[Headline: Receiver Voltage]
[Subhead: Even the most robust radio systems are worthless without a good, clean power supply]
[Author: Cal Orr]
[Photo credit: Cal Orr and Jay Smith]
I once asked, “Why do we have to take all these pills? Make one pill that takes care of all our ailments.” The reply was, “Okay, then you make a black box that will benefit any electronic circuit.”
A good, clean power supply is important. Our RC radios typically require a DC voltage of 4.8 to 6.0 volts. On an oscilloscope, this DC voltage would appear as a straight line, so in this case, a flat line is a good thing. An oscilloscope is a voltage meter that displays the voltage with respect to time.
In the first photo I have included with this article, 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 as time.
The vertical divisions in this example are set to 1 volt per division and the horizontal divisions are set to 5 milliseconds per division. Count up from the bottom line (ground or 0 volt) to the trace. There are approximately 51/2 divisions so the scope is displaying roughly 51/2 volts. Because the trace is a straight line (time is not a factor here), we are measuring a clean DC source.
What will happen if a load is put on the battery? When I attach a small lightbulb to the battery, the trace on the scope slightly drops and so does the voltage.
The load of the lightbulb is a constant so the display on the scope may still look as it does in the first picture, but slightly lower. The voltage can also be measured with a digital voltage meter which shows 5.42 volts with the lightbulb on. This means that the battery under this small load dropped the voltage approximately 1/10 of a volt.
I connect the same lightbulb to another 4.8 volt battery, but with 2,700 mAh. It has the same voltage but a higher capacity. The difference between its open circuit and under load is 2/10 of a volt. You may have figured out that the larger capacity battery translates into longer flight times and less of a voltage drop under load.
What happens when the load (lightbulb) turns on or off? I made an oscillator circuit that only turns the light on or off. All of the 1/10 of a volt fluctuations are visible on the scope. There is no longer a straight line.
What happens when the load is no longer a small lightbulb but a servo or several servos operating together? In photo #4, the scope is set up the same as before except now the load is a receiver with four standard servos that are continually being operated with the transmitter sticks end to end. All of those voltage fluctuations are dirty power that the radio system has to survive and operate in.
Note: There are a couple of voltage dips down to 3.2 and 3.4 volts for about 1 millisecond (1/5 of a division or 1/1000 of a second). How low can the voltage go and still operate the radio system?
Two systems must be powered in models—all of the servos and the receiver. If the voltage to the servos is decreased, we lose torque and speed. Many servos can function down to 3 volts, and the electronics in some servos can operate on slightly less voltage.
The torque at this low voltage may be so low that when connected to a control surface, the surface does not move. When the voltage is restored, however, the servo quickly resumes normal operations.
Our 2.4 GHz receivers have what is referred to as a “brown out,” in which a voltage drop may briefly turn the receiver off. When the voltage is restored, the receiver must relink.
Although the relink can be quick, we may feel it when flying. I have tested several receivers with a variable regulated power supply. In this test, I am only trying to determine where the receiver fails by observing its output on an oscilloscope.
I slowly lowered the voltage until the receiver no longer operated. One brand dropped out at roughly 3.2 volts and many of the others dropped out at approximately 2.7-2.5 volts. A voltage drop that does not fall below 3.3 volts typically is not a problem.
What about a higher-voltage battery? If a 4.8-volt battery is replaced with a 6-volt battery, the receiver will have a bigger cushion with which to work (4.8 volts down to 3.3 volts vs. 6 volts down to 3.3 volts). There also will be more torque and speed with the servos as long as they are designed to operate on the higher voltage.
Keep in mind that as the voltage to any circuit is increased, the current also rises. The increase in current translates into shorter flight times and more heat (larger voltage drops) if there is resistance in the wiring or connectors.
The biggest reason for voltage drops is resistance, which is in wires, connectors, switches, and even battery packs. The large voltage drops shown in the pictures with this article were caused, in part, by a high-resistance cell in my old, 4.8-volt, 500 mAh battery, yet the battery looks fine during a voltage check and when cycled.
I typically would not see voltage drops that low with a 500 mAh battery operating four servos, but four servos operating together is a large load and some drops in voltage will occur.
When electronics fail, the failure is typically because of a mechanical connection, switch, connector, etc. Most of the time, our RC systems are powered by 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 of the servos. If there is high resistance in the wiring, switching from 4.8 volts to 6.0 volts could make the voltage drops worse.
Make sure that the switch harness is of good quality and that both poles of the switch are in parallel to make or break the connection to the red wire.
The connectors need to have good connections and a tight fit—both the plastics and the pins. Your battery pack needs to be an adequate size and in good condition. If you are flying larger airplanes with more than five or six servos, you may want to use multiple battery connections to the receiver or even multiple battery packs.
Many ESCs have a BEC that allows you to power your receiver or servos from the motor battery. This saves weight and makes installation and charging easier. Make sure to use a good-quality BEC and don’t exceed its current output with the size or number of servos.
With larger models, many people opt to use a separate receiver battery instead of the BEC. You can use both. I prefer 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. Because of the diode’s voltage drop, the receiver will receive slightly less than 5 volts from the BEC at the throttle port.
I then install a five-cell (6-volt) battery with the normal switch harness plugged into the receiver battery port. My five-cell battery is the primary source of power for the receiver/servos because its voltage is higher than that from the BEC. If my 6-volt battery drops down, the BEC gradually steps in to help with the load. I figure that if I have a BEC onboard, I should put it to work, even if it is intended for backup power.
Are you flying an electric twin-engine airplane? You will have two ESCs and maybe two BECs. Both ESCs are typically plugged into a Y harness and then plugged into the receiver’s 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 allow each BEC to power the receiver/servos without the two BECs fighting each other.
The voltage into the receiver’s throttle port will be roughly 5 volts. The primary power to the receiver/servos is from the five-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 volts and the primary battery must be a good five-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 the past 15 years was the “Radio Spectrum” column in RCM magazine. If you have topics or questions that you would like me to address, send your ideas to MA Editor-in-Chief Jay Smith or email me.[dingbat]