Understanding RC Electronics
The growth in the number of electrically powered radio controlled model aircraft being purchased is quite amazing. Most newcomers to electric rc model flying have virtually no knowledge of electrics to back up their choice. This can lead to frustrations when things start to go wrong and components cause problems.
I believe that understanding rc electronics in its simplest form is important for your success in flying electric rc model planes. To this end I am going to give you some basics to help you along the road to your success.
We can measure electricity in a number of different ways, but there are certain units of measurement that are particularly important.
The voltage is a kind of electrical force that makes electricity move through a wire and we measure it in volts. The bigger the voltage, the more current will tend to flow. So a 12-volt car battery will generally produce more current than a 1.5-volt flashlight battery.
Voltage does not, itself, go anywhere: it’s quite wrong to talk about voltage “flowing through” things. What moves through the wire in a circuit is electrical current: a steady flow of electrons, measured in amperes (or amps).
Together, voltage and current give you electrical power. The bigger the voltage and the bigger the current, the more electrical power you have. We measure electric power in units called watts.
The electric power in a circuit is equal to the voltage × the current (in other words: watts = volts × amps). So if you have a 100 watt (100 W) light and you know your electricity supply is rated as 120 volts (typical household voltage in the United States), the current flowing must be 100/120 = 0.8 amps.
If you’re in Europe, your household voltage is more likely 230 volts. So if you use the same 100-watt light, the current flowing is 100/230 = 0.4 amps.
The light burns just as brightly in both countries and uses the same amount of power in each case; in Europe it uses a higher voltage and lower current; in the States, a lower voltage and higher current.
Power is a measurement of how much energy you’re using each second. To find out the total amount of energy an electric appliance uses, you have to multiply the power it uses per second by the total number of seconds you use it for.
The result you get is measured in units of power × time, often converted into a standard unit called the kilowatt hour (kWh).
If you used an electric toaster rated at 1000 watts (1 kilowatt) for a whole hour, you’d use 1 kilowatt hour of energy; you’d use the same amount of energy burning a 2000 watt toaster for 0.5 hours or a 100-watt lamp for 10 hours. See how it works?
The Difference Between Electrics and Electronics
Now we need to differentiate between Electrics and Electronics.
Electronics is the technology of electrical circuits that are made up from active electrical components such as transistors, diodes and integrated circuits.
It is distinct from electrical technology which deals with the generation, distribution, switching and conversion of electrical energy into other forms (e.g. light and motion) using wires, motors, generators, batteries, switches, relays, transformers, resistors and other passive components.
Electronics is the knowledge of how individual components work and how to assemble them together to make a working device, while electrical technology is the knowledge of how to connect together different devices to turn electrical energy into useful stuff like heat, light and motion.
This means that anyone with a basic knowledge of ‘model electrics’ can install the necessary circuitry to make a model plane do what he (or she) wants it to do without actually understanding the clever workings of the electronic bits in the circuit.
Our radio control electrical circuit involves a power supply, conductive wiring, a load and a switch. The circuit is dead until either the switch is made or the power supply is connected directly to the circuit, at which stage a current of electricity flows through the wiring of the circuit and energises the load.
The load does whatever it does for as long as the power is supplied and can be anything that consumes electrical energy such as a receiver and servos or a receiver, servos, ESC and motor. When the switch is returned to its original position or the power supply is disconnected, the circuit becomes dead again and the load stops whatever it was doing.
What is a Circuit Schematic Diagram?
A circuit diagram, or a schematic diagram, is a technical drawing of how the various components of the system are connected together to achieve the end result.
Each electric component is drawn connected to the next by a line or lines representing the conductive wires that link the installation together as in the diagram on the right here.
Let’s now deal with the individual components of our circuit installation one at a time:
This is our power source and is essentially a store of electrical energy. It has two poles, or terminals, which are termed negative and positive.
The battery has chemicals inside it which react to produce little particles of electrical charge called ‘free electrons’ and drive them all toward the negative pole. This is insulated internally from the positive pole, which has a deficiency of electrons and whose sole function in life is to grab them from the other end of the battery.
If a metallic conductor, in the form of a thin metallic rod called a wire or a bundle of very thin rods called a cable, is connected between the two poles then the electrons will flow along it from negative to positive. This flow of electrons is called a current.
All batteries are identified by two very important properties. The voltage it holds puts a value on the electrical ‘pressure’ exerted between the negative and positive poles, i.e. the higher the voltage then the more electrical pressure it can exert on a load.
Increasing the voltage will, for example, makes a bulb glow brighter or a motor turn faster. The battery also has a ‘capacity’ which is a measure of how much electrical energy it holds and can supply before it becomes discharged, or ‘flat’.
The rate of flow of electrical current along the conductor is measured in Amps or, to give it its full name, Amperes.
The more amps a load draws from the battery then the quicker it will discharge.
The value of a battery’s capacity is the arithmetical product of the current and the amount of time for which it can be supplied and is quoted in Amp-Hours (AH), or Milliamp-Hours (mAH) which are 1000 times smaller.
A battery which can supply one amp for one hour has a capacity of one amp-hour. To give a figure more typical of an rc model plane application, a 3000mAh (3AH) battery will supply a current of 2 Amps for 1.5 hours (1.5 x 2 = 3).
I hope you will have noted that this does not depend upon the voltage of the battery. This is a different thing altogether.
There is an arithmetical relationship between voltage and current in a circuit and it’s called Ohm’s Law and states that the voltage across a load divided by the current flowing through it is called its resistance and its formula is:
R=V ÷ I.
Also by re-arranging the formula in line with the triangle:
V = I x R and I = V ÷ R
R = Resistance, V = Voltage and I is the usual symbol for current. For most practical purposes you won’t need Ohm’s Law on a day to day basis but it’s useful to be aware that it exists, if only to know what ‘resistance’ means.
Batteries For RC Model Aircraft
Basically there are two types of battery in common use for rc airborne applications. These are Nickel Metal Hydride (NiMH) types and Lythium-Polymer (LiPo) types.
Nickel Metal-Hydride (NiMH) batteries are generally made up in packs of end-to-end joined-up cells. This arrangement is called ‘series’ connected.
Each cell has a nominal voltage of 1.2 volts and the cells are available in different case sizes. The smaller case sizes have the smallest capacity; typically around 850mAH for an AAA pack, while the largest cells, ‘F’ size, go up to 10000 mAH (or 10AH).
NiMH cells can be fast-charged at the field from a portable charger. When fully charged, a NiMH cell can have as much as 1.55 volts across the poles. This will quickly settle down to its ‘nominal voltage’ of around 1.2v and stay at that level until it is almost fully discharged where the voltage starts to drop quickly and should not be allowed to go below 1v per cell or damage will occur.
Lithium Polymer (LiPo) batteries are the latest technolgy and are lighter and more powerful even than NiMH cells. Their nominal voltage is 3.7v per cell, BUT they do have to be carefully handled and monitored. Careless handling or overcharging/discharging can in the extreme case, cause them to catch fire and even destroy your model.
They have become the main choice of power source for electric flight. Treated with care and respect, they deliver excellent power to weight ratio so long as their sensitivity to over-discharge problems is taken account of.
For this reason it’s always advisable to use a speed controller which will monitor the battery voltage and cut off the power to the motor before it reaches a critically low value.
That said, LiPo batteries are in use all over the world and instances of such accidents are becoming rare.
It is essential that you purchase the correct type of balancing charger and follow the instructions, then you will have no problems.
LiPo packs come in multiples of 3.7v and are quoted in the form ‘XS’ where X is the number of cells. Thus a ‘3S’ pack is 3 x 3.7 = 11.1 volts.
The capacity is quoted in mAHs, and the maximum current which can safely be delivered is given in the form ‘YC’, where Y is the value of the capacity in mAH/ 1000 = AH
As an example, a 1700 mAH LiPo pack has a corresponding ‘C’ value of 1.7 and so a pack rated at 20C can supply a maximum of 20 x 1.7 Amps = 34 Amps.
Previously I described how there is a flow of electric current when a load is connected between the two poles of a battery. Unfortunately some loads are sensitive to which way around they are connected (polarised) while others aren’t.
Examples of non-polarised loads are conventional bulbs, relays, switch terminals and fuses.
Practically any unit which includes semiconductors (transistors, Integrated circuits, etc.) will be polarised, so make sure that you connect items such as speed controllers and receivers the right way around. They are usually marked with + (positive) and – (negative) signs/labels or at least the instructions will tell you how to connect them.
Be warned, if you connect up a polarity-sensitive device the wrong way round, even for a split second, then you will probably damage it and often fatally.
All wiring used to connect the various components of our rc and power system are forms of conductor and usually of the insulated flexible variety. This enables complicated circuits to be accommodated inside our fuselages and wings with lots of formers, ribs and electronic gizmos to be negotiated.
A wire, or cable, is made up of a central conductive core, usually multiple strands of thin copper wire, and an insulating outer sleeve.
The sleeve is typically either PVC in the case of the rc linkages or a silicone-based compound, which is more flexible and resistant to heat for the motor power circuitry.
The most crucial factor as far as wiring is concerned is to use the right thickness/diameter. Too high a current passing through a thin cable will increase its temperature to the point where the insulation breaks down and melts and the conductor inside can short out against other ‘live’ components.
Circuits in rc planes are typically of two types; those which provide power to motors and those which don’t. The former will be subject to high currents while the latter will only carry a few amps at most.
The accepted standard is to use thick multi-strand silicone-coated cable for power wiring, and thinner multi-strand PVC ‘hook-up’ wire for such items as receivers and servos. Most products bought ‘over the counter’ will come fitted with the appropriate gauge and type of wire and often with the appropriate connector attached.
The gauge of the silicone covered wire is usually quoted in AWG (American Wire Gauge) and the following table will be useful when working out what gauge of wire you need:
AWG Conductor dia.(mm) Max current (Amps)
10 2.59 55
12 2.05 41
14 1.63 32
16 1.29 22
18 1.02 16
20 0.81 11
22 0.66 7
I know it may seem obvious but as we are discussing all of the major components in the system, we should also include these items.
Switches are devices usually operated manually and used to turn electrical circuits on and off.
In its simplest form, a switch has a lever which moves a conducting piece of metal into a position where it makes contact with another fixed conductor, thus bridging the two and allowing an electrical current to flow between them.
Each of these conductors is called a terminal (or pole) and is connected to the wires within the circuit, thus controlling the current in that circuit,
slide switches are usually fitted by radio manufacturers in the wiring harnesses that they supply to connect receivers to battery packs.
Series versus Parallel Connection and Loads
At this point I will have to explain those confusing terms parallel and series. There are two very simple rules here.
Loads connected ‘in parallel’ go alongside each other in side-by-side (parallel) cables which are then connected at each end (see diagram right). The voltage across each load will equal the total voltage across the circuit, Figure 6.
Loads connected ‘in series’ are connected one after the other along the same cable. The voltages across each load will add up to the total voltage across the circuit.
The terms ‘series’ and ‘parallel’ are also applied to connecting batteries, but beware! While you can connect two batteries with different voltages and capacities in series, you must NEVER connect two batteries of a different voltage in parallel or one will discharge into the other, with potentially serious damage to both batteries and possibly the model.
The total voltage of a series pair of batteries will be the total of the two battery voltages and the capacity will be that of the individual batteries.
The voltage of a parallel pair will be the same as the two individual batteries and the capacity of a parallel pair will be twice that of the individual batteries.
Be sure to always make sure that the two batteries in the pair are the same type (e.g. NiMH or LiPo) and the same capacity (e.g. 3000mAh), irrespective of whether you are wiring them in series or parallel.
If you are using two battery packs of the same voltage and capacity to power a plane, you should disconnect pairs of batteries from each other when charging. Charge them separately as there’s always the risk that one will charge at a different rate and that one will become the weaker of the two if you charge them together as a pair.
Over recent years there has been a major revolution in model motor technology with the advent of brushless motors. As the name suggests, these don’t rely on fixed brushes to transmit the power to the central armature. Rather they are made ‘inside out’ with the wire coils fixed around the inside of the motor casing and a permanent magnet rotating the shaft inside them.
The power to energise the coils is transmitted by some very clever, fast-switching electronics outside the motor case. Because there are no brushes, these motors are much more efficient than their brushed cousins, so a small and light brushless motor will do the same job as a bigger, heavier brushed one.
To learn more about brushless motors visit my main website page on Electric Motors where everything to do with these motors is explained in detail.
Electronic Speed controllers
There are two distinct parts to the inside of an ESC; the logic circuitry and the power circuitry.
The logic components are the ones which are connected to the receiver via the 3-wire lead with a plug on the end. Its operating voltage is dictated by the receiver and is generally of the order of 4.8v.
The purpose of the logic circuitry is to detect and decode the signal coming from the receiver and to switch the high-speed, high-current semiconductors in the power circuit which control the speed and direction of the motor.
The ESC manufacturer will state in the technical information the range of main motor battery voltages that the ESC is designed to handle. They should also state a value for the maximum motor current (in Amps) which the ESC will handle under continuous operation.
You should be guided by those two values when choosing your ESC, after first ascertaining the working voltage of the motor and its current consumption under load as described earlier.
I’ve emphasised the words ‘under continuous operation’, because the current rating often causes confusion . When you switch on an ESC and bang open the throttle there will be a sudden inrush of current from the battery to the motor to get it spinning. This will always be a larger value than that consumed when the motor is running at full speed, i.e. continuously.
Modern switching semiconductors therefore have two current rating values; one for continuous and one for ‘inrush’ or pulsed currents. The value we should be concerned with is the continuous current rating.
My rule of thumb is to allow a 30% margin over and above the motor manufacturers recommended maximum continuous current rating. E.g. If the maximum recommended current for a particular motor is stated as 30Amps I would select an ESC that has a continuous current rating of at least 40Amps.
Always buy a known unit from a reliable source with a good reputation and a clearly stated service/warranty policy. Incidentally it’s quite okay to run a low-current motor on an ESC which has a much higher rating, but not the other way around.
The power circuit connections of an ESC will always comprise a pair of thick battery cables. These are usually made in red and black, for positive and negative connections respectively. Always ensure they are correctly connected (+ to + and – to -)
Brushless motor ESC’s have three motor wires. It doesn’t matter which way round you connect the motor wires to the motor unlike the battery connections which should NEVER be reversed.
If you wish to reverse the direction in which the motor is rotating then just swap over any two of the three wires from the ESC to the motor.
Most standard ESCs for brushless motors will have been set by the manufacturer to a pre-determined operating mode and, when you switch on, will ‘Auto-set’ themselves.
If changes to operating modes need to be made, these can be done from your transmitter following the instructions supplied with the ESC or you may require a separate programming card to make changes to their working parameters.
Many ESCs contains a battery eliminator circuit (BEC) If it does, then you must NOT connect an additional power supply to the receiver.
To learn more about ESCs and BECs go to my main website page about ESCs & BEC circuits.
In this post we have covered just about everything you need to know about the control and power system of your electric rc airplane. If you familiarise yourself with the content you will have the basis of understanding rc electronics. There should be very little else you will require other than the information provided by the product manufacturers.
If you have found this post helpful why not visit my main website www.rookiercflyer.com for all the help you need to become a competent flyer. Any comments will be appreciated or I will be happy to answer any questions you want to ask via the comment facility below.
Happy New year everyone.