Best and easiest way to identify a tripping over temp switch

[SixOn]

I have two NC over temp switch in a circuit and they are connected in series one after the other.
Each is connected to a different side of the compressor and they are suposed to open when the compressor is over heating.
One of these switches keeps tripping out the compressor almost 5 times a day.
I want to add a mini circuit to the current 24DC one that alerts will alerts me the side of the compressor with a problem so that has a problem.

 

[MikeHalloran]

That circuit could also be tripped by a broken/breaking/loose wire.
I'd eliminate that possibility before adding complexity.



Mike Halloran
Pembroke Pines, FL, USA

 

[OperaHouse]

That could be a problem. If I understand, you have two switches in series and it is a 24V DC line.

If these feed into something like an opto22 input in the existing control, the current will be fairly small. Any sensing across the two switches (like another opto22)would draw enough current so that existing controller would never see the open switch.

The work around this would be to put a 200 ohm resistor on the existing controller input to provide a greater load. Then sensors across the switches would likely work.

 

[MiketheEngineer]

Check the wiring as mentioned. If they are cheap enough - swap them out for new ones..

 

[ScottyUK]

Once you confirm this isn't a wiring problem, you need to figure a way to monitor the contacts individually. An easy solution would be to use each thermal switch as the retaining contact of a latched relay. You'll need an additional NO contact to arm the latch initially. Use a NO contact of the relay to mimic the behaviour of the thermal switch contact in the original circuit.

If this is a PLC-based controller and you have a spare input you can do the same in ladder logic.


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If we learn from our mistakes I'm getting a great education!

 

[IRstuff]

Seems to me that you ought to be able to instrument both switches with voltmeters and just monitor them for a non-zero voltage drop.

TTFN
faq731-376

 

[ScottyUK]

Depends if you can sit there watching or not, and if the switches self-reset. Most compressors I work with are not pieces of plant I would care to sit around for long, but certainly a workable solution in some situations.


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If we learn from our mistakes I'm getting a great education!

 

[DRWeig]

In the olden days, we used to jumper a Sylvania Blue Dot flashbulb in parallel with each contact. The one that opened would blow its lamp. You do need a bit of current (but not much) but adding a load resistor can get you there if it's an analog input.

Or, an appropriate micro-fuse can be chosen. But not as colorful as the Blue Dot solution.

Good on ya,

Goober Dave

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[SixOn]

I actually done some thinking a thought a simple resistor and a red LED would do the trick just fine.

I was thinking that since the TEMP switch is NC, I can use the NO contact for the LED circuit. I'll use the resistor to limit the current in the LED circuit to about 1A & I expect the red LED to light up and stay so every time the TEMP switch trips.

I'm not sure if this would work but I'll be testing it out soon.

 

[IRstuff]

What kind of LED are you talking about? 1 amp is a lot of current for a single LED

TTFN
faq731-376

 

[OperaHouse]

I've got some. That actually isn't a bad idea. I bought some mini 10W 9-24V LED sealed flood lights for under $20. You need about that much power for them to be seen in an industrial site.

 

[btrueblood]

The problem with wiring a lamp or LED in parallel to the temperature switch is if the lamp/LED passes enough current to spoof the circuit it's attached to into thinking the temp. switches are still closed. A better approach if that's true is to wire a momentary switch in series - to check the temp. switches, push the momentary and see if the associated lamp lights up. Still not much use if the temp switches are self resetting; I like Dave's method the best.

 

[MikeHalloran]

I'm thinking a high value resistor across each temp switch, with two different resistor values, would minimize wiring changes near the compressor and still allow an add-on circuit at the panel to decode which switch went open. ... but that depends on the nature of the primary alarm, so far not revealed.



Mike Halloran
Pembroke Pines, FL, USA

 

[SixOn]

If I were to use a relay that monitors the temp switch single NC contact (the temp switch is only wired to a NC contact and no free NO contact around that closes when the NC opens), how would I wire this up.

I understand I would have to use some ladder logic circuit. I'm just not quite sure how to monitor a single NC contact that blows open shuting off the compressor & waiting for a manual reset.

 

[SixOn]

ScottyUK mentioned something similar in the 5th post & I'm intrigued to know how it works

 

[ScottyUK]

Sixon,

Google how a three-wire motor starter works, one with a start button and a stop button. You would basically build a miniature version of it using a relay rather than a contactor.

The temp switch is the 'stop' button, the 'arm' or 'reset' contact is the 'start' button. Use one of the relay contacts to replace the temp switch in the circuit. Obviously you will need a source of power for your relay.

When the switch opens the relay drops out and causes the machine to trip as it does now. The scheme requires a manual reset so you have a latched indicator of which switch caused the fault which will sit there until you reset it. No need to sit for hours waiting for a trip.


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If we learn from our mistakes I'm getting a great education!

 

[SixOn]

ScottyUK

Are you saying I must do something like this?

-----------NO------temp switch(NC)--------coil--------
|
|
-------reset----|


--------------------NC--------buzzer------------------
|
|
|----lamp--------------------

 

[ScottyUK]

Yeah, something like that. You might additionally want to use one of the relay contacts to connect in the position the temp switch normally occupies so you don't lose your protection function.

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If we learn from our mistakes I'm getting a great education!