Author Topic: Digital Multimeter (DMM) - Part 6  (Read 3360 times)

Offline K9DJT

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Digital Multimeter (DMM) - Part 6
« on: May 12, 2014, 02:03:00 PM »
Last month we looked at making current measurements (A for amps) in circuits which were 10 amps or less.  What if you want to measure larger currents up into the hundreds or even thousands of amps?  Is that really possible?  Yup…by using an accessory referred to as a Current Clamp.  The concept originally came from a company called Amprobe who manufactured a clamp-on instrument used just to measure current hence the name.  Those type of meters (clamps) are still manufactured with the addition of a lower level voltmeter incorporated.  We’ll discuss those at a later date.

A current clamp accessory is typically available in two flavors.  One being a transformer device, pictured on the left, used to measure AC current, and the second is a Hall-effect device used to measure AC or DC current which we’ll discuss next month.

The current transformer, depending on its rating, will convert 1A to 1mA.  The connection to the DMM (Digital Multimeter) is the same as if you inserted the meter in series with your load.  The black lead is placed in the negative-jack and the red is connected to the mA-jack.  You’ll need to turn the rotary knob to AC mA, and in addition interpolate the reading in your mind from mA to Amps.  The mA reading is direct, i.e., there is no decimal point.  You might see a display of 25 mA, not .025 Amps, which makes it pretty easy to see your circuit has a current flow of 25 Amps.  This is one of the reasons there is a mA range on the meter.  The floor lamp on the left is drawing 1.83 amps, but you will notice the range is mAAC.  What do you do if you don’t have a mA range?  You may still use the clamp in the current mode of your meter as long as you remember 1 Amp = .001 Amp.  Resolution is usually lost and you won’t have the “direct” read out on the display.  Remember the measurement you are making is the current flow which is occurring in one leg of a circuit.  One of the biggest mistakes I used to see was someone who wanted to know how much current an appliance was drawing by placing the clamp around the line cord.  Guess what…the draw was zero!  That’s because the clamp was around both wires, and the opposite flowing current cancelled the other.  You need to have a means of capturing the current in just one of the wires as shown using a current clamp adapter (pay attention to the rating of the device) for line cords.  The one shown in the picture is using a x1 opening while the other opening is a x10 which may be of help with measuring an extremely low current.  The other neat benefit to using a current clamp with a DMM is that you’re able to make use of the Min-Max feature of the instrument.  That means you are able to capture the inrush current of a motor or appliance at start up.

73, Gary