Inductive Proximity Sensors

[tomshirvo]

Hi All,

I have got a problem with Inductive Proximity Sensors. I build a motor control box that uses Inductive Proximity Sensors and I have been having trouble with the Arduino picking up the Sensor being on or off.

The Mosfet I am using is a BSS138.

I have attached the specs and the setup of how they are connected.

I have 4 of these boards with two sensors attached too them. Only 2 of 4 boards work with the sensors.

On the setup picture I have marked M1 and M2;

Sensor off; M1: 8.8v , M2: 5v
Sensor on; M1: 0.625v , M2: 0.68v

This is just one of the boards that doesn't work but the other boards are all different in this measurement.

One board that works the M2 when sensor is off is 24v which is what I thought it should be for all of them.
I just can't understand why the 24v pullup isn't bringing it to 24v. All I can see is that the Mosfet is stuffed.

I have been stuffing around with it for days and I don't understand why some boards work and others don't

I think the biggest problem is that when the Sensor is ON it doesn't pull down to GND it just brings the voltage down to 0.625v.  So this must be the problem but I don't know where to go from here.

Have I made a terrible design for how the limits hook up or is it just the Mosfets are faulty?

Any help would be great.

Thanks

[mij59]

Hi,

Take a look at the data sheet of the BSS138 , max. Vgs is 20 V, no need to drive the gate at more than 10 V.

[tomshirvo]

So you think I have damaged the Mosfets?

[AndyC_772]

It's easily done. The gate of a MOSFET is insulated from the channel by a very, very thin layer of silicon dioxide, which can be damaged by any voltage above the rated Vgs. They're particularly sensitive to static discharge for the same reason.

[mij59]

So you think I have damaged the Mosfets?

Yes probably, you can check it with  a multimeter, but even if they test good I wouldn't trust them any more.
In your circuit a Vgs of 5V will enough to switch the mosfet.