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stacaz822's avatar
stacaz822
Explorer
Dec 11, 2017

24V AC hour meter on generator

I got this hour meter and connected internally to the 120V AC outlet, and it seems to work fine, even though is it labeled for 24V AC 50/60 Hz. Do you think this will be an issue? I put a 250K ohm resistor in series with the meter, and I get about 68V AC across the resistor, and about the same across the meter. The meter works fine. This would mean it's only drawing 0.02 watts. You can hear the seconds tick off, and it's spot on after a couple minutes..

https://www.amazon.com/Hour-Meter-6-Digit-2-31-Round/dp/B00DWIE4IQ/ref=sr_1_3?s=industrial&ie=UTF8&qid=1513029725&sr=1-3&keywords=AC+hour+meter

7 Replies

  • DrewE's avatar
    DrewE
    Explorer II
    Dec 12, 2017
    I have a few hour meters in my house that operate for a year or so on a sing AA battery. I hang them on the walls of various rooms. :B That works out to something like 1.5 or 2 mW per clock, very roughly speaking

    It's not too surprising to me that the meter uses 20 mW or less.

    You might also be able to power it with a capacitor rather than a resistor as the dropping element. That should in theory be more efficient...not that it matters much in practice in this case since you're using next to no power and have plenty available.
  • road-runner's avatar
    road-runner
    Explorer III
    Dec 12, 2017
    I'm also surprised that the meter uses that little power, as well as impressed. I see zero safety risk with this. Even if the meter were to be a dead short, I crunched out that the resistor would be dissipating about 0.06 watts. I'm paranoid about things like this and generally would put a small value glass fuse in series with the whole thing, quarter amp or less. In this case however, if the meter is mounted on the generator with short wires, I'd probably skip the fuse.
  • stacaz822's avatar
    stacaz822
    Explorer
    Dec 12, 2017
    LOL. None taken! Just wondering why this little meter operates as it does, on such small current at 68V AC...

    MEXICOWANDERER wrote:
    Was just a general comment not a critique...
  • stacaz822's avatar
    stacaz822
    Explorer
    Dec 12, 2017
    At first I tried a 25K resistor, then 100K, and then 250K. They all worked the same (basic small 1/4 watt resistors), dropping the voltage down from 80, to 70, to 68. And when you apply the math, you end up with only 0.02 watts through the resistor, AND the hour meter. I just wonder why the meter still operates on such small wattage...My goal was to get the AC voltage down to 25 or so, based on the specs for the meter. Apparently, it works otherwise...and at 0.02 watts, it will *never* get hot.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Dec 12, 2017
    If it will endure overnight it will endure forever. The resistor must NOT get hot or even warm. Size the wattage of it to achieve no temperature gain, and you're almost home free...

    Don't permit generator vibration to yadda yadda yadda the resistor. Sometimes when I use resistors on krap that vibrates I'll choose a ceramic resistor. Huge wattage overkill.

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