Author Topic: Digital Multimeter (DMM) - Part 7  (Read 1749 times)

Offline K9DJT

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Digital Multimeter (DMM) - Part 7
« on: May 12, 2014, 02:09:45 PM »
Last month we looked at making AC current measurements using a Current Clamp in the form of a “Current Transformer” which extends the capability of a DMM (Digital Multimeter) to measure up into thousands of AC amps depending on the specifications of the clamp used.  This month we’ll take a look at the yellow clamp at the right in the picture.  It also measures AC current, but in addition it will measure a DC current.  It is not a transformer type of device as the grey one on the left, but rather a “Hall-Effect” device.  Instead of a transformer, an active component is used in this type of clamp and  therefore requires a battery to power it.  The “Hall-Effect” was discovered by Edwin Hall in 1879 while working on his doctoral thesis in Physics.  He learned there was a voltage difference across an electrical conductor which was transverse to an electric current in the conductor and a magnetic field which is perpendicular to a current.  At the left is an illustration  of the makings of a clamp showing the “Hall-Effect” sensor.  Because we are looking at a voltage generated in reference to the current, we will be plugging this clamp, red plug into the millivolt jack on the DMM rather than the current jack.  I would expect you to know the black plug goes into the negative jack.  (The pictured clamp will help you plug it in correctly)  We then need to interpolate the mV reading into a current measurement.  This particular clamp will generate 1 mV per Amp.  Therefore, if the display shows 150 mV, you will have 150 Amps of current flow.  Pretty easy!  This is one reason the Fluke brand has a separate mV switch position.  I will discuss the other reasons in future articles.  So what do you do if you have a DMM without a mV switch position?  Well, you are still able to use the clamp as long as your DMM has mV resolution, i.e., .001 display capability.  Now for the same measurement, your display will not be a direct read out, but rather show .150 volts which as we know is 150 mV’s, or 150 Amps when we interpolate.  The need of plugging the clamp into a voltage jack, instead of a current jack, is not the only thing you need to be aware of.  The clamp, which uses a 9 volt battery needs to be turned on and/or off.  It also makes sense that you want to ensure the battery is good.  The other thing is that when you originally connect the clamp to the DMM, but before it is placed on a wire to be measured, the clamp requires a DC “Zero” adjustment.  Quite easy to do with the knob on the clamp.  You just look for a Zero value on the display and then proceed to place the clamp on the wire you want to measure after adjusted.

73, Gary
K9DJT