Issues with LEDs on PLC outputs

[NetNathan]

I have a love hate relationship with LED status lights. I like them because they last a long time...BUT... They have a problem with PLC outputs.
I have to use Relay outputs PLC modules. If they are Triac out, the LEDs cause the output to partially energize??
They are very dull till output is on, then they are bright. If I use a small relay on the PLC output, the LED that says relay is energized, causes same issue.

Does anyone else have this issue?
I do not really want to have to put a resistor across the Output (1K OHM / 10watt) to ground. This does fix it, but the resistor gets real toasty when the output is on.

 

[TheWaterboy]

It's simply the nature of semiconductor voltage thresholds and leakage current that requires you need pullup or pulldown resistors matched the load. In a purpose-built device these are already built in because the designer knows what the load is. In this case you are the designer.

Having said that... I'm not sure why it was decided to leave low speed I/O unbuffered in this industry. I get it with highspeed IO in the khz range, but with normal DIO in the PLC related industry that's the exception rather than the rule.

BUT . . . you should NOT need a 10 watt resistor for this. There is a document somewhere that details the resistor requirements but 1/2 watt 4.7k to 10K resistor should get that job done. If you are warming up a 10 watt resistor there is something incorrect going on.

BTW Triacs are for AC signals, SCR or transistor is for DC. Perhaps that plays a role here?

 

[arpus4KM]

I have a love hate relationship with LED status lights. I like them because they last a long time...BUT... They have a problem with PLC outputs.
I have to use Relay outputs PLC modules. If they are Triac out, the LEDs cause the output to partially energize??
They are very dull till output is on, then they are bright. If I use a small relay on the PLC output, the LED that says relay is energized, causes same issue.

Does anyone else have this issue?
I do not really want to have to put a resistor across the Output (1K OHM / 10watt) to ground. This does fix it, but the resistor gets real toasty when the output is on.

Just use interposing relays for outputs and you'll get a two fold effect, you can make sure there is a large enough load on it that it will show OFF when OFF, and you protect the card with surge protection on the relay coil.

 

[Firejo]

http://www.plctalk.net/qanda/showthread.php?t=93584

 

[NetNathan]

Just use interposing relays for outputs and you'll get a two fold effect, you can make sure there is a large enough load on it that it will show OFF when OFF, and you protect the card with surge protection on the relay coil.

I am using interposing relays... but the relays have LEDs on them to show when on and that LED causes same issue.
I already replaced the the LED lights with Incandescent and that solved the lights that are on direct outputs, the interposing relays are the ones causing the issue.
The PLC is 24VDC out and I am controlling some 120V devices through the interposing relays.