nuts & bolts
craft & technique Charging from Afar Build an expanded-scale analog voltmeter to help troubleshoot charging woes W I L L IAM RYNONE
O
n occasion, the plane engine doesn’t want to start due to slow engine cranking. This typically occurs when it is cold, so of course you’ll have little desire to work on the plane. Sometimes, however, you will want to do some troubleshooting. A feeble click or a rapid buzzing of the starter relay usually is an indication that the battery is nearly dead. Assuming you haven’t left the master switch on, that generally means one of two things. Either the battery is tired and won’t hold a charge any more, or it is not being charged when the engine’s running. There is a wide variety of troubleshooting techniques that can be used to spot the problem, but a good way to start is to build and use a home-brewed voltmeter that may be used in elementary troubleshooting of the alternator/regulator combination. Fortunately, aircraft alternators and regulators tend to be discrete units, unlike automotive alternators that tend to have the regulator built into the alternator case. Therefore, if the regulator is defective on the airplane, it’s not necessary to remove the alternator. That’s a good thing, because for many planes removing the alternator requires pulling the prop. Evaluating whether the alternator/regulator pair is providing adequate charging voltage can be done with some
degree of confidence by using a DC voltmeter. If the regulator tests bad, you’re spared the hassle of removing the prop. Before examining voltmeter use, I would like to discuss different types of common test equipment. Quite often, a voltmeter is packaged with an ohmmeter and ammeter and is called a multimeter. A multimeter (sometimes referred to as a VOM: volts, ohms, milliamps) is available either as an analog or digital unit. Each has advantages, and an advantage of one is usually a disadvantage of the other. The user of a digital unit is less likely to make an erroneous reading. The digital unit is more complex, and due to circuit miniaturization, it can be smaller than an analog unit. Since it does not have any moving parts, it may be more capable of withstanding shock or vibration. One of the primary values of an analog unit is its ability to display trends, and this is where it is useful in testing an electromechanical charging system. With changing voltages, a digital meter display can be expected to change rapidly, which may make it difficult to obtain useful data. However, an analog voltmeter connected to the alternator terminals makes it unnecessary to read rapidly changing numbers as you advance the throttle; noting the needle swing is adequate.
An analog voltmeter connected to the alternator terminals makes it unnecessary to read rapidly changing numbers as you advance the throttle.
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Large display means you can hook up the meter outside the airplane and read it from the cockpit while you monkey with the throttle.
The diagram shows the design of an expanded-scale voltmeter that incorporates a meter movement from a “junk parts” box, some resistors, an enclosure, a ubiquitous 741 op-amp, a pulse diode, zener diode, and banana jacks. EAA Sport Aviation
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The expanded-scale voltmeter was built from simple parts, first on a proto-board, then on a perf board with point-to-point wiring (above). If you want to go one step further, create a printed circuit board (below) to clean up the final product significantly.
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craft & technique
The enclosure is easy to make from materials you already have in your workshop.
If the battery charge level is low, the charging voltage will be high (perhaps greater than 14 volts) when the engine speed is increased to cruise rpm. So we only need to increase the engine speed and monitor the needle movement. There are other considerations, such as the effect upon the alternator output voltage of a defective diode; however, we must again place this anomaly in the list of “to be explored” possibilities. Analog meter movements can be quite expensive. Perhaps the rise in popularity of digital meters has had an adverse effect on the sales of analog meters, and so fewer sales results in higher prices. Therefore, should you decide to construct an analog voltmeter, it makes sense to use a readily available meter movement. If you are interested in monitoring only a narrow voltage range, and if you assemble an analog voltmeter that only displays that range, you will be able to view the meter scale at a
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greater distance. That means you can leave the meter outside the airplane and still see the indications from inside while you rev the engine. The diagram shows the design of an expanded-scale voltmeter that incorporates a meter movement from a “junk parts” box, some resistors, an enclosure, a ubiquitous 741 op-amp, a pulse diode, zener diode, and banana jacks. In addition to the schematic, a layout for a simple printed circuit board is shown. The initial circuit was tested on a proto-board, superseded by point-topoint wiring on a perf board, and then followed by a printed circuit board. If you decide to construct the unit and desire to make a printed circuit board, send a stamped, self-addressed envelope to me and I will return it with a 1-to-1 black and white photocopy of the P.C. board layout. Mail to: William Rynone, Ph.D., P.E., Rynone Engineering Inc., P.O. Box 4445, Annapolis, MD 21403.
