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+-- Thread: Reading DC voltages (/showthread.php?tid=7366235)
Reading DC voltages - fredhargis - 11-27-2021
Our doorbell transformer died. I discovered this while checking voltages for a doorbell camera. Anyway, using a cheap VOM I checked the voltage at the door and got zero, then went to the basement and checked the transformer...also zero. After figuring out what voltage I needed (16V, in this case) I bought a replacement...although this was a multi voltage version that had 8, 16, and 24 volts. Installed the new one and tried checking the voltage at the 2 terminals I needed and again read zero. (chit!) so I went and got my Fluke digital and checked...zero again. Undetered, adn not understanding what was going on, I connected the doorbell and it works. So why can't I read the voltage (Yes, I had the VOMs set on DC volts). there must be something here I don't understand, although the original transformer must have been bad for the doorbell to not work.
RE: Reading DC voltages - JIMB49 - 11-27-2021
(11-27-2021, 11:51 AM)fredhargis Wrote: Our doorbell transformer died. I discovered this while checking voltages for a doorbell camera. Anyway, using a cheap VOM I checked the voltage at the door and got zero, then went to the basement and checked the transformer...also zero. After figuring out what voltage I needed (16V, in this case) I bought a replacement...although this was a multi voltage version that had 8, 16, and 24 volts. Installed the new one and tried checking the voltage at the 2 terminals I needed and again read zero. (chit!) so I went and got my Fluke digital and checked...zero again. Undetered, adn not understanding what was going on, I connected the doorbell and it works. So why can't I read the voltage (Yes, I had the VOMs set on DC volts). there must be something here I don't understand, although the original transformer must have been bad for the doorbell to not work.
Switch the VOM to ac
RE: Reading DC voltages - blackhat - 11-27-2021
Voltage is measured relative to a zero Neutral typically in AC and ground in DC. At a switch, you won’t have the zero conductor, you have potential to the switch which sends that same potential to a load. You were correctly measuring the transformer which has power and zero terminals but not the doorbell button.
In addition, the doorbell system should be AC and not DC. Typically.
RE: Reading DC voltages - fredhargis - 11-27-2021
Dayumm, it was that simple.....thanks guys!
RE: Reading DC voltages - TDKPE - 11-28-2021
(11-27-2021, 12:51 PM)JIMB49 Wrote: Switch the VOM to ac
What he said, and while your transformer (AC) showed zero VDC, there are situations where there is both AC and DC on the same conductors, whether it’s wanted or not. So you can use your DMM or VOM to check for noise on DC wires, or DC on AC or signal wires (like coax with DC power to an amplified TV antenna through the same coax that carries the signal).