I bought a Chinese digital ammeter on eBay like this one:
It requires a 5VDC power supply. The vendor initially suggested you
could pull the power from the leads the ammeter was monitoring:
I am using the meter with a 13.8V system. Without the shunt leads
connected, the meter draws 40ma. As soon as the shunt leads are
connected (with no actual current imposed on the shunt), the meter
current draw went through the roof. Fortunately, I was testing with a
current-limited lab supply.
The vendor then said that his drawing was wrong and that the shunt had
to be grounded if the monitored lead was used to power the meter.
Obviously, an ammeter that must ground its shunt is of limited
Moreover, the method did not work. The meter showed "-00.4" when
repositioned on a lead to ground when the load was open and there was
Finally, I switched to a separate power source -- a 9V battery into a
7805. Now the meter works OK, but runs the battery down in ten hours.
I had hoped to put two meters in an enclosure, which means a too-short
five-hour run time.
Apparently, there is an unnecessary and undesirable connection between
the power side of the meter and the sensing side.
Is there any way to isolate the 7805 circuit and still use one of the