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Workbench Wednesday 24: No-Contact Current Measurements

 

 

One of the safest ways to measure current is with a clamp. Clamps do not require you to break the circuit and they safely measure wires with thousands of amps. In this video, we measure DC current in the lab, AC current of a reflow oven controller, and the DC current in a car using a Tenma 72-722672-7226 True RMS Clamp Meter.

 

Bill of Material:

 

Product NameManufacturerQuantityBuy KitBuy Kit
72-722672-7226 - Clamp Meter, Auto, True RMS, 600 A AC, 600 V AC, 600 V DC, 28 mm Jaw Opening Max.Tenma1Buy NowBuy Now
Anonymous

  • Hi James,

     

    I purchased one of these meters from Newark and I have been quite happy with it. I have also found another use for it. Over the years my experiments have often involved magnets. Sometimes the polarity of the magnet has been important to the experiment and I have always struggled to determine which pole is the North and which is the South. By experimenting with the DC current function on the clamp meter I was able to determine that the position of the magnet inside the clamp very quickly determines which end is the North and which is the South.  Here is a diagram of how the meter reacts to the position of the magnet.

     

     

    The strength of the magnet can also be gauged by how high the current reading is and magnets can be compared to one another.

     

    John

  • in reply to kb3gkc

    Hi Mitch,

     

    There are specially configured Op Amps called Comparators that might work well for your application. They will not only trigger at a tipping point like the 350 mA that you mentioned but they also have some hysteresis so that you get a definite On - Off from them. I have used the LM 339 quite a bit but here are others that you can look at as well.

     

    John

  • Great episode! Thank you.

     

    I'm more of a tech (hack) than an engineer so I need some help with a project.

    I need to build an over current alarm circuit for AC. The current in the circuit is typically between 70mA and 200mA. I'm going to have it alarm around 350mA.

    I've been looking at different current sensors and I'm not really sure what to use. Hall effect or Current transformer.

    Then there's the actual circuit to make the alarm. I'm assuming a differential opamp but I'm a little lost as to how to configure it.

    I've been researching it but I'm a bit confused. My plans is to drive a small piezo buzzer.

    I would rather not break the circuit and would like to use a split core CT for ease of installation. But I'm not sure if that will give me enough sensitivity/resolution.

     

    I'd really appreciate any advice or pointing me in the right direction.

    Thanks!!!

    Mitch

  • in reply to jw0752

    That is really interesting John. Great experiment.

  • Hi James,

     

    Your blog inspired me to run an experiment. I had in stock this current measuring board with a ACS712T ECL20 Chip on it.

     

     

    I cut a notch in a ferrite core that was just big enough to slip over the Hall Effect IC

     

     

    Now I configured the circuit so that circuit with the Hall Effect sensor was in the gap of the core and I also had a current carrying wire going through the core

     

     

    I ran two test sequences with the polarity of the magnetic field reversed from one sequence to the next. The chip also got moved slightly in the transition which may explain the slight shift in scaling (delta V) from one sequence to the other.

     

     

    Current in the Test Wire          Voltage Output Seq 1          Delta V                         Voltage Output  Seq 2                    Delta V

     

              0  Amps                                  2.495V                                                                      2.524V

                                                                                                   0.024V                                                                                0.028V

              1                                             2.471V                                                                      2.552V

                                                                                                   0.025V                                                                                0.027V

              2                                             2.446V                                                                      2.279V

                                                                                                   0.024V                                                                                0.027V

              3                                             2.422V                                                                      2.606V

                                                                                                   0.026V                                                                                0.028V

              4                                             2.396V                                                                      2.634V

                                                                                                   0.025V                                                                                0.028V

              5                                             2.371V                                                                      2.662V

     

    I also did a little digging in the physics book and in an ideal situation the magnetic field induced in the ferrite core will remain constant regardless of the position of the wire inside the core. (Ampere's Law)  The shape of the loop of the core should also not have an affect. The position of the wire should still be irrelevant to the field produced.  Of course with the clamp meter we probably don't have a consistent continuous loop which may allow the position of the wire to affect the field.

     

    John

  • in reply to jw0752

    Hi John, here are my thoughts.

      wrote:

    I assume that centering is only crucial for the DC current measurement, is this so?

    As far as I knew, it should apply to both AC and DC. But I could be wrong.

     

      wrote:

    How do you think they are getting a DC current measurement? Perhaps integration of multiple Hall effect devices in the clamp?

    It's inhereint to the hall effect sensor works. It is just measure the strength of the magnetic field in the gap of the core.

     

      wrote:

    How did the meter do for accuracy below 1 Amp in the DC mode?

    I haven't really used clamp meters for less than 1 amp. My guess would be, they aren't very effective.

     

    I have used the hall effect for my oscilloscope on stuff in the 100 mA to 1 A range. I know that below 10 mA, it the measurement gets into the noise floor of the probe.

  • Hi James,

     

    Nice episode. I would think it would be relatively easy to build a centering bushing for the clamp.

     

    I assume that centering is only crucial for the DC current measurement, is this so?

     

    How do you think they are getting a DC current measurement? Perhaps integration of multiple Hall effect devices in the clamp?

     

    How did the meter do for accuracy below 1 Amp in the DC mode?

     

    John

  • in reply to koudelad

    Hey David, for low currents, that is a good way to improve the overall measurement accuracy. I did not mention it in this video because I was focused on getting closer to the meter's maximum current.

     

    But yeah, as soon as I read your comment, I thought "aaahhhhhh, I forgot."

     

    Thanks for the tip!