Originally Posted by hellosailor
"I wanted only 30 amperes to be displayed so I installed a divider across the shunt's output terminals"
foggy, I'm not quite waht you've done but can tell you this is NOT THE WAY TO DO IT.
The way meters and shunts are set up, it doesn't matter what range you want to read. All that matters is that the mv-per-display has been matched between them, and that you don't exceed the maximum amperage that the shunt can carry. Whether you want to read 10 amps, 25 amps, or 50 amps...if you have a meter and 50a shunt, they are set up the same way regardless.
I would strongly suggest that what you've done now is simply to distort all the readings. And, perhaps, that's related to the shunt overheating as well.
Some of the bargain stuff from China, which is SO attractively priced, almost scares me. The shunts are visibly different from the pricey domestic ones, and the all-in-one digital meter displays seem to vary quite widely in quality. (I've got two that never agree, even though they are supposedly calibrated and way exceed their display accuracy.)
50A shunt, 50A meter, put it back to the original configuration and it will read 30A scales just fine. Really.
GEEZ, I don't know what the heck I was thinking! Did this exercise late at night but thats a poor excuse. I scaled the 75mv for 30 amperes completely ignoring that the voltage drop of 75 mv depends on the current flow I through a resistor of value R. I just removed the divider, and you are embarrassingly correct, it will read 30 amperes if the shunt is correctly calibrated and of course, current flow is 30 amperes. Thanks for pointing out my screw up, I just deleted that mess from the previous post.
As to the stuff for China-- you pays you money, you takes you chance. Something as simple as a meter/shunt in a non critical application is fine.