It's Still an Analog World
Using basic electrical theory to troubleshoot digital component problems
By Jim Sparks
The answer to the Ohm’s law question that I was looking for was one that addressed the voltage consumed by a device is based on its resistance to electrical flow multiplied by the amount of current flow. Really, Ohm’s law is only one of several principles that are the foundation for understanding, and more importantly, successfully troubleshooting, electrical problems. Probably the biggest drawback in understanding is that electrons are invisible and it is hard to figure out something you can’t see. Plus, there are two contradicting theories on how electricity flows. The key word in the above statements is FLOW. Electrical current flows, rivers flow, hydraulic fluids flow and even sometimes, beer flows.
Flow is flow
The truth is flow is flow no matter what the substance and for the most part, the rules are the same. In fact, Kirchoff’s Law for current simply stated says, "... any electrical current that flows into a circuit, no matter how complex, must also flow out of the circuit." Obviously, if what flows in does not all depart at the normal exit, some must have gotten out elsewhere. This can be applied to even the most basic hydraulic circuit. An example would be a pump capable of moving 10 gallons per minute would deliver fluid to a motor through a two-inch pipe. The motor is rated to operate at a specific RPM at the 10 gallons per minute flow rate. If a two-inch "Y" pipe is connected at the outlet of the motor to send fluid back to the reservoir with each of the branches being a one-inch pipe, the motor could still operate at normal speed. When one of the two "Y" branches is obstructed, then the motor will only turn at half speed, and flow through the entire circuit is constant. Should a restriction to current flow be encountered in any electrical connection, current flow through that entire circuit will be affected.
The Kirchoff law also applies to circuit potential. This means that whatever source voltage is applied to, a circuit has to be consumed prior to getting back to the source.
This too can be visualized using a hydraulic pump that delivers fluid to a motor. The return line of the first motor is connected to the input of a second, identical motor whose outlet goes to the system reservoir. Now when the pump runs, the pressure drop across each motor is equal and when they are added, the sum equals the delivery pressure of the pump. What this means in the electrical world is resistance in any line carrying electricity will result in a voltage drop across the resistance. This voltage drop translates into less power available to the primary components in the circuit.
So, what is it exactly that computers look for from the outside world? Variable voltage, changing current flow, and frequency are all methods of communication.
The area of operation will often dictate the nature or types of sensors used by specific computers. For example, an electronic cabin temperature controller will often use temperature-sensitive resistors (thermistors) in conjunction with a temperature selector potentiometer. The programming of this type computer will observe different voltage levels based on selected temperature versus actual temperature provided by a specific voltage drop across a thermistor strategically located with in the aircraft cabin. This type device can supply troubleshooters with some unique challenges. For example, some materials used to manufacture thermistors possess a positive coefficient to temperature change while others have a negative coefficient. What this means is some sensors will see an increase in resistance with an increase in temperature (positive coefficient) while others will see a decrease in resistance with an increase in temperature. (Negative coefficient). Typically, these probes are housed in a metal case with the sensor and necessary wiring well-insulated from contact with the housing. In some situations, exposure to extreme temperatures or even as a result of age, insulation resistance may begin to deteriorate. This weakening may even be aggravated by vibration. Thus, the result may be some of the control current flowing through the sensor escaping to surrounding electrically conductive panels. So, the signal being monitored by the computer is now corrupted. If the damage is significant, the processor may identify a hard fault or a component operating out of assigned limits. The possibility also exists where the insulation breakdown may cause deterioration of the monitored signal and ultimately cause the computer to issue a command that is in complete contrast to what should actually occur. If one of the connections of this probe become corroded, the additional resistance will also have an affect on the controlling device. Hence, the application of the Kirchoff principles will lead to a rapid solution. By testing the output voltage from the computer to the respective sensor, and then testing the voltage consumed by the thermistor and comparing the two will often lead to a detectable discrepancy. Monitoring voltage on the downstream wire from the sensor may even help to narrow down the possible culprits.
Many information processors will use multiple temperature sensors and will often compare them. A common tool for comparing resistive components is the Wheatstone Bridge. The principle is similar to a river that divides into two identical branches and then joins back together somewhere downstream. Should an obstruction (resistance to flow) occur in either branch flow in the other will increase so that the sum of the flow in the two channels is equivalent to the total flow in the river. A true bridge, in fact, has four sides. Any change in resistance in any one section will have a noticeable impact on all three remaining sections.
Methods for transmitting signals
Monopoles, Permanent Magnet Alternators (PMA), and tachometer generators are all methods for transmitting velocity signals to computers. The unique thing about these devices is that they are all capable of producing electrical signals with no external electric stimulation. More importantly, a signal that consists of pulses can be laid over a timeline.
Monopoles – The monopole is a simple, yet reliable, device consisting of a permanent magnet surrounded by a coil. When a ferrous material comes within close range of one end of the magnet, the flux field will collapse across the coil. This results in a pulsed direct current output. In many cases, this type sensor is placed in close proximity of a gear. Subsequent rotation will cause one DC pulse per gear tooth movement past the magnet. By knowing how many teeth are used in the gear, the device that senses the monopole can calculate exact speed of the gear. Strength of the magnet along with the number of windings in the coil will determine the strength or amplitude of the signal and often, this strength is almost as important as the pulse so the receiving computer has a means of validating the pulses. What could possibly go wrong with a component as simple as this? With the coil, there are really two options; short and open. Often, specifications will exist spelling out values such as coil resistance, insulation value and even the normal value of the operating voltage. Frequently, failures may be the result of vibration or even heat and both factors should be considered when troubleshooting. Deterioration of the magnet is another possibility. This may also be the result of heat or possible mechanical shock associated with dropping or otherwise mishandling the monopole. Another consideration is mechanical contact while installed. In fact, if any device containing a permanent magnet is allowed prolonged contact with a ferrous material (sitting on top of a metal toolbox) the magnetic properties will diminish, plus, exposure to a coil with alternating current is a known method of degaussing (demagnetizing).
Permanent Magnet Alternator – A Permanent Magnet Alternator works on a principle similar to a monopole, but with one distinguishable difference. In this case there is movement between the coil and magnet resulting in both a positive and a negative pulse per one complete revolution of the magnet. Thus, a PMA produces an AC output.
Tachometer Generators – Tachometer Generators employ the same principle as a PMA, except they use three independent signals in unison. When troubleshooting pulse or AC generators used as computer sensors, it needs to be understood that just like a ripple in a pond, the further the wave travels, the risk increases for signal loss. The wiring can even be a factor in deterioration. Should a shielded wire be used and the insulation becomes compressed by excessive clamping pressure, significant signal loss can occur.
If a computer must accomplish a task involving, perhaps commanding, a valve to open or close, it once again takes the order from the digital realm and must somehow convey it to the motor on the valve. This is a job for a driver. As computers like to be in full control even when not commanding devices to move, the circuit powered by the driver may be monitored by a small constant current flow. This is something else that can be checked using basic analog test equipment such as a voltmeter and maybe a breakout box.
Specific troubleshooting techniques are often needed when isolating computer related sensors. Many times, these devices require significant forethought prior to tackling the problem.
Think like a computer
Often, when computer systems report discrepancies, the diagnosis is based only on specific observation, such as total loss of signal. When troubleshooting a circuit based on a computer diagnostic, it becomes necessary for the technician to think like the computer. In effect, think: "How does the computer know that this is a failed component?" Once this question can be answered, routine test equipment can be employed to resolve the issue.
A computer lacking operating power in an aircraft serves the same purpose as ballast. In fact, the context of that last statement has genuine meaning for me as I am currently riding in an airliner at 37,000 feet and the battery light on my laptop computer has been flashing for the past 20 minutes. I sure hope it doesn’t go de——