Author Topic: Dgital Multimeter (DMM) - Part 4  (Read 3464 times)

Offline K9DJT

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Dgital Multimeter (DMM) - Part 4
« on: January 20, 2014, 07:49:30 AM »
Let’s take a look at something I believe is pretty simple, but yet have been asked many times of what two different symbols on a DMM (Digital Multimeter) rotary switch are used for.  The first one is a small cone shape having multiple curved lines, and the second is a schematic symbol for a diode.

The cone shape with multiple curve lines is representing a speaker, and is used to measure continuity.  Most of us have done continuity measurements using an ohmmeter, which is fine.  But what if you just want to know if something is making contact or not, without having to look at the display of the DMM?  Let’s say you’re looking at a pair of contacts on a relay and it is taking both your hands and eyes to active the relay in some manner.  Well, by using the “Continuity Check” you can do just that.  You connect your test leads across the contacts you want to check; activate the relay, and LISTEN for a steady tone.  When you release the relay, the tone should disappear.  You may check this function of the DMM just by touching the probes together, or if you’re in the need of a Morse code practice oscillator, here it is.  Just connect the probes across the key terminals and you’re ready to go!!!  But wait, what if you are also interested in the integrity of the relay contacts?  What if they’re highly resistive?  At this point you do need to look at the display which will indicate a resistance as high as 600 ohms.  With perfect contacts the DMM should indicate 0 ohms as in the picture at the above right.  If the contacts are in poor shape they might indicate a resistance like the picture to the left.  In both cases, take note to the position of the rotary switch and the symbol on the left side of the DMM display.  It’s the little speaker symbol, and not the ohm meter.

OK…So why is there a “Diode” selection on the rotary switch?  In the past when we would test a diode with a conventional VOM, we would use the ohmmeter in its lowest range and place the probes across the suspect diode, first in one direction, and then the other.  With the negative on the cathode there would be a needle deflection indicating forward bias of the diode and therefore conduction.  Reversing the probes should indicate no conduction, i.e., a good diode.  If there was conduction in both directions, the diode is shorted.  That still holds true using a DMM except for one thing.  In many cases, the DMM, because of the processor it uses, doesn’t provide enough voltage to forward bias a diode while in Ohms.  That is why there is a separate “Diode Check” on most DMM’s.  The neat thing though is that it just doesn’t indicate a conduction when forward biased, but it will display the amount of voltage it takes to do it.  Take note to the picture on the left showing the voltage and a little diode icon to the left of it.

Can’t remember if it is the long or short lead on an LED which is the cathode?  Why not check it with the DMM?  Just connect the test leads across the LED and see if it lights up.  If not, reverse the leads and take note to the negative lead and the length of the LED lead it is connected to.  (It will not be full brightness and therefore you must be sure you are looking at it straight on and not off to the side as in the picture.)  Again, the DMM will display the voltage it takes to forward bias and turn the LED on.  In this case it is 1.63 volts which is enough to turn it on but not to full brightness which is usually around 2.2 volts.

Next month we’ll discuss the various current measurement capabilities and options available with a DMM.

73, Gary
« Last Edit: January 20, 2014, 07:52:34 AM by K9DJT »