@johnrr6 posted:My multimeter is going nuts and giving me jumping readings in about the 13 volt range but never stays a solid reading. And I get the same readings attaching the probes to either wires. I simply can’t tell the polarity difference on the two leads after I snipped the barrel connector off.
I looked at the power supply and it shows that the inside connector of the barrel connector is positive.
OK….But then that makes no sense if the barrel connector cord that plugs into the power supply is reversible. And that cord can be plugged in either way.
I am trying to wire that 72 W power pack into a power distribution board. Haven’t worked much with DC. Does it matter in this case?
John, Do you have the meter set to read DC volts?
The cord that's reversible is the 120V power line input to the power supply and connects to the power pack transformer's primary side. Inside the transformer that voltage is stepped down to the transformer secondary winding and then rectified to a DC output. Reversing the 120V AC input polarity has no effect on the reified DC output polarity.
When you have the meter set to DC Volts, connecting the probes in one orientation to the power pack output will give you a positive DC reading on the meter and swapping the probes will give a negative reading.
Whichever reading gives a positive result, then the red probe will be on the Power Pack positive output and black will be ground.
