Wiring 3 proximity sensors in series

Question

I have a stepper motor controller which controls 4 fairly powerful steppers and has input pins for 3 proximity sensors. The controller requires 48v so I have the power supply for that. The 3 NPN proximity sensors I have operate at 10-30v and draw 150mA. I initially connected the out pins of the sensors to the controller and powered the sensors from a 12v psu I had lying around. I then connected ground from both power supplies. I was told this was incorrect because if one power supply failed it might wreck other parts of the circuit.

My problem then was how to get it all working correctly. I thought about a relay for each sensor but my preference was for powering all sensors from the 48v supply. I thought about stepping voltage down from 48v to 30v but it requires knowledge I don't have. I could use resistors to do it but I get the feeling that isn't a good idea. It would only draw a maximum of 450mA-500mA but steeping down from 48v to 30v would generate a lot of heat correct?

My other idea was to wire them in series but that's not working as planned. The voltage across sensor 1 is 22v, across sensor 2 is 15v and sensor 3 is 11v. Sensor 1 is operating correctly, when metal passes it the sensor triggers. Sensors 2 and 3 are always on which is confusing because they're clearly getting power but for some reason think they've been tripped when they haven't. When I read the voltage between the + of each sensor and its out pin I get whacky results. Sensor 1 is reading 0v which is correct. Sensor 2 and 3 read 8.33v and 25v respectively.

Since sensors 2 and 3 are trigger, I would've thought those readings would've been the voltage applied across them (15v and 11v) with slightly lower values due to resistance. But, seeing as they're triggered without anything near the sensor then it's strange they have a voltage across them at all.

Is there a way to get the 3 sensors working in series or should I tackle it another way? I'm wondering if there's something I can do to split the voltage into 3, thus delivering 16v into each sensor without any fancy electronics.

Sensor doco (PL-05N)

Answer 1

You might be able to make this work but there are some sticky issues. First, if the ground connections of the sensors are not connected to the same ground as the controller then the sensors will not be able to pull their outputs down to the controller ground. As a result the controller will never see a logic low level from the sensor. One way to correct for this is to add an optoisolator at the output of every sensor. The output of the sensor would then activate an LED in the optoisolator and the phototransistor in the optoisolator could activate the controller input. This is pretty easy and inexpensive.

The harder problem is providing a stable supply voltage to the three sensors. When circuit elements are connected in series we know that the current through them must be the same but the voltage across them can be different. You need a way to make sure that the voltage across each sensor remains relatively constant even though the current they draw varies over time. I'm thinking that it may be enough to put a 16V, 1W, zener diode and a sizable capacitor in parallel with each sensor's power connections.

If you're not comfortable doing this then I would suggest just creating a 12V or 15V power supply using a linear regulator (say a 7812) from the 48V supply. You only need about 30mA to operate the three sensors, and I can't think of any problem that would occur by doing this.

Answer 2

Yea you can't do this. The proximity sensor you have is using an NPN transistor topology. The transistor inside looks like this:

This is NOT a linear device. You cannot wire these up as you would LEDs in series. As this device reaches its saturation, you get a defined gain which intrinsic to the specific device. Basically this thing is a little amplifier. So if you hook them up in "series" you are going to be amplifying the previous output in the current proximity sensor and so on for however many you have.

You have to hook them up separately.