Forum Discussion

icodeintx's avatar
icodeintx
Explorer
Oct 12, 2020

AC Amp/Voltage monitor

Today I installed one of those "Amazon" amp meters. Mainly because I'm a pro at tripping the 30amp circuit. So I decided to install this thing so I can see my amps while different devices are running.



Lessons Learned:

The photo you see is actually upside-down. The meter is about 18 inches off the floor which is possible to bend down and see the numbers. However, the display range on the LCD is in the wrong direction for my application. This display was designed to be eye level or up high showing the numbers vividly. But when slightly above looking down one can only see a blank screen.

So smart me just turned it upside-down (for now) and when I bend over to look the numbers are actually right-side-up.

I love the monitor but don't really like the installation so I think I'll build a small installation box and mount the display inside the bench it is currently mounted in. Nevermind the unfinished bench. It is under construction.

  • DrewE's avatar
    DrewE
    Explorer II
    Oct 12, 2020
    I put one of those in a few years ago. It's decently handy. Note that the power (watts) display seemingly is actual power, not apparent power, which is kind of nifty. That is to say, the wattage is not simply the voltage multiplied by the current, but properly takes into account the power factor. Power factor isn't displayed, but you can compute it roughly by dividing the displayed power (814.1 watts here) by the product of the voltage and current (121V * 6.79A), which in this case is 99%--very good.
  • Matt_Colie's avatar
    Matt_Colie
    Explorer II
    Oct 12, 2020
    I have been trying to figure out how to do that so it will still work when a 50 amp coach is plugged into a 30 Amp post.

    Matt
  • DrewE's avatar
    DrewE
    Explorer II
    Oct 12, 2020
    Matt_Colie wrote:
    I have been trying to figure out how to do that so it will still work when a 50 amp coach is plugged into a 30 Amp post.

    Matt


    For this style, you'd need two meters, one for each leg, and then have to manually add up the current values. I suppose alternately you could put the current sense transformer around the neutral line, and the voltage sense between the neutral and one leg; that would give accurate readings for a 30A service, but be nearly useless for a 50A service.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Oct 12, 2020
    I use a hall effect watt meter to monitor my loads.

    To make life simpler I upgraded to a hybrid inverter/charger that does load support. So I can draw about 3000 "extra" watts (26 amps) from the battery bank. I can limit my draw on the shore power to pretty much what ever I wish. I normally run at a 2800 watt limit (or 24 amps).
  • MrWizard's avatar
    MrWizard
    Moderator
    Oct 13, 2020
    Matt
    Put both hot legs thru the pickup coil
    Put them thru from opposite directions
    Like you were going to tie a knot
    This way the electron flow from both legs, is always going thru the coil in the same direction as the sinewave cycles
    Remember 230vac residential is single phase, the neutral is the center tap on the transformer secondary

    There is one issue with this
    On 30 amps it will cancel out and read zero amps zero power, because two hot legs connected by 30>50 adapter will be carrying current in opposite directions thru the pickup coil sensor
  • pianotuna's avatar
    pianotuna
    Nomad III
    Oct 13, 2020
    My watt meter came with 2 coils. So no cancellation would happen.
  • MrWizard's avatar
    MrWizard
    Moderator
    Oct 13, 2020
    Just a note:
    DC power meters, amp meters etc..
    Use Hall Effect sensor to detect direct current magnetic field, a solid state device

    AC power meters use CT coils aka Current Transformers
    Very old technology,
    Alternating current is easy to detect and measure
    For many years Direct Current could only be measured with a Shunt using voltage drop over the calibrated resistance,
    Hall Effect sensors are not needed for measuring AC, i won't say they can't be used, or have never been used for measuring AC,
    Usually pretty easy to tell them apart , small CT that might come with a meter we RV ers would use, normal have just two lead wires and either wire can go in either of the two connection points
    They can look like a wrapped up wire coil, or they can be molded in plastic
    The HE sensor will be molded and will have a multi pin connector on pickup or on the wire pigtail and be connecting often with a flat multi strand ribbon cable

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