A multimeter is my go-to instrument for initially figuring out what is going on or going wrong in a circuit.
The “multi” in “multimeter” refers to the tool’s ability to read voltage, current and resistance. You may have seen one in my toolbox in a Radio World article some time back titled “What’s in YOUR Toolbox?”
Combining multimeter readings of voltage, current and/or resistance will give watts of power when using Ohm’s Law. You may remember the article I wrote last year, “Look What You Can Do With Ohm’s Law.”
The Beckman Circuitmate DM10 in Fig. 1 is no longer in production but is an example of what I have in a toolbox. This little workhorse was not the cheapest on the market but relatively inexpensive. Small size for a hand-held meter is the major consideration. If it is too big to carry in your toolbox, you need to find a smaller meter or a larger toolbox.
Fellow RW writer Buc Fitch’s “carry on” toolbox has a Fluke 111 (Fig. 2), which has now been replaced by Fluke with several newer and more versatile units that are about the same size. Other manufacturers in this area include Sperry, GB and Klein Tools, just to name a few.
Of course, there are the Chinese brands. I stay away from $20 units in favor of something in the $100 class. If and when I need to replace my meter, it will likely be a Fluke 17B+ for about $125. The price is right.
When asked why I don’t use a large, almost lab-grade multimeter at a broadcast facility, I reply saying that 95% of the time the small meter tells me all I need to know. My truck always carried the larger high-spec multimeter for more precision work (Fig. 3). It was the same one I use in the shop where there is bench space, nestled on top of an oscilloscope.
A better multimeter might be required when measuring potential over 1000 volts while connected to a specialized probe (Fig. 4). This particular one is rated at up to 50,000 volts. Best be very careful when using one of those!
Some multimeters measure capacitance and can even do peak hold on voltage and current readings. A few can measure power line frequency, such as from backup power generators, as seen in Fig. 5. Many uninterruptible power supplies want to see incoming line voltage and frequency that is close to what they get from an electric utility. A multimeter can help in getting a generator adjusted.
I find auto ranging of the digital meter scales a bit distracting. Current clamps for reading high-current AC and temperature probes are often available as accessories. They work, but I prefer using specialized equipment for that. On rare occasions, I have needed to see a voltage going up and down several times a second. In that case, an old analog meter with a traditional scale pointer that moves, such as on a Simpson 260, is my choice.
Getting the work done
The first thing I do when troubleshooting equipment is to check the power supply voltages. Only when those are right do I move on to problem diagnosis. If you skip this first step you can waste time and get frustrated.
When all should work but still doesn’t, I start by looking at individual components. A resistor may physically look normal but might be open when checked for resistance. I have seen that especially with older carbon composition resistors.
Wall-warts
Let’s say you are going to check a wall-wart transformer. If the rating plate says 12 volts at 0.5 amperes, that is 6 watts (Ohms Law, 12 volts times 0.5 amperes = 6 watts). Ohm’s Law also tells us we can test it by connecting a 24-ohm resistor at its output (12 volts divided by 0.5 amperes = 24 ohms). If the voltage remains at or about 12 volts with the resistor connected, you know you have a good one.
I remember one that read 12 volts until a load was connected, then it dropped to 2 volts. If the wall-wart has a traditional transformer with a diode rectifier and capacitor, it might start out at 16 volts DC with no load, then drop to 12 volts when fully loaded. This is normal and OK.
Remember that 6 watts I mentioned? If all is well, the resistor is going to get 6 watts hot. If it is a 2-watt resistor, it will get hot in a hurry!
Audio consoles
If you are working on a traditional analog audio console, chances are there is a + 12 or 15 volt power supply and a – 12 or 15 volt power supply. Supply rails, as they are called, power analog audio integrated circuit (IC) chips, of which many are operational amplifiers.
I drew a simplified diagram of that in Fig. 6. The audio output should have nearly 0 DC volts when no audio is present. There is always a small offset of less than a tenth of a volt that you can normally ignore. Audio will make the output voltage move up and down between the supply rails.
If you know a console has problems, it is a good idea to measure DC output voltages of the audio chips. Do that when no audio is going through. A multimeter should read near volts to signal ground. Under failed conditions it might go to the + or – power supply rail. When that happens, it is time to find some replacement IC chips. Hopefully they are the plug-in type for easy replacement. I have used that technique successfully on many occasions, it’s a real timesaver.
True RMS
Root Mean Square: You are unlikely to need this feature unless you are measuring a voltage that is controlled by a switching power supply with a duty cycle other than 50%. That could happen if you check power going to a variable speed motor. A standard non-RMS AC voltmeter or ammeter assumes it is seeing a sine wave when giving a reading. True RMS metering gives an accurate number regardless if the waveform is sine or square. Think of RMS as an accurate measure of AC and its ability to do work in a circuit.
Battery testing
Measuring the voltage of a dry-cell battery can be misleading. Nearly dead batteries show near-normal voltage.
One trick I use, believe it or not, is to test batteries with the ammeter function on a meter. It might be anything from an AAAA to a D cell or a 9-volt battery.
I start with a new battery and measure how much current it can deliver directly to the meter in its DC current function for just two seconds. It might be around two amperes. Then I check the unknown battery the same way. If it is capable of one ampere, then it still has about half of its life left.
Every meter will see this a bit differently, but the new vs. unknown is a valid test. Basically, it is a measure of the internal resistance of the battery plus resistance in the test leads. That is close to a short circuit. Don’t try this with larger batteries.
Remote controls
Just when you think you don’t need a multimeter, remember the remote control at the transmitter. You need to check logic and sample voltages to determine if they are right for the remote control. A quick voltmeter measurement can save a lot of headaches when trying to figure out why something is not working.
I keep a multimeter in my toolbox even when on vacation so I can diagnose a dead car battery or some other problem that would otherwise require outside help.
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