Author Topic: Digital Multimeter (DMM) - Part 2  (Read 4042 times)

Offline K9DJT

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Digital Multimeter (DMM) - Part 2
« on: December 02, 2013, 10:50:40 AM »
Last month I touched on how most digital multimeters (DMM) will “Auto-range”, meaning they will choose the correct range for the most accurate reading for the value you’re measuring.  So why would you want to manually range the instrument?  Well, actually there are two reasons.  The first would be to increase the response time; meaning it will provide you a value faster.  The second would be to better utilize the ”Analog Bar Graph” as a peaking or nulling tool.  What happens when “Auto-ranging” is engaged, the DMM needs to take a moment for it to think.  It needs to look at the value you are measuring and make a decision on what range it should select for you.  We are not talking about a real long time.  It usually takes only a second or two depending on the brand but that might be too slow to capture an intermittent or let’s say a momentary switch closure.  I once met a customer who carried a DMM and an old analog meter with him.  I asked him why he still bothered to carry the analog with him?  He explained he serviced a piece of equipment were one of the tests was to momentarily see 120 VAC on the fly.  It presented itself for maybe a second or two and the DMM was trying to figure out what it was going to display…and by the time it was done, it was gone…in others words, he couldn’t tell if it had been there or not.  He said, “with the analog meter, I can see the needle swing up and back down.”  I asked him how the DMM reacted when he manually ranged it?  He said, “what do you mean, manually range it?”  I showed him the manual range button on the DMM, and chose the 600 volt range.  He then conducted the same test, and the DMM numbers not only came up immediately, but the “Analog Bar Graph” display went up and back down just as fast as the old analog meter.  In some cases, it might even be faster.  Guess what…he only carries one meter with him now.

As I mentioned at the beginning, the bar graph can be used for peaking and nulling. You are able to make adjustments using the bar graph just as you would an analog meter.  The thing to remember though, is to manually range the DMM first.  Depending on the brand, the bar graph should respond within all functions of the DMM, i.e., voltage, current and resistance.  Just for fun, measure the resistance of a potentiometer sometime and watch how the bar graph changes as the shaft is turned.

Now with all this being said about a DMM “Analog Bar Graph”, and being a senior citizen myself, there is still something to be said for the “Feel of watching a needle swing.”  Just as all of us have test probes, and even hand tools, which have a certain feel to them, we will all have a preference or feel for a certain type of meter for a particular measurement or adjustment.
Next month I would like to discuss the application/use of “Min-Max” and “Auto-Touch.”

73, Gary

Offline N9LOO

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Re: Digital Multimeter (DMM) - Part 2
« Reply #1 on: December 03, 2013, 08:30:07 PM »
Great information Gary.  Thanks for the reminder on the use of the 'manual' range to be more responsive to quick changing signals.