher temperatures the resistance is closer to 4.8 ohms when 'hot to the touch'. So this is a change based on temperature of around 6% or over 400 times more change than the relay you are measuring. Thus the relay itself resistance could be changing by that amount due to nothing more than temperature changes. This reminds me of how we should not look at changes on a measuring device that is not capable of measuring with this much accuracy. We must know when it is safe to ignore such microscopic changes. Not that this is the cause of what you are observing but it is possible. Part of the problem is digital display meters: They make these small changes look real, where an actual analog display showing 14Kohms would not even move for this small of a change and the user would not even observe it.
 

Alec_t

Joined Sep 17, 2013
13,229
Are you measuring the coil resistance in circuit or out of circuit?
Does the relay have a built-in diode for back-emf protection?
 

Ya’akov

Joined Jan 27, 2019
6,856
As @WBahn has said, the cause of the drift may be the inductance of the coil. Though you pushed off the question of how back EMF works, it may help to understand that very thing to see why what you are seeing could happen.

In a very simplified form: an inductor is an electromagnet. When current is applied to it, a magnetic field forms parallel to the coil. This field takes energy to create and the field is effectively lossy storage for that energy. An inductor has a specific capacity to create such a field, so after a while it stops using the energy to create the field and starts letting the current pass through. So long as the current is supplied the field will be maintained though at a much lower cost than creating it.

When the current is cut off, the field can’t be held up any more, but it has to go somewhere. Just like an electromagnet takes current and makes a magnetic field, a generator takes a magnetic field and makes current. When we stop holding up the field by supplying current, it collapses.

When the field collapses it doesn’t care how long it took to get the energy in, it collapses as fast as it can. This results in a much higher voltage in the opposite direction from the current that created it, hence back EMF. The resulting voltage spike can be prodigious. It easily destroys things like transistors that might be used to switch the coil on, as in a relay.

The effect is also used to advantage in boost converters where an inductor is charged up to some value, then intentionally allowed to collapse into a capacitor that can store the higher voltage output. This can then be used to trade current for voltage and boost a DC voltage where something like a transformer doesn‘t work.

The foregoing is very simplified and should only be considered a sort of hint to further study. All analogies break down as some point and only if yo understand it can you see where that is the case. I think the general concept is useful, though, to get you in the ballpark and point you in the direction of further investigation.
 

Thread Starter

dw85745

Joined Oct 5, 2015
32
Thanks to everyone for their replies.
WBahn: Thanks that is what I had surmised, just first time I've seen it, but normally use (and prefer) an Analog meter.
dcbingaman: I'm using a UTI-61E (with a Borne POT upgrade that really stablized the meter and improved accuracy.).
AlbertHall : Relay is 240 volts. I'm attaching a copy of the circuit.
Alec-t: Measuring Out of Circuit. Regarding diode -- unknown -- but my guess NOT.
Ultimately, I had hoped to find a way to test in circuit, but so far because of BackFlow EFT taking it out of circuit within a second or two, no luck. Circuit diagram below.
Ya'akov: Thanks for the explanation. Lot to digest there. Will take me a bit -- AND as stated just a start point.
 

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MisterBill2

Joined Jan 23, 2018
13,778
The change in apparent relay resistance due to relay coil inductance all happens withing a very few milliseconds and so whatever additional change is seen after that is due to some other cause.
In many cases the change is due to the change in the probing contact resistance.
 

MrSalts

Joined Apr 2, 2020
2,617
1) Check the battery on your meter. The ohms reading usually create a constant current for each range. If the battery is low, the current isn't constant and you get iffy readings.

2) If the relay is old or stored in a drawer in your garage for some time where the unidirectional was high or any salty air from the ocean, you can have some surface corrosion on the leads of the relay. Moving your probes around on the oxidation can cause the voltage to jump around.

note: 230vac coil with 14k ohms yields about 15mA of current. This is in the typical range for a 230vAC coil (3 to 60 is about the limits I've seen).

I'd replace the battery, polish the leads on the meter and the relay with a bit of scotch-brite and try again.
 

DickCappels

Joined Aug 21, 2008
9,512
2 Ohms out of 14K. I agree with dcbingaman in post #3. Do not worry about it. It could be as simple as oxidation of the surface your probe is contacting. In any case, this should not be a problem in any consumer product I can think of.

By the way, is this relay operated from 120 or 240 VAC?
 
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