I did further testing and I think I know where the problem was , I had to do with its excessive internal resistance in mA mode. All I would like to know now is weather this is normal for this Extech model or my unit is defected..
I measured current through a 270 ohm resistor, All results were correct up to 270mA until the resistor burned.
I did the same thing with a 2 ohm resistor. All results were about half of the values they were supposed to be in all ranges. It seems reasonable but c'mon is it about the limit already ?
Before I became aware of this, I was building a circuit that draws 250mA from 4V (values as displayed on Power supply). So impedance was about 16 ohms. After the DMM in mA mode was connected, the current drawn was 130mA which was way off from 250mA. Why does Extech meter fail in this case ? Is it because this is "Heavy duty industrial model" which is supposed to work on line voltage and things like that , not on low voltage measurements ?
I have taken 2 pics of the board, can you tell which resistor is the shunt resistor ?
In the 2nd pic, there are R36 (white-white-black-gold) and R37(black-black-brown). I noticed R36 solder joints were almost bridged together, leaving a tiny gap as you can see in the pic. The interesing thing is that where the mid point broke off , the part of the soldering is white ceramic instead of silver ! Should I do something about this ?
Lastly,
In the manual there is no specification on internal resistance, say I would buy a new meter, what sort of specs should I look for if I want to use it for low voltage, low impedance circuit ?