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Problem with stuck relay

maker_dwonnyr1_1663361508

Sep 16, 2022
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I'm not sure if this is the right area for this; if it's more appropriate elsewhere please let me know.

I have an alarm set up that is used to alert the end of a process. It consists of a Procon PT4RO hooked up to a light/buzzer, that when it receives the signal is supposed to close the loop and light up. With the original light that never had any issues, but after switching to something more robust (with the same electrical requirements) the relays in it (NT74 is the model number I've found if that's helpful) are getting stuck. Regardless of what command is sent to the relay the light will remain on and the circuit stays closed. Sometimes I can still hear the click of the relay moving and sometimes not, but with both there is no change to the circuit and light. It will eventually go away and resume normal functioning, but there doesn't seem to be any rhyme or reason to it doing so.

Is there any solution to this? Or should I be looking for some other kind of relay instead?
 

maker_dwonnyr1_1663361508

Sep 16, 2022
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The initial lights were fairly cheap and the LED on them tended to go out. The new ones are just physically sturdier and from a more reputable brand. The relay in question says 10A/250VAC on them. The light has an operating voltage of 120 VAC and 175 mA current (which I would assume would be well within it's capabilities).

I did find the sheet that came with the new lights and it mentions a Surge with a 50A inrush for 260 microseconds. Could this be enough to overwhelm the relay and fuse the contacts? If so is there a way to insulate the relay before the lower operating current is reached?

This may or may not be a correct understanding of the problem - I would be the first to admit my understanding of circuitry is that of a novice at best. If any further information could help shed some light on this please let me know
 

Bluejets

Oct 5, 2014
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5,883
Show a drawing of your circuit and include any and all power supplies.
 

crutschow

May 7, 2021
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it mentions a Surge with a 50A inrush for 260 microseconds. Could this be enough to overwhelm the relay and fuse the contacts?
Possibly.
A resistor in series with the contacts (i.e. 10 ohms) would limit the surge, and shouldn't affect the normal lamp operation.
 

maker_dwonnyr1_1663361508

Sep 16, 2022
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Here is a very basic diagram. Basically all that we need to have happen is for the relay to receive the alarm signal and to close off and complete the loop. Would a resistor just go across the C and NO contacts for the relay? Would a snubber be something to consider? I've seen a few other forums putting that forward as an idea
 

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maker_dwonnyr1_1663361508

Sep 16, 2022
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I'll give that a try then. So just a 10 ohm resistor basically in between the light and one of the terminals? Should it be wired up in any special way?
 

Bluejets

Oct 5, 2014
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The normal approach would be to use a series resistor and then switch it out of circuit (bypass) once the surge has passed.
To calculate any series resistance wattage by leaving it in circuit, the load current would have to be known.

I think you have other problems and this will lead nowhere.
For example, what powers your Procon PT4RO...?
Typically why a drawing of the circuits was called for originally.
 
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maker_dwonnyr1_1663361508

Sep 16, 2022
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The PT4RO has a converter that supplies it with 24 V DC power for the logic side. As far as I can tell this is still working as intended. The PT4RO communicates with the computer and the lights for each relay respond to commands. With the previous lights the relays still work just fine (and initially with the new lights as well). It's only continued use with the new lights that seem to lead to the relay getting stuck and the circuit remaining closed regardless of what command is sent.

I've attached the full wiring diagram with every wire in the system (minus ethernet cables). I obviously don't have a firm grasp on typical notation, so if anything needs clarification please let me know. If this is something that's being affected by a wiring flaw of mine that would be great as I wouldn't have to get any additional parts. But if this is caused by a greater surge of current from the new light what series resistor would I need (if that is indeed what would fix the problem?

Thanks everyone for your continued help
 

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Bluejets

Oct 5, 2014
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What is the 24v dc converter, any link or detail specs?

Did the original lights require an earth...?
 
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