How does an A/C 240v Led work (for use as a reverse polarity indicator)

[fuss]

I have tried to measure the resistance but it is an open circuit, I thought they would have a resistance of around 50,000 Ohms so they would work with 240v.

After a bit of research it seems that ac leds use some sort of blocking/ voltage limiting diode, does that sound right? not that I understand what that means.

The reason that I ask is , I want to put a reverse polarity indicating led (red) between the ac neutral and ground, and a correct polarity indicating led (green) between live and earth.

I see, after some reserch, that this may create a potential earth leak unless an 8000 Ohm resistor is used.

The initial reason for using 240v leds was that I figured they would already have the resistor built in anyway, and now it seems they dont work that way.

All I want to achieve is to have the read and green indicators working without causing any new potential problems.

Can anyone give me any help with this

thanks John

 

[old_salt]

It's not worth messing about trying to make your own.
Get the tester from here:-http://www.screwfix.com/search.do?fh_search=socket%20tester&cm_mmc=Google-_-Electrical-_-Test%208%20Detection%20-%20Socket%20Testers-_-mains%20socket%20tester?P=P&gclid=CISSvKfckZ4CFUYA4wodJjsMog&source=aw and use it in a socket to check may be it could be left in a socket connected permanently but I don't see any point in doing that once you are on a shore supply and checked it it's not going to alter just at the drop of a hat.

More here :-http://www.maplin.co.uk/searchtemplate.asp?criteria=MAINS%20POLARITY%20TESTER

http://www.test4less.co.uk/results.asp?menuselect=e&CategoryID=25&gclid=CIHjru_fkZ4CFZoU4wodQ0Zupg

 

[fuss]

I have that tester now, but what I want is it all built into my panel that I am making.

 

[halcyon]

After a bit of research it seems that ac leds use some sort of blocking/ voltage limiting diode, does that sound right? not that I understand what that means.

Normally use a neon, not LED's.

Neon indicators have resistor built in, so just wire neutral - earth.

One problem can be fitting after RCD, as the current flow if you have reverse polarity can be above the RCD trip value. So if you have reverse polarity the RCD trips, and the light does not work. So we used to fit them before the RCD.

Brian

 

[rogerthebodger]

I fitted red neon between neutral and earth as a correct phase indicator (on shows a fault) and a green between live and earth (on shows a correct earth connection)

a neon only draws 2/3 mA where as a LED may need 10/20 mA to show correctly and this would be too close to the RCD tripping point.

 

[mattw]

LEDs only switch on when there is more than 1.4v between anode and cathode. Multimeters use a tiny voltage to measure resistance so they will just see an open circuit. Some multimeters have a dedicated diode test function which is useful for sussing out the polarity of LEDs as well as checking that they work.
There is no correct polarity between earth and anything else so, as others have said, you're better off using neons instead. The voltage between live and earth should oscillate between +339V and -339V. An LED will be switched off when the voltage is negative, while the neon will lit the whole time that there is sufficient voltage.

 

[fuss]

LEDs only switch on when there is more than 1.4v between anode and cathode. Multimeters use a tiny voltage to measure resistance so they will just see an open circuit. Some multimeters have a dedicated diode test function which is useful for sussing out the polarity of LEDs as well as checking that they work.
There is no correct polarity between earth and anything else so, as others have said, you're better off using neons instead. The voltage between live and earth should oscillate between +339V and -339V. An LED will be switched off when the voltage is negative, while the neon will lit the whole time that there is sufficient voltage.

If the 240v leds show an open circuit when no power is flowing then it seems that these are ideal for use as polarity indicators. My main concern is the connection that is made when the 240v red led is connected from neutral to ground and the green led is connected from live to ground.
So i dont understand why 240v Leds would not be better than neon.
Or am I still missing something?

I've wired it all up and it works fine, but I am still a bit concerned.

As I said before, the info on polarity indicators state that you must insert an 8000 Ohm resistor to stop earth leaks. Does this mean then that they cannot contribute to an earth leak which also can trip the RCD?

Thanks again for all the replies