Damaged digital V/A meter

[lordvader88]

I have an ebay 100V 10A meter, and I've damaged it from not using it right, like mixing up wires more than once over a year so I don't remember..
https://www.ebay.com/itm/DC-100V-10A-Voltmeter-Ammeter-Blue-Red-LED-Dual-Digital-Volt-Amp-Meter-Gauge-/312118220700

The voltage works fine, but the current reading reading is not linear anymore. Spec's are

Working voltage of Meter itself:DC 4 - 30V(The maximum input voltage can not exceed 30V. Otherwise the meter will be burned)
Working current of Meter itself:≤20mA
Voltage Measuring range: DC 0 - 100V
Current Measuring range: DC 0 - 10A
Voltage resolution (V): 0.1V
Current resolution (A): 0.01A
Measure accuracy: 1% (± 1 digit)
Display: 0.28" digital tube, Two color blue and red
Refresh rate: about 300mS / times

It used to be off a bit, but now its like it's off a few at 10's of mA and by 0.120A actual it reads 0.20A, 0.42A reads as 0.68A

Reminds me sort of like when I burned out a resistor in the divider chain of a DMM. I have the tools and even a digital microscope.

How do these work ? Anyone know where to find a schematic ? I think they use a pretty generic chip for the V/A section.

[james_s]

Did you connect the current shunt across a voltage source? It sounds like you may have damaged the shunt resistor, that will cause the current reading to be wrong.

Generally these things are just a microcontroller or ASIC with two voltage inputs, one of which reads the voltage across a current shunt.

[lordvader88]

The 1st time I used it I never read the diagram and probably did wire the shunt across the DUT.

There's 2 ways to wire this. 1 is for a 30-100V, and the meter is pwer'd by another source under 20V. The other way is for 0-30VDUT and u can pwr the meter from the same source. And the shunt is wired different each time.

I mixed that up too.

Just today tho it's way off. I'm sure it was't like this last week.

I can't see any letters/#'s on the chip, it's a fine pitch 20pin thing . It runs the LCD too. It uses an LM358 but most of it's pins are going to via's.

These thing are a pain to take fully apart without melting stuff, and plus all the pins on the LCD would have to be de-soldered, doh


IDK if it's the resistors I can see that are vital or if it's internal stuff in that chip that would be damaged.

Nothing looks damaged.

[lordvader88]

I kid u not, if I turn the meter on it's side, with no load, it's going from 0.03-0.09A, just moving it around. It's shunt wires on the PCB of the PSU are in the middle of nowhere, so can't be touching anything wrong. How can the reading be changing from me moving the meter? Unless it's 60Hz AC noise in my room affecting it,



seems the shunt connector is loose, moving it seems to have stopped the reading I just had moving it around.

It's zero'ed again, but 0.57A reads as 0.78A