Inductive sensor problem on plc input

Hi, yesterday i wired a no brand Chinese NPN inductive sensor on allen bradley input card, so the input on the plc was always true even if the sensor was inactive, it does'nt changed state. I checked the sensor with a multimeter and the switching goes on an off so the sensor is OK unplugged to the input card. I changed for an Omron one and it work perfectly. It's probably due to the impedance of the chinese sensor, can someone would explain more in details that? Thanks

fons04 said:

Hi, yesterday i wired a no brand Chinese NPN inductive sensor on allen bradley input card, so the input on the plc was always true even if the sensor was inactive, it does'nt changed state. I checked the sensor with a multimeter and the switching goes on an off so the sensor is OK unplugged to the input card. I changed for an Omron one and it work perfectly. It's probably due to the impedance of the chinese sensor, can someone would explain more in details that? Thanks

Click to expand...

what input card?

KISS, is the input card pnp or npn?

janner_10 said:

KISS, is the input card pnp or npn?

Click to expand...

I don't remember exactly the number of the card but it was a card with each input isolated sink/source possibility,each input have its own Com (GND), so if you want sourcing input (NPN) you have to wire on the card terminal GND1 the sensor, and the IN1 the +24Vdc powersupply, with the Omron sensor it work perfectly

Leakage?

Mickey said:

Leakage?

Click to expand...

That would be my guess also. It is fairly common for inductive sensors to have high leakage currents and require a load resistor.

That should be where in the circuit and how determine the value? Thanks

Measure the voltage at the PLC terminal in both states. When the sensor is not detecting a target, the voltage needs to be lower than the minimum value required by the module to switch to the off state. You may need to add a resistor in series to get below the minimum level. Make sure the wattage rating of the resistor is sufficient to handle the heat that will be generated.

fons04 said:

...allen bradley input card...each input isolated sink/source possibility...terminal GND1...IN1...+24Vdc powersupply...

Click to expand...

Based on the information you have provided thus far, I'd hazard a guess at this input module being a 1756-IB16I?

If so, then the following are the main Technical Specifications for this particular digital input module...

Inputs - 16 individually isolated

Voltage category - 12/24V DC sink/source

Operating voltage range - 10…30V DC

Input voltage, nom - 24V DC

Input delay time (screw to backplane)

Off to On - Hardware delay: 1 ms max + filter time
User-selectable filter time: 0 ms, 1 ms, or 2 ms

On to Off - Hardware delay: 4 ms max + filter time
User-selectable filter time: 0 ms, 1 ms, 2 ms, 9 ms, or 18 ms
Current draw @ 5.1V - 100 mA

Current draw @ 24V - 3 mA

Total backplane power - 0.58 W

Off-state voltage, max - 5V

Off-state current, max - 1.5 mA

On-state current, min 2 mA @ 10V DC

On-state current, max 10 mA @ 30V DC

Input impedance, max 3 kΩ @ 30V DC

Note: Only reference the above On-state values if you have confirmed that the input module is a 1756-IB16I.

So for this particular input module, any value at or above 2 mA @ 10V DC will switch the input channel ON.

Regards,
George

It looks a good number, thanks to all for your support

The best solution I have found for leaky sensors and such is an old school incandescent indicator light as a load. At low currents they act as almost a short. Once the sensor turns on the impedance of the indicator will go up. Of course you need to make sure the sensor can power both the input card and the incandescent indicator. Mostly I have used this on piezo alarms that squeak on leaky output cards. I find that this works a little better than just a load resistor. YMMV.