indicator lites on relays not lighting relays pulling in

[realolman]

Yesterday I connected 3 110VAC relays to the same triac output on a PLC. when I tried it, I looked in the cabinets where the relays were located expecting to see the indicator lights (LED's?) lit up. I didn't see anything so I thought I had something screwed up.

Turned out that all three relays are energized - they pull in quite solidly just like normal - but the indicators on none of them light up. there are two from one manufacturer and one from another.

I can't figure out how the relays pull in, but the indicators don't light on any of them.

 

[petersonra]

most of the relays we use have a mechanical flag that indicates when it is on rather than an LED. you can't see it in the dark usually.

 

[realolman]

these have the flag also.... I'll have to look closer... sure looked to me that there was a tiny circuit board in there and a window on the front of the relay ...even with a little magnifying lens.

I was just wondering if there was any reason the triac PLC output wouldn't allow an LED to conduct... seems to me it should.

Would there be some sort of a (I don't know exactly what I'm trying to talk about here (again)) a forward voltage drop on the triac that would be larger than that of the LED?

 

[Besoeker]

Would there be some sort of a (I don't know exactly what I'm trying to talk about here (again)) a forward voltage drop on the triac that would be larger than that of the LED?

Unlikely I would have thought. In the on state a triac would normally have under 2V across it. Your nominal 110Vac would drop to 108V at worst. The relay has sufficient voltage across the coil to energise it.
The LEDs I've seen for indicator lamps usually just have a resistor in series to limit the current and are fairly insensitive to voltage at least in terms of giving some indication.
Maybe your relays have neons?

 

[petersonra]

these have the flag also.... I'll have to look closer... sure looked to me that there was a tiny circuit board in there and a window on the front of the relay ...even with a little magnifying lens.

I was just wondering if there was any reason the triac PLC output wouldn't allow an LED to conduct... seems to me it should.

Would there be some sort of a (I don't know exactly what I'm trying to talk about here (again)) a forward voltage drop on the triac that would be larger than that of the LED?

some relays have an RC network on the coil that you might be seeing. I have even seen some that have an RC network across the contacts, usually reed relays.

 

[realolman]

some relays have an RC network on the coil that you might be seeing. I have even seen some that have an RC network across the contacts, usually reed relays.

could be ... I'll hafta look closer... sure expected to see a light, though.... thanks

Maybe your relays have neons?

Neons ARE weird ... why would they not light due to being switched by a triac?

 

[GoldDigger]

Yesterday I connected 3 110VAC relays to the same triac output on a PLC. when I tried it, I looked in the cabinets where the relays were located expecting to see the indicator lights (LED's?) lit up. I didn't see anything so I thought I had something screwed up.

Turned out that all three relays are energized - they pull in quite solidly just like normal - but the indicators on none of them light up. there are two from one manufacturer and one from another.

I can't figure out how the relays pull in, but the indicators don't light on any of them.

Simple questions: When you connect one of the relay coils to 120VAC instead of the PLC output, do you see anything light up?
And are you sure those are triac outputs rather than SCR outputs? The relay would pull in fine on 1/2 wave rectified DC, but the LED would only light with one polarity.