Load indicator light "stuck" on

WrongWay · Jun 8, 2012

I was called out to a piece of equipment today because a new sensor that was installed was not working properly. I found an inductive prox, 3 wire, PNP NO that had the load indicator light made with no target present. When I presented a target the intensity increased. I removed the prox and put in a tester and the green NPN light lights until I make the switch and then the PNP light makes. Voltage on load wire without target is about 14VDC and 26VDC when made. This sensor is from auto direct, I tried a new one and had the same results. I switched to an AB prox and had no problems. I have seen this before with a balluff sensor but never bothered to figure out why. Could it be that some 3-wire prox's need a resistor on the load?

jkerekes · Jun 8, 2012

Too much leakage current? What input module are you using?

Thomas · Jun 8, 2012

I've seen this before when the input impeadance is too high to allow the device to switch off completely. A "bleeder" resistor on the load side will usually draw the voltage level down enough.

WrongWay · Jun 8, 2012

The sensors behave the same on a prox sensor tester, which simply has a place to land +,-, and Load. There are no internal resistors on the tester, just lights a the NPN or PNP leds depending which side of the Load the switch is on. I just wondered why particular sensor would light the NPN when not made and the PNP when it is.

WrongWay · Jun 8, 2012

Thank you, I suggested that but the maintenance manager felt we shouldn't have to do that. I'm just trying to understand in what application this sensor would work without having to add additional components?

Goody · Jun 9, 2012

I have also seen this with certain brands of proxy's - and it was the power supply.
By that I mean not pure 24V DC. If there is 'ripple' in the 24V some proxy's just don't like it and work as you described.

It's usually been when the 24V is supplied from a transformer and then rectified.

WrongWay · Jun 9, 2012

Thanks Goody,
This particular sensor has a input voltage range of 6-50VDC, fairly liberal. My tester uses 3 nine volt batteries for power.

Goody · Jun 10, 2012

It's not the voltage range of the sensor, it's the actual un-clean 24V DC.
How is the 24V DC made - through a pre-made voltage stabilizer unit or transformer and rectifier.
If it's the latter then there is often (in USA) 120HZ ripple in the 24V DC supply. That is enough to partly trigger the proxy or not make it work at all.

wimpiesplc · Jun 10, 2012

I don't think the tester he mentioned has any ripple though...

Isn't there any manual available with connection examples? you said you tried a new one. didn't it come with any kind of connection schematic?

Goody · Jun 11, 2012

It read to me as though it worked when he used a battery voltage tester

WrongWay · Jun 11, 2012

Goody when I wrote
"The sensors behave the same on a prox sensor tester, which simply has a place to land +,-, and Load. There are no internal resistors on the tester, just lights a the NPN or PNP leds depending which side of the Load the switch is on. I just wondered why particular sensor would light the NPN when not made and the PNP when it is," They behaved the same as in the indicator light on the prox is on with no target and intensifies when made. As far as a connection schematic, just your typical +/-, load drawing with the corresponding brown/blue, and black.

wimpiesplc · Jun 11, 2012

It read to me as though it worked when he used a battery voltage tester

uhm..

The sensors behave the same on a prox sensor tester, which simply has a place to land +,-, and Load

Lancie1 · Jun 11, 2012

I just wondered why particular sensor would light the NPN when not made and the PNP when it is,

To answer that question, I would need to see the internal circuit of the tester - to find out what triggers the "NPN" light, and what triggers the "PNP" light. A guess would be that that are two transistors, a NPN transistor connected to the NPN light, and a PNP connected to the PNP light. Of course these transistors have opposite polarity and thus either switch voltage to a load, or serve as a sink for a voltage source.

Load indicator light "stuck" on

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