Author Topic: Digital Multimeter (DMM) Part 1  (Read 3198 times)

Offline K9DJT

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Digital Multimeter (DMM) Part 1
« on: October 21, 2013, 08:49:12 PM »
Now that we have reviewed some history on the VOM and VTVM, I think it’s time we got into some of the current technology used today, i.e., the Digital Multimeter (DMM).  Not only is a DMM easier and safer to use, it also provides a whole host of what I call “Convenience” features.  First, when I say safer to use, I mean it is less likely you will damage it or literally blow it up due to a misapplication.  With that said, anyone working within a high voltage, high current situation needs to take necessary precautions…most importantly, THINK!  Be aware of what you are measuring, and if you do not know what you’re measuring, why are you doing it?  So what makes a DMM safer and easy to use?

Using the Fluke brand as an example, you simply turn a rotary knob to the function you are interested in measuring, e.g., AC-DC voltage, AC-DC current, resistance, diode check, frequency or capacitance.  (With some models, there are even more parameters which can be measured.)  If you like, you can choose a range like the older instruments, but the neat thing is that you don’t need to.  The meter will “Auto-range” and select an appropriate range for you.  If you are making a DC voltage measurement, you want to watch the polarity of your test leads (+/-) but again, you don’t have to. The meter will display either a negative or positive voltage in reference to the way your leads are connected.  If you had been using a VOM or VTVM you might have damaged the meter movement because it was pegged in the wrong direction.  Resistance measurements do not require a manual range selection either.  You turn the meter to ohms, connect the leads and the DMM will choose the correct range for the best resolution.  For those of you who have used a VOM/VTVM in the past, how many times have you left the meter in the ohms position, picked up the probes and made a voltage measurement???  Did you enjoy the aroma of the resistors cooking, or was a major arc?  With a Fluke DMM, as long as the voltage you apply in the ohms mode does not exceed the highest rated voltage of the meter, nothing happens.  No smoke or arc!!!  The only thing you will notice is an “OL” (Out of Limit) displayed on the meter.  How cool is that?  This holds true for whatever function you left the meter in.  The current mode operates just a little differently.  You need to physically move the positive test lead (red) to the appropriate jack, i.e., milliamps (mA) or the 10 Amp when making a current measurement.  Both jacks are fused for safety.  In this case you turn the rotary knob to either mA or A (Amps) and choose either AC-DC.  If you know for fact the current will be less than 10 Amps, but not exactly sure of how much less, it would make sense to start by placing the probe in the 10A jack, and if it is way less, .4 Amp (400 ma), you can move the probe to the 400 mA jack for more resolution.  Again, there is no need to select a range.  It will automatically select the best range and it will also provide the polarity (+/-) in reference to the way you connect the leads.

Next month I will explain why you might want to manually range the meter and will get started with some of the convenience features.

73, Gary