Thirty-nine years ago, a fellow I know fairly well wrote an article for the EAA magazine on homemade ammeters and voltmeters. This is the second installment in that series of articles in 2019; who knows, but maybe the third and final chapter will be written on the 50th anniversary in 2030 to coincide with the planned human landing on Mars!
But I digress. I’m about to show you a digital voltmeter-ammeter that is every bit as easy to work with as the analog meters from 1980, much more reliable, and (in constant dollars) about half the price of those old moving-needle meters.
Here’s the deal: The one we are going to work with today is what I’ll call the “junior” version as it has a good 4- to 30-volt input range, but the current is limited to 10 amperes. I use one of these in my lab ra-dio rack to keep the backup battery voltage monitored and also the current used by the aircraft-band lab radio, as well as the ham-band 2-meter rig.
(Left) The voltmeter (13.8 volts) and load meter (0.11 amps) display. (Right) The voltmeter-load meter with the transmitter keyed, showing the increased current draw of 0.41 amps.
In airplane talk, we use the term “ammeter” to show both charge and discharge of the various loads on the battery/alternator setup. A “load meter” shows the actual load current being drawn from the electrical system. Cessna prefers an ammeter while Piper prefers a load meter. Me? Since this digital meter includes a voltmeter that tells me whether or not the alternator is sending current into the battery (12.6 volts with no charge, while 13.8 volts is normal when charging), I much prefer the load meter.
If you are looking for this voltmeter on Amazon or eBay, the part number is YB27VA or DSN-VC288, and it comes in several versions: red digits for the voltage and blue digits for the load meter or red digits for both volts and amps. There’s a 10-amp version, a 50-amp version, and a 100-amp version. You’ll pay somewhere between $10 and $15 depending on who has the best deal of the day. The best deal I’ve found is the McLgLcM meter on Amazon for $8 and free Prime shipping.
The back of the meter showing the two heavy red and black wires, and the three thin red, yellow, and black wires.
The version I’m using for this article (and the one that I use on the lab-radio setup) is the red-blue 10-amp version of the DSN-C288. It takes just a single glance to see what the meters are reading with the different colors, and the meter has an internal 10-amp shunt. The 50- and 100-amp versions (next month, I promise) require you to use an external shunt. You can buy a shunt from the same company, but I’ll show you how to make one out of plain old house wire.
There is one sticky wicket with this meter that you will have to solve. For plastic airplanes, it’s pretty easy; for metal airplanes, not so much. Here’s the deal. The ammeter circuit in this little gem requires that the shunt be placed between the negative supply line and the negative lead of the load. In a plastic airplane the negative bus is not connected to the fuselage as the fuselage is nonconductive. In a metal ship the negative lead of the battery and the case and ground of the radio or other load are usually both connected to the metal fuselage.
Alright then, time to figure out how to hook this little gem up. There are five wires coming out of the back of this unit: thin black, red, and yellow wires, and thick red and black wires. (The accompanying schematic is the visual version of these instructions.)
- Heavy black goes to the negative side of the battery.
- Heavy red goes to the airframe ground. The negative lead of the load goes to the same airframe ground.
- Small red and yellow, and the positive side of the load go to the positive battery terminal.
How does it work? The thin wires are the voltmeter wires, which draw very little current (microamps to perhaps a milliamp or so). The thick wires are connected internally to another circuit that also measures voltage down in the millivolt level. They are also connected to the internal 7.5 milliohm shunt (silver wire in the image). This shunt is calculated to drop 75 millivolts (0.075 amps) at the full-scale 10-amp rating of the meter. This means that for every ampere of current, the shunt drops 7.5 millivolts (2 amps is 15 millivolts, 4 amps is 30 millivolts, and so on). While the front panel meter is calibrated in amperes, the internal circuitry is actually measuring the voltage drop across the 7.5 milliohm shunt.
It is sort of funny as I reflect over the past 38 years of progress in electronics. Moving-pointer meters were cheap ($5 or so each), and digital meters were dozens to hundreds of dollars each back in 1980. Now moving-pointer meters are in the dozens of dollars, and dual meters are down in the $6 to $8 range. Digital meters are 10 times as rugged as the pin bearings on the pointer meters and can be calibrated 10 times more accurately.
The little voltmeter-load meter occupies a place of honor in the center of the second shelf from the bottom.
Like I said, I’m going to be working on a higher amp meter for your airplane (probably one of the 50-amp devices), and I’m going to try (no promises) to find one that can be used with a shunt in the positive battery lead instead of the negative lead. Until then…Stay tuned…