Leakage current is too high

NetNathan · Feb 5, 2015

I have an IFM Efector 1/4 turn valve limit switch (IN0177). Operating on 115VAC.

Problem:
The PLC input is energized with limit switch not energized.

The Honeywell HC900 PLC input module is rated 80-264 VAC and manual shows minimum "On" current of 0.3 mA and maximum "Off" current is 0.2 mA.
The IFM Efector limit switch manual shows leakage current of <0.8 mA.
I believe the leakage current is energizing the input.

How do I solve this?
A resistor across the PLC input or in series between LS and PLC input
What size?

I have test wired in a relay (instead of the PLC input) and it works correctly.

jrwb4gbm · Feb 5, 2015

If you have space for the relay that is a good method for isolation. Just use the contacts to switch the actual input. I personally don't like using resistors for this purpose.

ASF · Feb 5, 2015

jrwb4gbm said:
If you have space for the relay that is a good method for isolation. Just use the contacts to switch the actual input. I personally don't like using resistors for this purpose.

I second that. Especially on 115V, I'd be steering well clear of resistors

NetNathan · Feb 6, 2015

I would with 1 valve.....but I have 10 valves and monitoring open and closed position, so 20 relays would be required.

Ken Roach · Feb 6, 2015

I'm far less familiar with using bleeder resistors in 120V AC circuits than in 24V DC circuits, but I just went through an exercise with ninety 2-wire DC prox sensors so I sympathize.

Go ahead and test with a resistor in parallel with the PLC input, so that it's connected between the output of the sensor and AC Neutral.

V/I = R in ordinary simple resistive circuits, so 120 / 0.0008 = 150,000 ohms.

The resistor has to be able to handle being connected to 120V AC all the time. Power = I^2 R in simple resistive AC circuits, so 0.0008 ^ 2 * 150,000 = 0.096 watts.

I try to be very conservative with wattage ratings, so start with 1/2 or 1 watt resistors.

NetNathan · Feb 6, 2015

Ken Roach said:
I'm far less familiar with using bleeder resistors in 120V AC circuits than in 24V DC circuits, but I just went through an exercise with ninety 2-wire DC prox sensors so I sympathize.

Go ahead and test with a resistor in parallel with the PLC input, so that it's connected between the output of the sensor and AC Neutral.

V/I = R in ordinary simple resistive circuits, so 120 / 0.0008 = 150,000 ohms.

The resistor has to be able to handle being connected to 120V AC all the time. Power = I^2 R in simple resistive AC circuits, so 0.0008 ^ 2 * 150,000 = 0.096 watts.

I try to be very conservative with wattage ratings, so start with 1/2 or 1 watt resistors.

Thanks....I will test out.

NetNathan · Feb 6, 2015

Using 150Kohm - 1/2 Watt
Works great....thanks Ken.

shooter · Feb 8, 2015

calculation is not correct and 150000 is very much to be sure, but if is enough just do it.
another way is to have some pilotlights over the plc inputs.
if mounted in panel you can see the states direct.

Ken Roach · Feb 8, 2015

I'm willing to have made a mistake, but Ohm's Law is straightforward in simple resistive circuits, both DC and single-phase AC.

Where is the calculation wrong ?

ASF · Feb 8, 2015

0.008 would be 8mA. "The IFM Efector limit switch manual shows leakage current of <0.8 mA"

Sherlock4me · Feb 8, 2015

IF I am not mistaken, ASF's post is in response to a post that read "Shouldn't it be .008 not .0008?" In case anyone was wondering...

I guess the poster deleted it after being correctly corrected??

I am looking forward to also learning something, because Mr. Roaches numbers seem on the money when I run them. Perhaps I am missing something, wouldn't be the first time...

Secpcb · Feb 9, 2015

shooter said:
another way is to have some pilotlights over the plc inputs.
if mounted in panel you can see the states direct.

I second the motion for a string of pilot lights. This way it helps the operator & serviceman see what's not on quickly.

I frequently get called to one line where the conveyor isn't running, and find one or more of the 23 required inputs not ready to allow the conveyor to run - and the 23 status's are not shown on the HMI.

I have suggested more than once to my boss a string of 23 pilot lights, but he keeps saying no - the operators will figure it out; but the operators keep calling me every time.

Ken Roach · Feb 9, 2015

On my most recent machine I have a series of HMI indicators that show green to indicate that a complex tooling fixture is clamped and verified at all points.

While describing the system to a group of operators, I slipped into Tom Clancy mode and said "your board is green, go ahead."

One of the operators, almost without thinking, responded: "Dive ! Dive !".

NetNathan · Feb 9, 2015

I am using pilot lights but they are on outputs. I blink them if the valve is in "fault" condition.

Yes the 150Kohm is wrong, it is too high. I get intermittent limit switch "on/off" condition when energized.
I am now trying different resistors.
I am now trying a 39 Kohm 1 watt. It appears to work okay.

shooter · Feb 9, 2015

why is 150000 ohm wrong, well if this is what is flowing the voltage over the resistor is 120 Volts, meaning the input is high (when the input is very high in impedance.
39 kOhm is 31 Volt and that is below the threshold.
in off it will be 0.025 Watts
in ON it is 0.4 Watts, double it and result is 1 Watt, just fine

Leakage current is too high

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