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Strange readings on the proximity switch

gubavac111

New Member
Hi,

So, I've connected a proximity switch to 24V (+ to BROWN wire and 0 to BLUE wire). Could someone please try to explain the following readings I've got:

VOLTAGE BETWEEN BROWN AND BLACK
VOLTAGE BETWEEN BLACK AND BLUE
SWITCH NOT ACTIVATED
(NO METAL OBJECT IN VICINITY)​
0.73V​
23.24​
SWITCH ACTIVATED
(METAL OBJECT IN VICINITY)​
0.79V​
23.22​


First of all, the switch is brand new and I've observed this kind of behavior on another new proximity switch of a different brand.

Please find some switch pictures attached.

1579675675565.png
1579675713896.png
1579675743881.png
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The output "switch" will be a transistor of some sort.

That may have some small amount of leakage, or there may be other components connected for internal monitoring or protection.

A normal digital multimeter has very high input resistance and does not take enough current to mask the leakage, as a normal load such as a relay or PLC input would.
(eg. imagine the prox sw has a 100K or 1M resistor from brown to black, while meter is typically 10 megohm)


The only thing that matters is that the function and voltages are reasonable when the prox switch is connected to its proper load, which will draw far higher current that the meter.

If they are not OK then, it's faulty..
 

gubavac111

New Member
The output "switch" will be a transistor of some sort.

That may have some small amount of leakage, or there may be other components connected for internal monitoring or protection.

A normal digital multimeter has very high input resistance and does not take enough current to mask the leakage, as a normal load such as a relay or PLC input would.
(eg. imagine the prox sw has a 100K or 1M resistor from brown to black, while meter is typically 10 megohm)


The only thing that matters is that the function and voltages are reasonable when the prox switch is connected to its proper load, which will draw far higher current that the meter.

If they are not OK then, it's faulty..
Thank you for your reply.

I've tried connecting 2 relays as a load (one at a time) - 24VDC solid state relay and 12 VDC mechanical relay.

Almost identical voltages as without load... And I've tried changing the position of the load (between black/COMMON and blue/0V and then between black/COMMON and brown/24V).

I sincerely doubt 3 proxy switches are faulty...
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The load should definitely be connected between black and blue, from the label on the cable sleeve.

Have the switches been connected at any time to a mechanical relay that did not have a "flywheel diode", either internally or externally?
That could possibly damage them.
 

dr pepper

Well-Known Member
Most Helpful Member
A totem pole o/p type switch would go 0v to 24v on actuation, the one you have as mentioned is most likely open collector.
There are NPN ones that switch -, and PNP ones that switch +.
I've tried ones on 12v that say they will work with that low a voltage, but they have not.
 

gubavac111

New Member
The load should definitely be connected between black and blue, from the label on the cable sleeve.

Have the switches been connected at any time to a mechanical relay that did not have a "flywheel diode", either internally or externally?
That could possibly damage them.
Hi,

I tested the switch first without a load and then connected it to the load.

The load was indeed a relay without a flywheel diode.

But the strange readings I got were prior and after connecting it to he relay.
 

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