Avionics Technology
ELECTRONICS
Troubleshooting Tips
The term avionics is often associated with flight deck displays, navigation systems, auto flight systems, and communication devices. In fact the direct and most basic definition is 'electronics as applied to aviation.” Aircraft being designed and produced in this generation are far more dependent on electrical devices than their predecessors. Good, bad, or otherwise the days of the cable and pulley are rapidly giving way to wires and electrons.
Testing and adjustment can often be accomplished without sophisticated equipment.
Contacting an avionics technician to assist in troubleshooting an electronically controlled windshield wiper is not always an option. In many cases a general understanding of the way various control devices operate may enable an aircraft maintenance technician to undertake resolving problems with electrical circuits. Like working with almost anything around aircraft, safety should always be a paramount issue.
Not only can electrical systems cause personal injury; improper handling and troubleshooting methods may also result in costly component failures. When cutting pieces for a sheet metal repair a common practice is to measure twice and cut once. When troubleshooting electrical problems, think twice before testing once. It is also essential to consider the effects of electrostatic discharge (ESD) on any electronic device. In most cases they are voltage and current sensitive. The mild shock felt after a stroll across the floor and contact with a door knob might be catastrophic to a semiconductor.
History and definitions
The word 'electron” comes to us from the Greek language. Back then the term was used when referring to what we today call amber. It was observed even in ancient times that when certain materials contacted amber, interesting properties were exhibited. In later years when atomic theory came to pass, electrons took on a more dynamic role. Atoms consist of a core made up of protons and are considered to be a positive charge. As they may be numerous and all of the same potential a strong bonding agent is needed to keep them all together. Neutrons may be best described as proton glue. The electron carries a negative charge and orbits about the nucleus in structured layers. It is the ability to interact with other molecules, which provides the means of conducting electrical power and the ability to control precise movement, that makes electronic control systems highly desirable.
Systems and components
When designing hydraulic systems, specific flow rates and pressures may be required to achieve a specific operation. Components such as restrictors, valves, one-way check valves, and accumulators may be used to create event sequencing and it is the precise control of fluid flow that allows successful system operation. If we can do it hydraulically, why not electrically?
Two types of materials are used in electrical control systems and they are conductors and insulators. A conductor is a device that will allow electron flow while an insulator will inhibit electron transfer. This is accomplished by using materials with exact atomic compositions where conductors have one to three electrons in their outermost orbit and insulators contain five or more. The fewer number of electrons contained in the outermost or valence orbit, the easier it is for electrons to freely move about. On the other hand a material with five or more electrons does not have as many free spaces or holes to allow electron movement.
Perhaps surprisingly those that design electronic components tend to have a strong background in chemistry to be able to realize the effects of different elements in a control circuit.
Semiconductors and current
Semiconductors provide a means of electron regulation. As the name implies it will allow the flow of electric current only some of the time and may be very specific. These devices include such components as diodes and transistors and there are numerous variations on these basic devices to accomplish exact tasks. Silicon is a material frequently used in semiconductor construction. And, interestingly enough, it contains four electrons in its valence layer.
A discovery that silicon was comprised of two distinct regions was found by the way they allowed current to flow. The area that favored positive flow was named 'P” and the area that favored the negative, 'N.” An understanding was reached about the properties that caused these tendencies in the 'P” and 'N” regions and that these characteristics could be reproduced consistently. With the discovery of the P-N junction and the ability to control its properties, the fundamental groundwork was laid for the invention of the transistor. This discovery which was made by Bell Labs was instrumental in the development of all semiconductor devices to come.
A semiconductor is a unique material with physical properties somewhere in between a conductor like aluminum and an insulator like glass. Examples include germanium, the semiconductor used for the first transistors, and silicon, the basic material in the integrated circuit. Researchers exploited the unique properties of a semiconductor to create the transistor effect. That is, the ability to change its conductivity properties using an electric current.
Diodes are the most basic of the semiconductors and are made up of two electrically different materials. Based on the applied charge or potential the device will either conduct electron flow or restrict it and for that reason is often related to a one-way check valve. These check valves may also be referred to as rectifiers. When they are connected in a particular manner or bridge circuit, they can convert alternating current (AC) into direct current (DC).
There are several types of semiconductor junction diodes:
Zener diodes: Diodes that can be made to conduct 'backwards.” This effect, called Zener Breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes.
Avalanche diodes: Conduct in the reverse direction when the bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes but break down by a different mechanism, the Avalanche Effect. This occurs when the reverse electric field across the P-N junction causes a breaking wave or an avalanche. This leads to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed.
Light emitting diodes (LED): As the electrons cross the junction they emit photons. In most diodes, these are reabsorbed, and are at frequencies that can not be seen (usually infrared). However, with the right materials and geometry, the light becomes visible.
Photodiodes: These have wide, transparent junctions. Photons can push electrons over the junction, causing a current to flow. Photo diodes can be used as solar cells.
SCR or silicon controlled rectifier (SCR): A diode that does not conduct until it is triggered by applying a very small voltage, usually 1 to 20 volts, to its 'gate.” Once triggered it can only be shut off by stopping the electric current flow. SCRs are very useful in the control of alternating current (AC), because they shut off automatically at the moment the current drops to zero and then can be 'triggered” once again as the current begins to rise during the next cycle. They are manufactured in a range of sizes, with power-handling capacities from a few watts to tens of megawatts.
Diodes and troubleshooting
The first diodes were vacuum tube devices (also known as thermionic valves), and were comprised of arrangements of electrodes surrounded by a vacuum within a glass envelope, similar in appearance to incandescent light bulbs. The arrangement of a filament and plate as a diode was invented in 1904 by John Ambrose Fleming, who was at the time a scientific adviser to the Marconi Company. Like light bulbs, vacuum tube diodes have a filament through which current is passed, heating the filament. In its heated state it can now emit electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or just the 'plate.” Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated.
Although vacuum tube diodes are still used for a few specialized applications, most modern diodes are based on semiconductors consisting of an N region adjacent to a P region, creating a P-N junction.
Troubleshooting diode circuits often requires some finesse. If the device is installed, the circuit will impact any testing accomplished using an ohmmeter. Basic testing using an ohmmeter is accomplished by placing the positive meter test lead on the anode and the negative test lead on the cathode. In this condition low resistance should be observed. When the leads are reversed a rise in resistance should be seen. Continuity checks while often used to check the basic functions of a diode to conduct or restrict current flow may not be a valid check of the component's true integrity. Testing this kind of semi-conductor is best done with the unit installed and subject to normal operating conditions.
Transistors
When checking a diode using an ohmmeter the only current the component is controlling is that provided by the battery contained within the meter. This milli-amp flow may give the false impression that the diode is working, whereas when under normal load a breakdown may occur. Checking using a voltmeter or amp meter often provides a more realistic view of the component's true operational characteristics.
Invented at Bell Laboratories in 1947, the transistor resulted from efforts to find a better amplifier and a replacement for mechanical relays. The vacuum tube had amplified music and voice during the first half of the 20th century, and it made long-distance calling practical. But it consumed lots of power, operated hot, and burned out rapidly. The telephone network required hundreds of thousands of relays to connect circuits together to complete calls. Network relays were mechanical devices, requiring regular maintenance. Cheaper to make than the vacuum tube and far more reliable, the transistor cut the cost and improved the quality of phone service and, seemingly overnight, spawned countless new products and whole new industries.
The transistor has many applications, but only two basic functions: switching and modulation which are used to achieve amplification. In the simplest sense, the transistor works like the lamp dimmer. Push the knob of the dimmer, the light comes on; push it again, the light goes out, just like a switch. Rotate the knob back and forth, and the light grows brighter or dimmer, thus a modulator or amplifier.
Different arrangement of transistors can create arithmetic and logic processors.
We often find these logic gates used to provide control or monitoring of complex aircraft systems. For example, the sequencing of movable landing gear and gear doors that during operation will occupy the same space. By using logic gates to determine the position of various moving parts damaging collisions can be avoided by controlling the order of operation.
Proximity switches are used by many airframe manufacturers to replace their mechanical counterparts. These electronic controllers require power to do their job. Contrary to popular thought, proximity switches are not magnetic. Instead they use an inductive material such as aluminum to act as a core for an internal transformer. The principle is not unlike a metal detector. Once the target is present an internal transistor is either turned on or off signaling the circuit to which it is attached. Once again an ohmmeter is not an effective tool in diagnosing problems with this type device.
Like with a diode, fault diagnostics of transistorized circuits is best accomplished with the circuit operating and utilizing test equipment to monitor voltage or current flow.
Semiconductors have limitations by design. Diodes for example include a peak inverse voltage (PIV) rating that describes the maximum voltage they can successfully block without damage. In addition there is a maximum current rating for both momentary surges and continuous operation.
The age of electronics has drawn a very fine line between where avionics stops and airframe or engines begin. In modern aircraft of today the system that controls the flow of oxygen to passenger masks may in fact utilize a microprocessor.
Just think this whole thing started with the transistor radio back in the 1950s. I think I'll go listen to some rock and roll.
