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AC/DC amps - Are they the same?

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  • AC/DC amps - Are they the same?

    I'm going to be building a stand-alone tool shortly and need to be able to read amps to one decimal place. The maximum will be three amps but I need to be able to read and set amperage to x.xA. I see meters for AC or DC. What is the difference? My application will be AC but what would a DC meter read for the equivalent AC amps?

    Regards, Popping Fuses.

  • #2
    Current is current. JRouche
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

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    • #3
      AC current changes directions. DC is one direction. You need a ac ampmeter to measure ac and a dc ampmeter to measure dc. You could measure the current by the voltage drop across a series resistor using an ac or dc voltmeter.

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      • #4
        AC current is usually measured as RMS which is the effective current assuming the waveform is sinusoidal which electrical AC is. The rms rating is the effective DC level which would produce the same power as the sinewave. AC ammeters are almost always calibrated for rms (root-mean-squared). There are some multimeters now that calculate the actual rms level on non-sinusoidal waveforms. 10 amps rms flowing through a resistor will produce exactly the same amount of heat as 10 amps DC. Using rms keeps it simple, however if the current is flowing through inductors or capacitors, it gets non-simple fast.

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        • #5
          It gets even less simple if the frequency varies. Very few meters measure current accurately if the frequency is very far from 60 hz.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            The difference is that AC current goes "both ways" and averages to zero, but the average of DC (unidirectional) current is the DC value.

            So what?

            So the usual type analog meter that reads DC will not read AC because it relies on a coil pushing to one side (very simplified) with the dc current, but the AC pushes it both ways, so often all it does is vibrate at zero.

            A similar effect occurs with DC digital meters.

            So, the AC meter generally has to include some form of rectification to convert the "bothways" AC to "oneway" DC.

            The simplest AC analog meter is a "moving vane" type, some of which are inherently insensitive to polarity. Used to be they were cheap, these days a cheap digital is often cheaper than anything mechanical.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Sounds like you need a cheap digital ohm meter wired in,then you can take your pick
              I just need one more tool,just one!

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              • #8
                Thanks guys but I think my head is going to explode.

                It looks like I would need two meters then if I were testing DC too. This is part of test stand but I guess I could use a VOM for the readings. I've got one that reads AC-rms as well as DC.

                I'll probably have more questions when I get it built and run a couple of tests.

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                • #9
                  I'm not sure of your application but the reference to heating a resistor to equiv. value is correct for AC RMS and DC.

                  If you're working in AC what is important is that value (peak, RMS,etc.) If you don't need to know the DC current focus on the AC and don't worry about it. What does the part manufacturer specify?


                  Originally posted by Evan
                  It gets even less simple if the frequency varies. Very few meters measure current accurately if the frequency is very far from 60 hz.
                  Freq. is not the problem, it is purity of sine wave. Lots of diodes are linear to high freq. Boonton and HP make meters accurate to Mhz.

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                  • #10
                    Originally posted by CCWKen
                    I'm going to be building a stand-alone tool shortly and need to be able to read amps to one decimal place. The maximum will be three amps but I need to be able to read and set amperage to x.xA. I see meters for AC or DC. What is the difference? My application will be AC but what would a DC meter read for the equivalent AC amps?

                    Regards, Popping Fuses.
                    Sound like what you are looking for is an AC Current Meter. (duh)
                    Typical AC current is measured with a current transformer and meter. The current transformer, like a big donut accepts a wire from the load through the hole in the middle. Two leads from the coil then go to the current meter, analog or digital. There is no direct electrical connection between the load and the meter.

                    A more recent method uses a linear Hall effect sensor mounted in a ferrite loop to sense current flowing in the wire to the load, much like the current transformer. The difference is that the Hall sensor gives a DC voltage output in proportion to the AC current flowing through the wire carrying the load.

                    For a dedicated test stand you may want to go with the digital panel meter and current transformer as it would probably be easiest to implement for someone with less electrical experience.

                    If the stand will see only occasional use, a digital multi-meter with an accessory clamp-on probe might work. This would be even less hassle, but not as rugged.

                    Wes
                    Last edited by Weston Bye; 08-15-2006, 07:42 PM.
                    Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                    ~Practitioner of the Electromechanical Arts~

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                    • #11
                      If your wanting to measure only AC current, I used a digital meter similar to this one on a customer's machine. You will still need a current transformer like the one below.

                      Last edited by JPR; 08-15-2006, 08:23 PM.
                      John

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                      • #12
                        Freq. is not the problem, it is purity of sine wave. Lots of diodes are linear to high freq. Boonton and HP make meters accurate to Mhz.
                        That capability isn't common. The Fluke 170 series is good to 1 khz on AC. That is their commercial/industrial line.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          I don't think you mentioned if your AC xuurent was "line current" or from an isolated source like a transformer secondary.

                          If it is "line current", then probably the current transformer is the best approach. It keeps you isolated from line voltage if you use a digital meter.

                          If its low voltage isolated current, then you can use a 0-5A meter directly, or with a shunt resistor if the meter reads less than the 3A full scale.

                          If you use an analog meter, like a standard 0-1A, then a shunt resistor will allow you to change the full-scale to any amperage you want. No isolation usually required for sensible voltages including line voltage, although there is nothing wrong with isolation either. These days, it is better to take precautions than not to.....
                          Last edited by J Tiers; 08-15-2006, 11:34 PM.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            There's no line current other than to a power supply for a variable speed DC motor. The motor will drive a magneto (16 pole) that outputs from 0 to 30v AC. At about 600rpm, the frequency will be around 4.8kHz and output will be around 10vac. This powers an apparatus that must be adjusted to 1.3A for for optimum function.

                            Forgot to mention, the load is inductive.
                            Last edited by CCWKen; 08-16-2006, 12:26 AM.

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                            • #15
                              Originally posted by Evan
                              That capability isn't common. The Fluke 170 series is good to 1 khz on AC. That is their commercial/industrial line.
                              Please, the 170 is a 6K count meter.

                              The 189 has a 100K bandwidth.

                              Again, it's not the freq. of the wave it's the harmonics that cause problems.

                              My HP 34401A, now that's an industrial meter!

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