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R/CAerobatics L


ast month, the pros/cons of using voltage regulators was discussed, along with the possible benefits of using high voltage (HV) servos


(suitable for 8.4 volts from unregulated 2S Li-Pos) versus standard servos (typically limited to 6–7 volts). Not discussed were the pros/cons of conveying the electrical power to the servos. Several sizes of wiring are commonly used


in R/C systems, ranging from 30 to 22 AWG (American Wire Gauge). The AWG system is used for both solid wiring (single strand) and flexible wiring (multiple smaller strands twisted together into a bundle). The actual diameter of flexible (twisted strand bundle) wiring is typically slightly larger as the AWG size is determined by the sum of the cross-sectional area of each wire in the bundle and there are void spaces between the wires. As a general rules of thumb, when the cross-sectional area of the wire is cut in half, the AWG increases by 3 (and the resistance doubles); when the diameter of the wire is cut in half, the AWG increases by 6 (and the resistance is quadrupled). Yes, lesser AWG equals larger wire. For RC applications, flexibility, current capacity, and weight are the driving factors in determining the “best” wire gauge. I listed flexibility first as this is very important, considering the often irreg- ular path of servo leads and the need to be vibration resistant (minor consideration with electric powerplants). 26 AWG is the gauge of wire most com-


monly found on standard/sport servos. Smaller gauge 30 AWG wiring is occasion- ally found on micro and submicro servos where weight savings are the priority and the current demands are substantially de- creased. 22 AWG “heavy duty” wiring is found on higher performance servos and heavy duty switch harnesses. In practice, 22 AWG is the largest wire that can be fitted to the industry standard 3-pin “universal” ser- vo plug, which is generally accepted to have a rating of 5 amps. 5 amps is sufficient for even the most current thirsty servo applica- tions (up to 40% aerobatic airplanes), how- ever, more substantial connectors (such as


PHOTOGRAPHY: DAVE LOCKHART


Servo leads that cover all sizes and types of models. The standard universal 3-pin radio connectors pictured from left to right with decreasing wire sizes: 22, 26, and 30 AWG.


Ultra Deans plugs) and wiring (16 AWG) are commonly found between the power source and the Rx (or power distribution system, if used). The job of the wiring and connectors is


quite simple; convey power to the servos with minimal drop in voltage (caused by re- sistance) while adding a minimal amount of weight. Minimizing wiring length and un- needed connectors are the best ways to re- duce voltage drop. The quality of connectors and wiring can vary substantially. If ven- turing beyond OEM items from Futaba, Hitec, JR, etc., close inspection of the con- nectors in particular is recommended. The connectors should be free of mold flashing on mating surfaces, have accurately aligned gold plated pins, secure crimps that are cap- tured in the housing, and fit snugly when plugged in. Wiring should be tightly bun- dled with the greatest number of small strands being preferred. The most common/simple radio installa-


tions consist of power to the Rx with all ser- vos plugged directly into the Rx, with per- haps 1 or 2 servo extensions from the Rx to servos in the removable wing(s). There is lit- tle opportunity to improve performance,


Table 1 — Voltage Measurements LOAD (in Amps)


Extension Type None—Vreg only


3″ heavy duty 22 AWG, 2.3 grams 3″ standard 26 AWG, 1.8 grams 18″ heavy duty 22 AWG, 7.5 grams 18″ standard 26 AWG, 5.2 grams


Three 3″ heavy duty 22 AWG, 6.7 grams Three 3″ standard 26 AWG, 5.3 grams


26


0.00 6.51 6.51 6.51 6.51 6.51 6.51 6.51


0.50 6.44 6.42 6.40 6.39 6.36 6.39 6.31


1.00 6.36 6.33 6.30 6.28 6.21 6.25 6.12


2.00 6.20 6.15 6.10 6.05 5.91 6.02 5.72


aside from shortening wiring lengths to the bare minimum to decrease weight and elec- trical resistance. In the instance that either maximum servo efficiency or minimal weight are priorities, the wiring gauge can be decreased to 22 AWG (larger and heavier, but less resistance) or increased to 30 AWG (smaller and lighter, but more resistance). Decreasing wire size should not be done ex- cept in instances where wiring length is short and servo current demands are small (< 0.50 amp peaks). In the interest of quantifying the amount


of voltage drop occurring in commonly used wiring and connectors, I did some simple tests. As a power source, I used a freshly charged 2S Li-Po regulated to 6.5 volts with a Tech Aero Flex Reg PLR5-E. I measured the output voltage at no load and with 0.5-, 1-, and 2-amp loads using a Hangar 9 digi- tal voltmeter. I then added a variety of ser- vo extension leads to evaluate the voltage drop at the different loads. Table 1 shows the results: While the equipment used is not reference


grade or calibrated, the measured values vary only slightly from published values and track relatively cleanly. Table 2 is the result


by dave lockhart You can reach Dave LockhartFLYING MODELS, via e-mail at davel322@comcast.net


VOLTAGE LOSS (at indicated load) 0.50 0.07 0.02 0.04 0.05 0.08 0.05 0.13


1.00 0.15 0.03 0.06 0.08 0.15 0.11 0.24


2.00 0.31 0.05 0.10 0.15 0.29 0.18 0.48


JANUARY 2012


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