Aero ‘lectrics

Multimeter musings.

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An analog multimeter on the bottom, the cheap Harbor Freight $6 multimeter on the top left, and a very good $50 digital multimeter on the top right.

Now that we’ve pretty well thrashed out how people make multimeters, let’s do some practical uses for our little meter. As I said when I started this thread, please buy one of those cheap $6 Harbor Freight multimeters to start. When you smoke one or two of them, it isn’t like you tossed away half an AMU (1/2 Aviation Monetary Unit = $500) or so. Later on you can buy one of the really nice meters.

Actually, Harbor Freight was giving away multimeters this last weekend with any purchase. That’s about the least expensive multimeter you will ever get. Not only that, but you could print out another one of the coupons and your significant other or junior birdman could have gotten a second one.

The cheap meter checker. Measuring from A to B gives 9 volts on the voltmeter. Measuring from A to C gives 9 milliamps on the current meter. Measuring from B to C gives 1.0K on the ohmmeter.

Before we start, let me give you a repeat of what I said in the first part of this series: You measure voltage and resistance across a part and current in series with the part. Keep this golden rule in mind and you ought to have multimeter happiness.

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Now, why did we get away from using the old “pointer” style meters in favor of the digital readouts? Price, for one. Expensive moving pointer meters required a magnet, a precisely wound coil of wire, a set of hardened (sometimes jeweled) pivots, a case, and other hardware. Cheap digital readouts are some electronic liquid “goop” sandwiched in between two pieces of plastic.

Even more so, all electronic meters require that we rob the circuit we are working on from a tiny bit of power to move the meter or change the digits. The pointer meters required a relatively large amount of this power, while the digital meters require somewhere around 5% of the best pointer meter’s consumption.

You measure current in series with the circuit to be measured and the voltage in parallel with the circuit to be measured.

Let me give you an example. Let’s simply measure the voltage of a 12-volt aircraft battery (12.6 volts fully charged). A moderate-quality pointer meter on the 15-volt scale puts around 300 Kilohms (300kΩ) as a load onto the battery. A relatively inexpensive digital meter puts around 10 Megohms (10 MΩ) as the load. This is a ratio of 33:1 or 3%.

Even more important is when you are working on a piece of equipment in a fairly high-impedance electronic circuit. Let’s take a simple transistor amplifier with a collector load of 100kΩ. Let’s also operate the transistor from a 10-volt supply and adjust the current through the transistor for 5 volts on the collector. Now if I measure what should be 5 volts on the collector with the pointer meter, we have a voltage divider that reads 3.8 volts, a whopping 1.2-volt error. The same measurement with the digital meter shows 4.95 volts, or a more reasonable 0.05 volt error.

The digital meter loads the battery very little, while the analog meter loads the battery many times more.

Now admittedly, the really cheapo Harbor Freight meter is only a 1-MΩ meter, but even so, the measurement on this amplifier would read 4.5 volts, which is a moderately reasonable 0.5 volt error.

So, reading voltages seems to be rather straightforward. In a relatively low-impedance world like batteries, light bulbs, and devices that draw moderate currents (say, 100 milliamps or more), the cheap HF meters are roughly as accurate as the expensive meters. When you start playing around with the innards of radios and such, you need to understand the limitations of an analog meter with an input impedance less than a megohm (1 MΩ).

On to current meters. Once again, the meter is going to rob the circuit of a tiny bit of power to operate. This time, though, instead of the voltmeter robbing current, the current meter is robbing voltage. There will be a tiny little voltage (about 0.1 volts) subtracted from the power supply voltage in order to measure the current. In general, unless we are making laboratory measurements, we don’t worry about this so-called “burden voltage.”

Using an analog voltmeter to measure a sensitive circuit gives false readings, while a digital high-impedance meter gives very close to the actual voltage.

And the last one of our trio is the ohmmeter, or resistance meter. Here is the secret of this little gem…you have to have at least one of the contacts on the device you are measuring completely out of the circuit. I can’t tell you how many people measure a resistor in the circuit and say, “Ah-ha, I’ve found the problem,” only to replace the supposedly defective resistor, then find that it wasn’t bad after all. Here are some other “gotchas” to watch out for:

• An incandescent light bulb will be somewhere around 50 to 90% lower resistance than you expect. Light bulbs go up in resistance when they get hot, so the cold resistance is far below what you might expect.

• If you are measuring a capacitor to see if it is shorted or leaking, be sure to discharge it by shorting across it before you make the measurement. If the capacitor is particularly large (say, over 1000 microfarads) or particularly high voltage (say, over 100 volts or so) then short the capacitor, wait ten minutes, and then short it again. Some capacitors have what is called “dielectric absorption” where the charged electrons hide inside of the insulating dielectric and then sneak out slowly to bite you during the measurement.

• Measure diodes with an ohmmeter by measuring in one direction, and then reversing the leads and measuring in the other direction. There should be at least a 100x difference in the two resistances, more like 1000x+ for a normal diode.

Here is a trick for measuring very large currents (like starter currents). You have to know the AWG gauge of the wire between the battery and the starter (or the master switch relay and the starter solenoid). Measure the length of the wire and then translate that into a resistance at the wire table calculator on rstengineering.com. Once you have the resistance of your battery-starter wire, you can crank over the engine while measuring the voltage drop across the wire. One quick Ohm’s law calculation (I = E / R) and you have the starter current. Here is an example:

• You have a 10-foot run of AWG 2 wire from battery to starter, which has a resistance of 0.0016 ohms according to the spreadsheet calculator. If you measure a voltage drop from one end of the wire to the other of 0.39 volts during engine start, then your starter current is 244 amps. Please note that it does not matter how long the leads are from the multimeter to the two ends of the #2 battery wire. A quick calculation shows that you could have (if you wanted to) not quite a thousand miles of #24 wire connecting the multimeter to the battery wire for less than a 1% error in measurement.

Measuring very heavy currents (like a starter motor) is easy by simply measuring the voltage across the battery to starter wire.

Back in August ’15, I gave you an elementary checker to be sure that your multimeter was telling you the truth. I’ve improved on that design a little and am re-presenting it to you as an indispensable little tool for your multimeter bag. I increased the voltage source from 1.5 to 9 volts, which also increased the current from 1.5 mA to 9 mA to take advantage of the 10-volt range on a lot of the available multimeters. The current set remains the same at 1.0 kΩ, so you can test the voltmeter part, the current meter part, and the ohmmeter part.

While at Oshkosh this year, somehow I managed to lose the rubber duckie antenna from my aircraft band handheld. I think I’ll make a replacement antenna out of Romex cable for a total cost (including connector) of somewhere around $5. After I get done with that, I’ll write up a couple of things I talked about at my Oshkosh forum and then maybe do a little bit of work on a nifty new cheap airplane whirligig with solar lighting. It’s going to be a good year. Stick around… stay tuned.


Jim Weir is the chief avioniker at RST Engineering. He answers avionics questions in the Internet newsgroup www.pilotsofamerica.com-Maintenance. His technical advisor, Cyndi Weir, got her Masters degree in English and Journalism and keeps Jim on the straight and narrow. Check out their web site at www.rst-engr.com/kitplanes for previous articles and supplements.

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