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Hello! I've built a small adjustable PSU with a buck converter (XL4015 module) and a cheap 10A digital voltmeter and ammeter. It worked fine for a few minutes but after that the voltmeter started fluctuating between the real voltage and something like half of that, most of the time showing the wrong value.

I've wired it following a schematic on the internet, but powered the voltmeter from the input of the buck converter (where as in the schematic the red -meter power- and yellow -Vsense- wire where connected) because the output will go as low as 1.5V, not being able to power the meter. I've tested the meter separately and confirmed that the problem is related to it. I do have a spare one, but I don't want to do the same thing with it. Is the wiring correct? Is there any chance I could fix the meter? Schematic (edit:) Sorry for the lack of details, it was my first post.
Links:

The shunt is connected to the thick black and red wires. And I've let the thin black disconnected because It's common with the thick one.

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    \$\begingroup\$ The meter module will have a shunt resistor to measure the current. It is unclear how that resistor is connected in relation to the supply voltage input and voltage measurement input of the meter. This information is crucial as it will dictate how the meter should be connected. You might have to do some reverse engineering here as these modules usually have no proper technical documentation. \$\endgroup\$ Commented Jan 26, 2018 at 14:59
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    \$\begingroup\$ Show the correct circuit if you want any help. Link the devices too because nobody here can read minds. \$\endgroup\$
    – Andy aka
    Commented Jan 26, 2018 at 14:59
  • \$\begingroup\$ The wire pair connecting between the output and input of the converter seem suspect, though this cannot be confirmed unless you include the V/A module's part number or a link to the documentation. \$\endgroup\$
    – Nedd
    Commented Jan 26, 2018 at 15:13
  • \$\begingroup\$ where is the DVM datasheet or spec.? \$\endgroup\$ Commented Jan 26, 2018 at 15:17
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    \$\begingroup\$ I just used it with XL6009 buck-boost converter, connected in a same way as you, and it's working fine. XL6009 buck-boost power source \$\endgroup\$
    – TomasHC
    Commented Nov 5, 2018 at 14:03

2 Answers 2

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It seems you have something close to this type of V/A meter, https://www.amazon.com/Paddsun-Digital-Voltmeter-Ammeter-Meter/dp/B01LSGP1UI

in which case the proper wiring connections are included on the left side of the amazon page.

Notably the thin red and black wires are the modules power inputs, while the yellow is the voltage sense contact. The wiring method depends on if you want the meter powered from the source power or the output power.

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  • \$\begingroup\$ That's how I wanted to wire it, with a common power adapter. But the meter works above 4V, while the output of the buck goes as low as 1.5v. So I can't connect both of them to the output of the converter. \$\endgroup\$
    – F14V
    Commented Jan 26, 2018 at 16:09
  • \$\begingroup\$ It's ok to have 1.5V between yellow and black. What you must have is 4V+ between red and black. There is no rule that says you must connect yellow and red to the same place. \$\endgroup\$ Commented Jan 26, 2018 at 16:13
  • \$\begingroup\$ That's what I assumed when I connected them like that, but something went wrong apparently. I know cheap electronics from china aren't always reliable, but I don't think it's that. \$\endgroup\$
    – F14V
    Commented Jan 26, 2018 at 16:17
  • \$\begingroup\$ An external current shunt (if needed) would be connected across the meter's thick red and black wires. But per the spec I see that an external shunt is only needed if measuring >10A. \$\endgroup\$
    – Nedd
    Commented Jan 26, 2018 at 16:39
  • \$\begingroup\$ I'll only be measuring a maximum of 5A, so the included one (a thick copper wire on the meter circuit pretty much) connected between the wires you've specified will work fine. \$\endgroup\$
    – F14V
    Commented Jan 26, 2018 at 16:42
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A lot of these amp volt panel meters have knock off 3.3volt voltage regulators to power the meter. I had a number fail if powered over 12volts. I think its an a1117 regulator. If you have wired it wrong you also may have blown a resister to the meter and the regulator. Changing the reg out normally fixes a dead meter.

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