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I Am building a DC to DC step-down variable voltage power module and have a questions. Answered

The power module or desktop PSU is fairly basic but I have a mini 3 wired power meter to use the meter I need to power it and to do soI have to follow these instructions. "Simply connect the red wire to a positive power supply from 4.5 to 28VDC, and brown or black to the common ground to power the display. Finally, connect the orange or white wire to whatever you want to measure the voltage of." This is a link to the actual power meter. https://www.adafruit.com/products/705
What I am not sure of is how to figure out what the common ground is and where to attach the green wire I am sure I can figure that out once it's powered so if anyone can tell me how to find this common ground that would be awesome thanks in advance for your time

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seandogue

2 years ago

The third wire (Orange, in this case) measures the voltage referenced to its power supply common (the brown wire). Therefore, one needs to remember that these three terminal devices are suitable only for measuring voltages that share a common with the power supply that feeds them. They will be useless for differential measurements and provide no isolation whatsoever.

red--------+V supply

orange------- Device under test (DUT)

brown------V supply (common with DUT "ground")

Vorange is the voltage between orange and brown.

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Jack A Lopez

2 years ago

The words "common ground", as used in Adafruit's description of this gizmo, simply mean the voltage source supplying power to the gizmo, and the voltage source whose voltage is to be measured, have their negative terminals tied together.

It seems complicated, in words, so I have attached a hand-drawn symbolic picture (circuit diagram) of the same thing. In this drawing, I am using the a circle with two terminals (one labeled "+" and the other labeled "-") , and this is the usual symbol for this. The Adafruit voltmeter gizmo, I have symbolized with a three terminal device that, kind of, looks like it does in real life, a little box with some 7-seg numbers on it.

The first picture shows this general case with two voltage sources.

The second and third pictures, show some less complicated circuits you could try, for fun, and or, to simply prove this voltmeter gizmo is working.

I forgot to make a note in the drawing, but for real life testing, the unknown voltage source, could be a boxy 9-volt style battery, since that will give a voltage in the range of about 0 to 9 volts DC, and is also capable of supplying current (3 to 4 mA, at 4.5 to 38 VDC, according to the product page) needed to power the meter.

Actually, if your collection of electronic parts includes a 9-volt style battery, and also some other battery (with nominal voltage in the range 0 to 99 volts), you could build the circuit I drew in the first diagram, if you wanted to, as an experiment, to help your understanding.

Polarity may be important for this gizmo. In the description list of bullet points, Adafruit put an exclamation point after the one that says,

"For use with positive voltages only!"

If you have an ordinary voltmeter, you can of course use that to check for correct polarity before hooking up the new gizmo. Also, if you're experimenting with batteries, most come clearly labeled with little "+" and "-" symbols on them. So that helps too.

common-ground-for-digital-voltmeter-module.jpg
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petercd

2 years ago

The red and black is to power the display, the white is the voltage read leg.

The black is common, so think of it as the manufacturer having saved on having 4 wires ie red/black and white/black.