What happens if I use the DC Voltage setting on an AC line? What about the reverse?

Question

I have a run of those low voltage lighting systems in the hallway. The kind that have a two parallel exposed wires, with fixtures that clamp on in various spots on the wires and then take an MR16 bulb.

I am curious what the voltage is across these lines, but I didn't install these, so I don't know if the transformer is outputting 12VDC or AC. I can't easily get to the transformer.

My multimeter has a single set of holes for voltage readings, then I select either VDC or VAC on the dial. What happens if I get this wrong? Is it dangerous? Assume the multimeter is rated for more voltage than I am going to find in this situation.

Answer 1

As long as the meter's voltage range is initially set to at least the highest that can be there, you won't damage the meter, especially if you make just a momentary initial connection to see what the meter does. What happens with the readings will depend in part on the type of meter, whether it auto-senses polarity, whether it is auto-ranging, and whether it has a digital display.

For example, a mechanical meter with a needle and scale will "peg" the needle below zero if the meter is set to DC, and you measure a DC voltage with the polarity reversed. It will peg the needle at the end of the scale if the voltage is higher than the setting, either AC or DC with the correct polarity.

With a digital meter that auto-senses polarity, you will get a negative accurate reading if you have a DC setting and measure DC with the polarity reversed. It will show zero or an error if the polarity is reversed and it isn't auto-sensing.

AC will display as zero voltage on a DC setting, regardless of the polarity. Reversing the leads and still reading zero will distinguish this from DC with reversed polarity if the meter displays zero for reversed polarity DC.

Measuring DC on an AC setting will depend on the meter. The meter rectifies AC to turn it into DC for a reading. DC with polarity in reverse of the rectifier will read zero. Reversing the leads should give you a reading, but it is likely to be inaccurate because the meter is scaled for the rectified voltage.

The combination of AC/DC settings and polarity reversal should allow you to figure out whether the source is DC or AC.

Answer 2

Power has both a DC and AC component

However in human made power, normally only one is created. The other one will give you useless/null redings.

Imagine your car has a hatchback with gas door lifts to assist raising the door, and a high mounted center brake light. There are no wires through the door hinges; positive and negative travel through the two gas door lifts. And now designers want parking lights there too. You can run wires through the door hinge, but they tend to break.

You put an electronic chopper on the parking light circuit, which pulses the power at about 1000Hz. Then you run this through a transformer tuned for 1000Hz, and 24VAC comes out the other side. This is paralleled with the brake light circuit, with an LC filter to keep the 24VAC out of the rest of the brake light circuit. These two signals share the two "wires" and run up the hatch lifts. On the hatch itself, you have another transformer which knocks 24VAC down to 12VAC to feed the parking lights - the incandescent bulbs don't care. However brake light DC cannot pass through the transformer so it won't work the parking lights. Then you have another LC filter to keep AC from lighting up the brake light at 24V.

Now if you were troubleshooting that circuit, you could be switching back and forth between AC and DC modes on your meter, quite a lot, without ever moving the wires.

However that isn't usually done, so usually the wrong mode will give you no useful info.

Answer 3

You'll get a wrong reading, likely zero or close to it

Either way around, your meter won't read correctly. I checked with my Fluke 77 III and it reads 0VAC on a DC source (Leader LPS152A bench supply) and gets stuck in an autoranging loop when set to DC while trying to read an AC source (wall outlet).

Update: I also checked this with my Brymen BM235, and it reads 0 or effectively 0 in both cases (AC mode on the same DC source as before, and DC mode while trying to read a wall-outlet AC source).

Answer 4

I killed my multimeter by doing this. I was testing my multi meter on an 115v outlet and it tripped the circuit beaker. the display still works but no measurement works (not even conductivity mode).

edit: I had it on 600v DC