A little over a year ago I purchased a mains voltage sensor module from from a local reseller of cheap electronics from China (eparnters) for $4.99 NZD. This is a small transformer (model ZMPT101B) with an amplifier on a small PCB.
Yesterday I returned to working on the project I bought this for, and after wiring it up, I found no signal on the output pin.
After a few minutes of frustration watching the scope and adjusting the gain pot, I pulled up the data-sheet for the OP07 op-amp on this board, and started probing the pins with my DMM and noting down voltages.
What I found made no sense. My first thought was maybe the package was soldered the wrong way around, so I redid my notes with the 180 degrees rotation, but that still didn’t make sense…
Getting a bit desperate, I started doing web searches for the ZMPT101B module, and came upon this site where I saw the following image of the same module.
Notice this module looks identical but the op-amp is an LM358. Pulling up the data-sheet, I realised the measurements would make sense if the dual op-amps of the LM358 were chained together:
So… let’s fix it. I picked up an LM358 at Altronics…
…and set about removing the OP07 with some hot air:
Getting the orientation correct for the ST Micro LM358 SOIC-8 package was a little tricky for me. Here is the drawing:
Note the slighly more beveled edge on the pin 1-4 side. It is actually visible in the photo if you look real close:
Some more heat and solder and the new op-amp is installed:
And happily it worked! I immediately got a signal, and it was easy to adjust the gain to prevent clipping with the scope attached::
In countries where buying hobby electronics supplies from the likes of Adafruit and Sparkfun incurs expensive shipping, online resellers buy bulk items on alibaba.com and re-sell them in-country with a stiff mark-up for impatient people (like me).
I suspect the factory (probably a CM using spare capacity to churn out these modules) made a mistake and loaded the wrong reel of op-amps into their pick-and-place machine. This might also have been a grey-goods (i.e., workers running the line at night) situation. Either way, the modules were faulty and probably offloaded at a steep discount to the NZ reseller (more profit!).
The seller probably didn’t know any better (I wouldn’t expect them to test everything they resell) and I fixed the module anyway (and had fun doing that) so all is well that ends well.