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  • Author Author: jw0752
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Cheap Chinese Volt Amp Panel Meter won't zero

jw0752

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A while back I bought 8 of these small 0 to 30 Volt 0 to 10 Amp panel meters from China. I like to use them in my builds because they have in the past worked well for my needs and they are very inexpensive. The other day when I needed one for a new project I found the 4 of the 8 would not zero on the Amp Scale. They would say 2, 3, or 4 in the least significant position and this number would add to the actual current that the meter was attempting to read. This put the meter outside the small trimmers range to calibrate. Further the trimmer had no affect on the meter's 0 level display. I can accept some inaccuracy from these inexpensive meters but I knew I could not tolerate a meter that had a reading when it should have read zero.

 

I mention this problem in case anyone else has purchased these meters too. Here is what the board side looks like:

 

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I have devised a patch fix to the board that clears the problem and allows the regular I ADJ trimmer to do its job.

 

I will be glad to share the patch but only if someone has some of these with the same problem. Let me know if you have had a similar problem and I will continue the blog.

 

John

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  • I began the process by looking at the board and identifying components that might affect the zero of the display. In the past I have had situations where this problem could be corrected by trimming the Null on an Op Amp. There was an LM 358 on the meter board. It is a dual Op Amp so this was my first target. I made a probe with 10K of resistance in series and connected one end to the circuit ground. Each of the Op Amp's active inputs and outputs was probed and the display observed. This had no affect on the display and the non-zero digit remained. Next I moved the probe connection to the Vdd of the circuit board. Once again the active legs of the OpAmp were probed. This time the display zeroed when the probe was put on leg #6 which was the inverting input of one of the Op Amps.

     

    The 10K ohm connection to Vdd was likely an extreme compensation so I built a 100K Trimmer with a 1 Meg Ohm resistor off the sweep. This would allow me to slowly increase the voltage level that was being applied to the pin 6 of the LM358. The trimmer and 1 meg resistor were attached to the meter board and while observing the display the trimmer was slowly turned from the ground side towards the Vdd side. At one point, later determined to be about 1.5 K above ground, the display zeroed. With the display now showing zero with no current passing through the meter I applied current and adjusted the meter calibration so that the meter reading matched the current showing on one of my better bench meters.

     

    Here is a schematic of the trim patch:

     

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    Here are pictures of the trimmer patch installed on the meter. The red wire connects to the meter positive power and the black wire to the ground. The 1 Meg resistor is soldered to the wiper of the trim pot and the yellow wire comes from the 1 meg resistor. The legs of the LM358 were small and difficult to get to so I traced the circuit and found a more convenient location to make my connection. In this case it was on one side of a 47K surface mount resistor.

     

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    I experimented with changing the supply voltage to the meter to see if this would affect the patch but over a reasonable range 6 volts to 18 volts it did not disturb the zeroing nor the reading on the meter.

     

    John

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