Low continuous output voltage

I have a 30 year old Symax PLC. Lately two of the outputs are putting out approx. 8vac continuously without the output being energized. All the outputs operate are indicator lights, which are glowing real dim (it's 120vac indicator lights, very low wattage).
What can I use to draw the voltage down to zero when the output is not in use?
I've replace the output card 3 times without any change to the output voltage.
Is it possible that the rack may be going bad and causing this low voltage output continuously? All the other outputs on the same card are reading zero volts.
Thanks for any help,
Dave

Solid state AC output devices have leakage. Review the specs for this card and compare it to the current required to light your indicators. Parallel resistors will help at the expense of wasting power. Possibly switch to relay output. Or possibly change to DC indicators (LEDs?) and output cards.

If you have replaced the cards & the fault is still there then it is unlikely to be the PLC/Rack. Normally outputs are isolated from the PLC bus by opto-isolators on the cards, one other thought is pick up on the cables, try disconnecting them & see if the voltage still exists. I assume these are triac outputs. there could be a leakage. Putting a larger load on them may reduce the voltage or perhaps fitting transformer type bulbs i.e. 110 LED type if you can still get them or a resistor but be careful, make sure the resistance is not too low and the wattage is high enough so they do not get hot.

fishenguy said:

I have a 30 year old Symax PLC. Lately two of the outputs are putting out approx. 8vac continuously without the output being energized. All the outputs operate are indicator lights, which are glowing real dim (it's 120vac indicator lights, very low wattage).
What can I use to draw the voltage down to zero when the output is not in use?
I've replace the output card 3 times without any change to the output voltage.
Is it possible that the rack may be going bad and causing this low voltage output continuously? All the other outputs on the same card are reading zero volts.
Thanks for any help,
Dave

Click to expand...

'Lately' sounds like it's a new issue. Aging electronics leaks more ... so that is reasonable.

As suggested, a parallel resistor (that will not melt when 120 VAC is applied continuously) is the cheapest solution. But it heats up and wastes power.
Something in the 10K, 2 watt range should do. I calculate 1.44 watts at 120 VAC so you would have an OK (although not great) safety factor on that.

If the 10K does not turn off your LEDs, you will have to drop the resistor value and increase the wattage. That may not be practical.

I'm surprised that the LEDs light at all with only enough leakage current to show 8 VAC at the terminals. Under 10% voltage ... should be *WAY* down the V/I curve ... is your room really dark? If not, maybe your LED lights have had their current limiting resistors cooked and those resistors need replacing?

What I do, because the correct resistor isn't usually around, is add a relay coil to the output.


I had one application where the TRIAC output powered an AC adapter and when it was off the leakage was enough to charge up the adapter and cause the connected controller to flash about once per second. A 8 pin octal relay (an ice cube relay might not pull down the leakage enough) added to the terminal strip connections solved it perfectly.