Probe Warning, Analysis and Fix

30 Jan 2021 05:58 #197119 by safarir
Hi all,

I recently purchased an pair of probe and toolsetter. After quickly connecting the probe and testing the wiring, I did my first probing and proceed to crash the probe :angry: . This really confused me because the probe sensed the contact, but my ethercat input module did not register it. I decided to do some probing around and realize something really alarming.

The documentation indicate that the output should be an PNP transistor that is normally close:

This is exactly how my connection was done: an 2.7K pull-up to 24V. However, after probing around, it look like my output terminal was only reaching ~13V when the circuit was open, just barely enough to sometime trigger my ethercat input pin. This behavior is not what is documented, so I decided to take my probe apart and do some reverse engineering to try to understand what is happening. This is the schematic I came up with:

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In general, the circuit seem pretty amateurish. The big error is that it look like the output transistor is not rated for 24V. Anything over 12V on the collector andt the current grow exponentially. It look like the 24v output support was just an afterthought and the designer forgot to update the specs of the output transistor.

The fix is quite simple and this is what I did in my case. You can simply replace the output transistor with an 2N7002P mosfet which is fully pin compatible and can take 60V. This is a SOT-23 chip which is small but totally doable by hand.

I hope this will prevent someone else to crash their probe.

Sorry, I forgot to take an picture of the PCB, I might add one once I replace the transistor on my second probe.
The following user(s) said Thank You: Mike_Eitel, tommylight

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04 Feb 2021 15:29 #197656 by Roiki
Thats pretty weird because that shouldnt happen. I think the common ”1” range for 24V logic is 10-24V so 13V should be enough to trigger it reliably. At Least the EL1002 that ive used triggered fine with 12V high signal.

Me thinks there was something else going on.

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