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Model Aviation February 2017 "Cessna Type Retracts"
Model Airplane News September 2016 "Final Approach"
Note: There was a typo in this article when it went into print. The third paragraph talks about a voltage drop of 0.1 volt and another voltage drop of 0.02 volt. The print version changed these numbers to fractions of 1/10 and 1/20.
0.02 and 1/20 is NOT the same number... Sorry for any confusion.
Even the most robust radio systems are worthless without a good clean power supply.
Our RC radios require a DC voltage of usually 4.8 to 6.0 volts. This DC voltage as viewed on an oscilloscope would be a straight line, so in this case, a flat line is a good thing.
An oscilloscope is nothing more than a volt meter that displays the voltage with respect to time. In the first photo, I have attached an old but charged 4.8 volt 500 mAh battery to the scope. Notice the grid on the oscilloscope’s screen, we can set the values of the divisions in the vertical direction as a voltage and the horizontal direction is time. The vertical divisions in this example are set to 1 volt/per division and the horizontal divisions are set to 5 ms (milliseconds) per division. We then count up from the bottom line (ground or “0 volt) to the trace. We have about 5 ½ divisions, therefore the scope is displaying about 5 ½ volts and since the trace is a straight line (time is not a factor here), we are measuring a clean DC source.
Now what will happen if we put a load on the battery - make it work. When I attach a small light bulb to the battery we see the trace on the scope drop sightly and therefore the voltage. The load of the light bulb is a constant so the display on the scope may still look like that of the first picture, just a little lower. For comparison, I can also measure the voltage with a digital volt meter which shows 5.42 volts with the light bulb on. This means that the battery under this small load dropped the voltage about 0.1 (one tenth) of a volt. Now I connect the same light bulb to another 4.8 volt battery but with 2700mAh, same voltage but higher capacity. The difference between its open circuit and under load is only 0.02 volts. You may have figured out, the larger capacity battery translates into longer flight times and less of a voltage drop under load.
Now what happens when our load (light bulb) turns on/off? I made an oscillator circuit that does just that, turns the light on/off. We can see all the 0.1 volt fluctuations on the scope, we don’t have a straight line anymore. So what happens when our load is no longer a small light bulb but a servo or several servos operating together?
Photo #4, the scope is set up the same as before except now my load is a receiver with 4 standard servos while I am continually operating the transmitter sticks end to end. All of those voltage fluctuations is dirty power that our radio system has to survive and operate in. Note: there are a couple of voltage dips that drop down to 3.2 and 3.4 volts for about 1 ms (1/5th of a division or 1/1000 of a second). So how low can the voltage go and still operate the radio system?
We have to power two systems in our models, all the servos and the receiver. If we decrease the voltage to the servos we lose torque and speed. Many servos will operate down to 3 volts and the electronics in some servos can operate on sightly less voltage. But the torque at this low voltage may be so low that when connected to a control surface, the surface does not move. However when the voltage is restored, the servo resumes normal operating very quickly.
Our 2.4 receivers on the other hand, have what is referred to as a “brown out” where a low enough voltage drop may actually turn the receiver off for a brief period of time. Then when the voltage is restored, the receiver must relink. While the relink can be very fast, we may be able to feel it when flying. I have tested several receivers with a variable regulated power supply. In this test, I am only looking to see where the receiver itself fails, by observing the receivers output on an oscilloscope - not a servo. I slowly lower the voltage until the receiver no longer operates. While I could publish a chart of where each receiver fails, our advertisers don’t like the comparisons. So I will say that one brand drops out around 3.2 volts and many of the other brands drop out at about 2.7 - 2.5 volts. That being said, a voltage drop that does not fall below 3.3 volts or so is not usually a problem.
What about a higher voltage battery?
If you replace your 4.8 volt battery with a 6 volt battery your receiver will have a bigger cushion to work with. 4.8 volt down to 3.3 volts vs a 6 volts down to 3.3 volts. You will also have more torque and speed with your servos as long as your servos are designed to operate on the higher voltage. But keep in mind as you increase the voltage to any circuit you also increase the current. The increase in current will translate into shorter flight times and more heat (larger voltage drop) if you have resistance in your wiring or connectors.
The biggest reason we have voltage drops is resistance. Resistance in our wires, connectors, switches and even battery packs. The large voltage drops shown in these pictures were caused in part by a high resistance cell in my old 4.8 volt 500 mAh battery, yet the battery looks fine with a voltage check and when cycled. Normally I would not see voltage drops that low with a 500mAh battery operating 4 servos. However, 4 servos operating together is a large load and some voltage drops will occur.
Usually, when electronics fail, the failure is due to a mechanical connection, switch, connector etc. Most of the time we power our RC systems with one battery, one switch harness and one connector into the receiver - a failure here and we are finished. In many cases the battery connector is the same size as one servo connector, yet the battery connector has to feed all the servos. If we have high resistance in our wiring, it is possible that switching from 4.8 volts to 6.0 volts can make the voltage drops worse. Make sure your switch harness is a good quality switch where both poles of the switch are in parallel to make/break the red wire. The connectors need to have good connections and fit tight, not just the plastics but the pins themselves. Your battery pack needs to be adequately sized and in good condition. If you are flying larger airplanes with more than 5-6 servos, you may want to use multiple battery connections to the receiver or even multiple battery packs.
Many ESC’s (Electronic Speed Control) have a BEC (Battery Eliminator Circuit) that allow you to power your receiver/servos from the motor batteries. This is done to save weight and ease of installation, charging etc. Be careful that you use a good quality BEC and you don’t exceed its current output with the size/number of servos.
With larger models, many people opt to use a separate receiver battery instead of the BEC, I would like to suggest that you can use both as a means of redundancy.
I like to use a BEC of 5.5 volts at a minimum of 4 amps. I then install a diode (of at least 4 amps) in the ESC’s servo connector (red wire) with the cathode toward the receiver. This means, due to the voltage drop of the diode the receiver will see just under 5 volts from the BEC at the throttle port. I then install a 5 cell (6 volt) battery with the normal switch harness plugged into the receivers battery port. My 5 cell (6 volt) battery is the primary source of power for the receiver/servos since its voltage is higher than that seen from the BEC. But if my 6 volt battery drops down then the BEC gradually steps in to help with the load. I figure if I have a BEC on board anyway, I will put it to work even if it is just as back up power.
Flying an electric twin engine airplane? You will have 2 - ESC’s and maybe 2 - BEC’s,
usually both ESC’s are plugged into a “Y” harness and then plugged into the receivers throttle port. In the throttle “Y” harness, add a diode (of at least 4 amps) to each leg of the “Y” in the red wire with the cathodes toward the receiver. The two diodes will allow each BEC to power the receiver/servos without the two BEC’s fighting each other. Again the voltage into the receivers throttle port will be about 5 volts. The primary power to the receiver/servos is from the 5 cell 6 volt battery through a standard switch harness to the receiver’s battery port..
NOTE: with either of the above systems, the BEC used must be 5.5 volt and the primary battery must be a good 5 cell 6 volt battery. The servos must be designed to operate on 6 volts.
I have written articles for most of the model magazines for many years. Much of my writing for 15 years, was the “Radio Spectrum” column in RCM magazine - the good ole days. If you have topics or questions you would like me to address, send your ideas here to Jay Smith or email me at email@example.com.
1) Oscilloscope display set to 1 volt per division vertical and 5 ms (millisecond) per division horizontal. The scope is connected to a 4.8 volt 500 mAh battery with no load. The trace is about 5 ½ divisions above my ground or 0 volt line indicating 5 ½ volts.
2) Digital volt meter indicating 5.42 volts from my 4.8 volt 500 mAh battery with the load of a small light bulb. The load of the light bulb dropped the voltage about one tenth of a volt.
3) Oscilloscope displaying some what of a square wave. I have an oscillator circuit turning on/off my small light bulb. The display shows the battery switching from open circuit to loaded where the voltage swings from about 5.5 volts (no load) to 5.4 volts with a load.
4) The oscilloscope is connected to the receiver buss displaying the voltage. The RX battery is 4.8 volt 500 mAh and there are 4 standard servos plugged into the receiver. I am continually operating both transmitter sticks end to end. Note: the voltage drops as all the servos operate back/forth. The left side of the screen shows a voltage dip to 3.2 volts and the center of the screen shows a dip to 3.4 volts.
5) Same as photo #4 except the battery is replaced with a 4.8 volt 2700 mAh. The lowest voltage dip is about 4.5 volts.
6) “Y” harness with diodes, plugged into the throttle port of a receiver. This “Y” harness would be used in a twin engine electric aircraft with two ESC’s. The two BEC’s would then act as backup power to our receiver/servos. Normally the diodes would be insulated in heat shrink tubing. (See text)
I have written many articles over the years for most of the RC Hobby magazines.
I started with "Scale RC Modeler" magazine in the late 1980's for about 2 years, my column was "Electronics Page".
Then "RCM" magazine from about 1990 through 2005, my column was "Radio Spectrum".
Since the close of RCM, I have written articles for "Model Airplane News", "Fly RC" magazine, "RC Report" and "Model Aviation".
While you can find several of these articles on line, I will also reprint some here.
Model Aviation August 2014 "Receiver Voltage"