Author Topic: Digital Multimeter (DMM) - Part 3  (Read 2255 times)

Offline K9DJT

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Digital Multimeter (DMM) - Part 3
« on: December 04, 2013, 01:58:58 PM »
This month I would like to explain two convenience type features available on Digital Multimeters (DMM).  One is the “Min-Max” and the other is “Auto-Hold”.  Although most brand DMM’s have buttons labeled as such, they do not all function in the same manor.  As in the past, I will be referencing the FLUKE brand with my examples.

The “Min-Max” button, when depressed while making a measurement will do as it implies, and that is retain the maximum and minimum values while the probes/leads are connected to your source or device you are measuring.  In addition, it will also compute an Average value between the minimum and maximum.  For example, if you would like to see how well your regulated 12VDC power supply is actually functioning, you would first turn the main switch of the DMM to measure DC Voltage, connect the test leads across the output terminals of your active supply and then press the “Min-Max” button on the DMM.  At this point, you may press the “Min-Max” button, and each time you do, it will first display the Maximum, the next press will display the Minimum and the third press will display the Average measurement.  (Holding the button  down will turn off the Min-Max function.)  Because you haven’t really drawn any current yet, the values should really be close to each other.  Now, without changing anything with your test setup, draw some current, e.g., transmit for a couple of seconds, assuming you are also connected to a transceiver.  You most likely will see a slight difference in the values when you toggle trough the Min-Max again, but nothing great.  If the regulator is poor or not functioning properly, you will notice the Minimum value much lower than the Maximum value.  Instead of monitoring voltage, you might want to measure current instead.  Just make sure the meter can handle the current in question.

“Min-Max” will work within all the measurement functions of the DMM, e.g., Voltage, Current, Resistance, temperature and frequency.  It is important though to remember the proper sequence in which you connect the DMM and turn on the “Min-Max”.  Always connect the DMM and start measuring your parameter prior to depressing the “Min-Max” button.  If you connect the DMM to the circuit with the “Min-Max” already turned on, your “Min” value will equal Zero!  That is because that is what the DMM was seeing prior to connection…zero!  Connected in such a manor will display the Max and compute an Average between the Max and Zero.  The same thing will happen if you disconnect the test leads prior to toggling through the “Min-Max” values.  As soon as you disconnect, the new minimum value is zero!

Use your imagination with this feature.  I’ve monitored my line voltage over a period of time and even made remote continuity measurements by using the ohmmeter on a pair of wires, going to the far end, momentarily shorting the wires and returning to the meter to see if my new minimum resistance went to zero or not.  It also works great when working on a trailer wiring harness.  Connect the DMM, go and depress the brake pedal and then return to the DMM.  Did you have a voltage on the correct lead?  See, no need to bother the wife or kids to step on the brakes!!!

The other feature is the “Hold” and “Auto-Hold” button.  While making a measurement, you might want to retain the reading on the DMM to record somewhere…especially if your short-term memory is starting to fail you.  All you need to do, while the DMM is connected, is depress the “Hold” button. Now it is stored on the display so you’re able to disconnect the DMM and carry it over to your PC or desk to record it.  To bring the display to normal, just press the hold button again.  Now you should be asking, “what’s the difference between “Hold” and “Auto-Hold?”  Have you ever worked around lethal voltages or currents?  Like those inside a breaker-panel?  How about an HV power supply for you amplifier?  These are places where you need to be extra careful of what you are doing.  One slip of a probe can damage components or worse yet be a “widow-maker”.  This is where “Auto-Hold” or “Auto-Touch” saves the day.  Set up the DMM for the proper parameter, in this case let’s say AC voltage.  Press the “Hold” button twice, and AutoHold or AutoTouch should show up on the display.  Now carefully go into your breaker panel while watching both hands, having no need to look at the DMM, and listen for a beep.  Once heard, remove your probes and look at the DMM.  The measured value was retained on the display!!!  Now, without changing anything on the DMM, go back in and make a measurement at a different point, wait for the beep, remove the probes, and again look at the display of the DMM.  The new value is now displayed!!!  So as you can see, each time you touch a different test point, the DMM will HOLD the last measurement without ever having to reach over and do anything with the DMM or even look at it while making the measurement.  It’s Automatic!   To turn the function off, just press the Hold button one.

Next month I would like to discuss the use of the “Continuity” and “Diode.” functions.

73, Gary
K9DJT
« Last Edit: December 04, 2013, 02:03:38 PM by K9DJT »

Offline N9LOO

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Re: Digital Multimeter (DMM) - Part 3
« Reply #1 on: December 04, 2013, 10:15:45 PM »
Now I want to purchase a new DMM with these features.  Thanks Gary!