Good day to all. I have a Carbon pile battery load tester and the problem is that it reads amperage all the time even with everything basically disconnected. I have it down to running on my benchtop power supply with just the bare necessities to light it up and it still displays an amperage with everything else disconnected. It generally uses an inductive pickup. Not sure where to start but suspect MAX138 may be problem. I have reflowed the board suspeecting broken solder joints but no change. Thoughts on where to look and why? Here are some pics and the MAX138 datasheet.

 

Is shunt on the board for current sensing or off. If its off, everything
disconnected, then A/D inputs are floating which will give you readings
all over the map.

Terminate those with a few K ohm (>> shunt R) one solution.

Regards, Dana.

 

Two questions. Which one do you consider to be the on-board shunt. I am looking at the diagram and expecting to see a 1M resistor connected to terminal 31 but I see nothing but the capacitor. Which leads to the second question. Why is there nothing else connected to 30 and 31 other than the 1uF capacitor? Where is it getting its input from?
On another note, if it wasnt picking up floating voltages before, why now? I don't want to add things to it. I want to figure out why it went stray all of a sudden.
One other thing I noticed. I was checking voltages on MAX138 and when I got to pin 31, the meter would drop in voltage and the ammeter would start to climb???

 

If you are measuring 10's of amps, that shunt is probably off board.

Speculating when you disconnect from battery is there a plug involved. Just thinking
plug may disconnect external while also connecting pulldowns ? Some mechanical
arrangement that happens when unplugging.

Pin 31 very Hi-Z when disconnected, so very susceptible to pickup. Is there dust/dirt/oil
accumulation on board ? Even fingerprint oils/salts......

Regards, Dana.

 

Hi Dana, okay sorry I misunderstood. I thought you were saying that there was an on-board shunt and I was scratching my head wondering where it was. The shunt is off-board and in the unit itself.
The ammeter is reading with the rest of the unit connected, or not. Right now it is just connected to a power supply. I am going to grab another unit and tear it down and do some comparison to see what is different. I will get to the bottom of it somehow. I thought there would be a few responses to this but maybe nobody works on this stuff. I just haven't had enough exposure to it yet. Fix lots of DVOM's. Guess the principles are basically the same.