How can we check DC polarity using multimeter?
If the DC cable (the positive and negative end) is connected in the cable rack (terminal block) how can one identify which cable is positive and which cable is negative?
Electronic – How to check DC polarity using a multimeter
polarity
Related Solutions
"Zip cord" or cable that has two conductors connected together via the insulating jacket, is frequently used for things like DC power supplies (wall warts), speaker cable, etc. Sometimes it will have a stripe, a texture, or printed characters on one side. The "- - -" should not be taken to mean negative. It's merely a marking. Always check which wire is connected to tip or ring using a continuity meter, and of course a volt meter to determine polarity provided by the supply.
If you post a picture or two of your PCB (close ups of input and surrounding components) it would be easier to help confirm things.
Often, there will be some indication on the PCB silkscreen at the power inputs of polarity. If not you can often tell by following the input trace(s) and seeing what it connects to. For example, if the input pad is connected directly to a large copper fill (you may see 2 or 4 "wheel spoke" type connections to the fill), then it's almost certainly ground (not always though).
You can also look for things like power filter capacitors and see if the negative side (usually a stripe on that side probably also polarity symbols)) is connected to the input pad (i.e. has 0 ohms on multimeter ohms range, or beeps on continuity setting)
Similarly, if there are any ICs on the PCB you can get datasheets for, look at the pinouts, see which pin is ground and test between this and the input pad.
For the positive input, you would look for the input to the other side of a filter cap, connection to an IC regulator input (example datasheet from which above pinout was taken), any LEDs (on a typical 5mm LED the flat side is the cathode or negative side, so the other pin may be connected to the positive rail, probably through a small value resistor - it's cathode may be connected directly to ground)
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Best Answer
I just measured a Lithium battery with unknown polarity with a digital multimeter and got the following measurement. The same results should apply for other DC sources:
I hypothesized that the negative result may have meant that I'd attached the positive (red) lead of the multimeter to the negative lead of the battery. To confirm this hypothesis I flipped the battery leads around and found further evidence this may be true:
So it appears that if you connect a multimeter postive lead to positive, and negative to negative on the circuit under test the meter will give a positive measurement. If the connections are reversed you will get a negative voltage reading.