Which are plus and minus leads from a Lionel 72 Watt DC power pack?

@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.

Note: based on the picture of a similar meter to yours, after selecting Voltage, you may need to press the orange button to put the meter into DC voltage mode.

@SteveH posted:

Note: based on the picture of a similar meter to yours, after selecting Voltage, you may need to press the orange button to put the meter into DC voltage mode.

Thank you sir! Pressing tthe little select button gives me a steady reading of 18 V and I was able to identify the positive lead. That wire also has a very slight rib on it which I did not notice at first which I guess indicates positive as well. Thank you for clearing that up! Great help!

Oh and I also feel like a dumb dumb even mentioning the reverse of the cord. I thought that cord was the one that led to the barrel connector not the wall plug lol.

I’m having one of those days…..

Thanks again!

John, I'm glad you got it all sorted.  Good luck with your project.