Why aren't I getting a voltage reading from my AC/DC power adapter

Question

I am new to electrical engineering.

I cut the barrel jack off the end of a 24V DC power adapter, exposing a red and white wire. I'm trying to determine the polarity of the red and white wires with a multimeter. Here's what I've tried, with my multimeter ends connected like so: black to COM, red to V/mA/uA/omega/Hz/Temp.

1. Set multimeter to VDC mode (as opposed to VAC)
2. Touch Black tip to white wire, red tip to red wire.
3. No reading at any range (including mV)

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1. Set multimeter to VDC mode
2. Touch black tip to white or red wire
3. Touch red tip to wall jack side of the adapter
4. No reading

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1. Set multimeter to VAC mode
2. Touch black tip to white or red wire
3. Touch red tip to the other wire
4. Steady reading of some random low voltage

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Why aren't I getting a voltage reading from my power adapter? The multimeter gives accurate voltage readings when tested with a battery. Note that I haven't plugged the adapter into a power source as I read this is dangerous, so I don't quite understand how I'm supposed to get a reading?

Answer 1

Red is likely to be the positive and white the negative. Note likely.

"Note that I haven't plugged the adapter into a power source as I read this is dangerous"

This seems to be your problem. You will have to energize the adapter to get a reading.

Your connections seem to be correct for the meter.

How to do this safely.

1. Have the adapter unplugged.
2. Strip enough insolation from the wire that you can wrap it around the multimeter leads
3. Warp the exposed wire around your probe leads.
4. Have the leads and wires on a desk or something where it cannot short
5. Setup your multimeter to first read high DC voltage
6. Energize your adapter. Pay attention to not short out the leads.
7. Work your way down the DC scale of your meter until you get a reading.

Answer 2

Safety is a very fair concern.

1. Search the adapter for any of these marks: UL, CSA or ETL. If none found, back to Amazon it goes. If it touches AC wall power it must have one of those marks. CE won't cut it! 99.9% of adapters made with a NEMA 5-15 plug are UL/CSA/ETL Listed, because who buys them? Manufacturers of low-voltage electronic gadgets who aim to sell in North America, where it's required. That makes them cheap commodities - no reason to accept a non-listed one. (Europeans: BSI Kitemark, TUV or other established 3rd party testing lab. CE is a self-certification widely faked by sellers located outside the EU. It is only trustworthy when it's positively from a manufacturer with seizable assets inside the EU.)

2. Get GFCI protection for the outlet you will be plugging it into. Plug in a loud radio and go around hitting "TEST" on every GFCI in your house. If the radio goes dead when you press one, you're GFCI protected. (Europeans: your whole-house RCDs aren't bad at 30mA protection).

3. DO NOT EVER, EVER connect any wire or probe to the AC wall power side terminals. Safety ground/earth should be safe, but in badly wired sites it isn't always (Europe: don't start me on your TN-C-S!)

OK, so those work together to create a safety zone you can work in with confidence.

Now, your adapter won't work without being plugged in... because it's a power adapter, not a battery.

So plug it in and retry your test #1 and you should now get fruitful results.