Advice on wiring of temp sensors

[Omatako]

I need to connect two temperature sensors to a control panel to monitor the temperature on the exhaust system of my home-made genset. The sensors that are already fitted were intended on the original engine to talk to a fancy electronic monitoring system which is no longer present. I want to use the original sensors and they are metal sensing (as opposed to coolant sensing) and are normally closed (i.e. they open the circuit when high temps are sensed).

This being the case I believe the solution to be a normally closed relay which will be open when it reads voltage coming from the temp sensors and the warning light and buzzer will not work. If a high temp is then sensed, the temp sensor opens the circuit, the relay will close and energize the lamp/buzzer.

Firstly does this make sense?

Secondly, I want to use a common buzzer on both sensors. Will the installation of diodes as shown in the diagram below achieve the desired result?



Thanks

[Faster]

No - sorry.

What you are doing is putting 2 loads (the coil and the light/buzzers) in series - that's not do-able because both 12 V devices now have to split that voltage and will not work.

Further, you're not even utilizing the contacts on the relays at all.

You can energize the coils with your 'temp sensors' (which are really temperature switches) and then use the appropriate relay contacts for your lights. Choose pilot duty relays because with your normally closed switches the relays' coils will be energized all the time.

Your temp switch to coils to ground cct will be fine; then you need to get 12 V to your relay contact common (center) terminals and wire your lights and buzzer to the N.C contacts. That way when your switches overheat and open, the coil drops out, and the contacts will close, energizing your lights and buzzer.

Most relays have two sets of contacts - or you'll easily be able to find some: In the sketch below you won't need diodes.



Too bad the switches (temp sensors) dont' have a N.O connection - then you wouldn't need the relay(s)

You'll also need to be able to cut the power to the whole cct to silence the alarms when not running.

[Omatako]

Perhaps I was not clear enough. The diagram was not intended to show every finite detail in the circuit, only to determine whether the position of the diodes would stop backfeeds to other lamps etc.

I'm fully aware that a relay won't work if it isn't connected . . . . the relay symbol was just to indicate where the relay would go.

But thanks anyway.

Please also see embedded comments below

Originally Posted by Faster

No - sorry.

What you are doing is putting 2 loads (the coil and the light/buzzers) in series - that's not do-able because both 12 V devices now have to split that voltage and will not work.

Further, you're not even utilizing the contacts on the relays at all.

You can energize the coils with your 'temp sensors' (which are really temperature switches) and then use the appropraiate relay contacts for your lights. Choose pilot duty relays because with your normally closed switches the relays' coils will be energized all the time. (I can't find any data on pilot duty relays that makes sense to me, can you elaborate please)

Your temp switch to coils to ground cct will be fine; then you need to get 12 V to your relay contact common (center) terminals and wire your lights and buzzer to the N.C contacts. That way when your switches overheat and open, the coil drops out, and the contacts will close, energizing your lights and buzzer. (That's pretty much what I expected)

Too bad the switches (temp sensors) dont' have a N.O connection - then you wouldn't need the relay(s) (That would've been too easy )

You'll also need to be able to cut the power to the whole cct to silence the alarms when not running.(Turning the power off would achieve both silence and non-running)

Sorry I posted this between your original and your modified posts. I need to digest your sketch and will come back.

[Omatako]

Your sketch still appears to have current flowing the whole time that temp is normal. Where do the "pilot duty relays" come in and what do they achieve?

Originally Posted by Faster

[Faster]

With normally closed temperature switches that's the only way this will work.

The coils are energized by the closed temp sw. That means that the contacts (shown normally closed) are in fact open and the lights and buzzers are off.

When the temp sw opens (at high temp) then the coil is de-energized and the contacts return to their (normal) state shown in the sketch.

The second set of contacts behave similarly, but being wired in parallel means they have an 'or' function, ie either relay (and by extension either temp sw.) will sound the buzzer.

Relays rated for "pilot duty" are meant to energized most of the time. For example, the relay used for your glow plugs would not be meant to be constantly energized, but rather only for short periods of time.

To recap: yes, current flows through the coils all the time temperature is normal.. however under those conditions the relay contacts shown are in fact open, and your lights and buzzer are off.

[Omatako]

Originally Posted by Faster

With normally closed temperature switches that's the only way this will work.

The coils are energized by the closed temp sw. That means that the contacts (shown normally closed) are in fact open and the lights and buzzers are off.

When the temp sw opens (at high temp) then the coil is de-energized and the contacts return to their (normal) state shown in the sketch.

The second set of contacts behave similarly, but being wired in parallel means they have an 'or' function, ie either relay (and by extension either temp sw.) will sound the buzzer.

Yes thanks, I get all that.

Originally Posted by Faster

Relays rated for "pilot duty" are meant to energized most of the time. For example, the relay used for your glow plugs would not be meant to be constantly energized, but rather only for short periods of time.

So a pilot duty relay is simply a higher rated unit that can handle continuous current not a different type of operation. Now I get that too.

And the layout you describe will solve my problem.

Thanks

[Faster]

Great! Good luck...

[Hartley18]

Originally Posted by Omatako

Your sketch still appears to have current flowing the whole time that temp is normal. Where do the "pilot duty relays" come in and what do they achieve?

If you're worried about current draw and don't mind electronics and soldering - have you thought of using an optocoupler in the NC switch circuit??

These use even less current than a pilot relay. If you're still wanting something you can wire up, Weidmuller have an extensive range of optocouplers and solid-state relays to choose from.

Hope this helps. Good luck.

[Omatako]

Originally Posted by Hartley18

If you're worried about current draw and don't mind electronics and soldering - have you thought of using an optocoupler in the NC switch circuit??

Thanks but it's not about the current that the unit will draw - this is a genset and while it is running it will be pumping anywhere up to 240 amps so an amp or two to keep the coils energized are not going to scare me.

It's more about heat with the constant current but if that isn't a problem and it seems that for a "pilot duty" relay it isn't then that's all I need.

Thanks for the help.

[Hartley18]

No problem. I have no idea how you're planning to run this, but just keep in mind that relays need a certain amount of voltage to 'cut-in'. If you're running this circuit off the gen-set output, whilst the unit is starting (not up to speed), your buzzer might go off..

Sounds like an interesting project.