RADIO CONTROL ELECTRICS
Bob Kopski, 25 West End Dr., Lansdale PA 19446
This column comments on Dump'r, suggests a needed E-product, leaps from "too little" to "too much," and continues the discussion of basic electrical terms and concepts.
Dump'r: reader response and clarifications
You did it! You made it happen! Never has there been a faster, larger reader reaction than in recent months regarding Dump'r. This versatile battery discharger, offered as a test topic in the June column, is a feature in this issue.
Your immediate and continuing flood of requests to publish Dump'r caused me to quickly compile all of the necessary information and offer it to Model Aviation for consideration. As a result, Dump'r is included herein for your consideration and construction. Thank you for your interest and vote of confidence in Dump'r.
Included in the many letters were numerous repeat questions—and misconceptions. Dump'r as designed and presented is for Ni-Cd and NiMH packs of four to 18 cells. It has a fixed discharge current, and no, it cannot safely be increased! But it could be reduced. Dump'r itself does not "cycle" packs—that's not its purpose—and it can't charge! Dump'r does not measure or record the discharge parameters, although because of its fixed discharge current, you could time its operation and deduce the information—but that would be very boring.
To summarize what Dump'r can and cannot do:
- Designed for Ni-Cd and NiMH packs of 4–18 cells.
- Has a fixed discharge current (cannot be safely increased; can be reduced).
- Does not cycle packs and cannot charge batteries.
- Does not measure or record discharge parameters (timing could be used to estimate capacity).
That covers most reader queries and comments about Dump'r. However, since so many readers inquired about measuring the battery's discharge (capacity), I have reviewed the possibility and can imagine a simple "add-on" to allow this.
Possible add-on for capacity measurement
Initially, I see a possible way to automatically and inexpensively get the "time" parameter, but this needs to be developed and tested. If it works out, I'll offer it as a simple supplement to Dump'r in a future column. In the meantime, I'm confident that you will find Dump'r to be everything it's described to be and that you will be quite happy with its intended operation. I have four of them!
The potential add-on—as I imagine it now—will not significantly alter Dump'r as presented. Please don't delay in building Dump'r because of this uncertain addition. If and when I can offer the extra timing utility, it will likely only require a small additional connector on the box; otherwise, I wouldn't consider it "simple." Besides, I don't want to mess up my own Dump'rs! And please don't request early info. I'm uncomfortable sharing design on the fly; I much prefer to share finished, documented, proven stuff.
Battery Eliminator Circuits (BEC): limitations and a wish for switch-mode
Despite the vastness of present-day electric technology and market offerings, there is one E-product missing. Most modelers know and appreciate the convenience of the Battery Eliminator Circuit (BEC) feature that is part of many Electronic Speed Controls (ESCs). A BEC allows one to power the radio system with regulated voltage derived from the motor battery; that is, a BEC substitutes for the receiver battery.
This has the clear benefit that you do not have to install and maintain the classic radio pack. However, current BECs have limitations; the main one is that they can only be used in "smaller" installations. Generally the BECs I've seen are limited in the number of cells allowed in the motor pack and the number of servos allowed in the system. This is a technical limitation associated with the power-handling capability of ordinary BEC circuitry. Thus, one may find a BEC specified for "6 to 10 cells" and "2 to 3 servos" (specifics vary with the product).
The reason for these limitations is that all ESC BECs I know of incorporate linear regulators. Linear regulators are inefficient and normally get rather warm to fairly hot, and this temperature rise must be held to safe operational levels. That is done by limiting cell and/or servo counts. I'd like to see the ESC/BEC idea implemented in efficient switch-mode circuitry rather than inefficient linear circuitry. This would eliminate heat-related concerns and could dramatically raise permissible cell and servo counts. But who will be the first ESC manufacturer to get onboard with this product wish?
Too much choice in modern electric technology
The vastness of today's electric technology and product array brings to my mind the earliest days of silent power—not because of the similarity, but because of the difference. Having "been there," beginning in the early 1970s—with nearly no E-product and no E-interest to be found—I can greatly appreciate the huge difference three decades have made. What a large and glorious change!
Not for everyone. For some, E-power still offers a challenge: the paradoxical challenge of too much choice.
In the earlier years of this column, I frequently addressed the common reader cry of "which motor, which model?" Nowadays it's more like "which motor"—because the model no longer matters as much. Now one has many choices of E-equipment for almost any airplane.
More and more I'm experiencing frustrated readers who have difficulty choosing between one product and others that are similar; there are so many that seem so much alike. It's like pulling into an empty parking lot and having to decide in which space to park.
Consider the contemporary motor marketplace. A look through paper or electronic catalogs for suppliers such as Hobby Lobby, Northeast Sailplane Products, New Creations R/C, and many more illustrates a veritable blizzard of motors and many seem eligible for the same application. How does one choose? What's more, this quandary is not limited to motors; it includes most other E-stuff.
One approach to this "new" problem is the same as for the "old" one, because it's really ongoing. The trick is to look for examples. The July column described the numerous E-info resources presently available to everyone. Included therein is an abundance of accomplishments by others that can become guidelines for you.
No matter your choice of E-project, you are highly likely to find something similar in the references, and from them you can more easily and more confidently choose a satisfactory power-system makeup; copy what was successful for someone else. Those same resources can suggest choices besides the motor; e.g., for speed controls, battery makeup, and even propellers. I ask again as I did in the July column: How do you think electric power got from so little to so much through the decades? The answer: more and more E-modelers making and sharing more and more E-accomplishments in the course of time.
Measuring current in high-current E-systems
The last two columns discussed common electrical terms such as voltage and current and illustrated how to use multimeters to measure these quantities. This knowledge can help you not only troubleshoot, but also optimize your E-power system.
Although getting into this detail is not a requirement to enjoy electric flying, having such knowledge and ability can add much to the enjoyment of E-power for those who are so inclined. It's a little like enjoying a sports-car ride through the countryside or doing the same thing knowing that you fine-tuned your own car.
Previously you learned that voltmeters are always connected across terminals—such as battery or motor terminals. Also, current (amp) meters are always connected in series in a closed circuit. You learned that good voltmeters are high-resistance devices that minimally disturb what they are measuring. Similarly, good ammeters are low-resistance devices that "use up" little of the available circuit voltage.
Now to complicate things a bit. Or maybe this is actually a simplification—it depends on how you see it.
In electric applications, because power systems normally have high currents present in a compact installation, one does not normally directly use the ammeter part of a multimeter to measure these currents. This is because of practicalities—not the principles involved.
For one thing, most Digital Multimeter (DMM) ammeter functions are limited to 10 or 20 amps maximum, and this is inadequate for higher-power E-systems. Furthermore, just connecting to such a DMM requires test leads that are much different from those commonly supplied. The three- or four-foot, light-gauge wires and probes that come with a typical DMM are not good with 20 amps flowing!
That is because this test-lead wire is added to a circuit that normally has short, heavy wiring within itself, and the additional length and associated electrical losses of light wire can significantly alter power-system behavior. Such measurement could alter what is being measured.
Using a typical DMM's ammeter function directly in an E-power system requires some custom, short, heavy, properly connected test leads. Although this is not a serious challenge, it does physically locate the instrument close to the "stuff" in the model—and the propeller—which is less than ideal. What to do?
This brings me to the more fundamental question of how the ammeter works. In the case of DMMs, the ammeter function is really the internal voltmeter (i.e., the DVM part) reading the voltage on a built-in shunt (sampling) resistor.
That is, when you use the DMM's ammeter option, you are actually routing circuit current through a sampling resistor internal to the instrument, and the internal voltage metering is reading the voltage that appears on that resistor. (Remember that voltmeters measure across terminals.) While it's a sampled voltage being measured, the instrument display reads out in amps.
If you are unsure at this point, appreciate that when current is flowing through a resistor, a resulting voltage appears across that resistor. This is exactly the same—though viewed differently—as current flowing through a resistor when voltage is impressed on it.
Expanding on this idea, but implementing it vastly better for our high-current application, one can insert a separate or external shunt (sampling resistor) in an existing motor circuit—say at a connector set—then use a DVM to read the voltage appearing on this shunt when motor current flows.
Hence in this application, a shunt samples current and presents the current magnitude as a small voltage that can be read "at a distance" since the lead length to the instrument does not affect a voltage reading.
A shunt is characterized with a descriptive constant usually expressed as millivolts per amp; a typical shunt constant might be 1 mV/A. This means that for each amp that passes through this shunt, one millivolt will appear; that is because this shunt is a precise 1 milliohm (1/1000 ohm) resistor that is capable of safely carrying high current.
Such a shunt, situated locally or in situ, introduces minimal resistance and voltage loss within the circuit being measured. A good shunt—one with short interconnects and a suitable low mV/A constant—imposes minimal burden on the circuit under test. Therefore, in a motor-circuit installation the presence of the shunt would be minimally impactful.
For those with a Model Aviation library at hand, this idea was detailed in the January 1993 column. I'll discuss it more next month. Later in this continuing miniseries we'll see how one dedicated product—the AstroFlight Whatmeter—does all of the preceding and much more in a simple and convenient way.
Please enclose an SASE with any correspondence for which you'd like a reply. Everyone so doing does get one!
Happy and numerous safe E-landings, everyone! MA
Transcribed from original scans by AI. Minor OCR errors may remain.
