It's Still an Analog World
Using basic electrical theory to troubleshoot digital component problems
By Jim Sparks
Digital technology in today’s world continues to advance with leaps and bounds. Computers are becoming more powerful and even more self-reliant – even for such things as self-diagnostics. As dominant as these digit-dealing devices have become, they are still incapable of doing any serious work without assistance from the analog world. In other words, what good is a computer unless it is plugged into a power source or its contained internal energy supply is depleted? In fact, analog devices obtain much of the information used by aircraft computers.
I was recently conducting a class on the use and maintenance accessibility of a specific Integrated Digital Avionics system for a small group of technicians. The experience level of this assembly was from 15 to 25 years and included specialists on avionics, engines and airframes. About halfway through the discussion, one of the participants made the comment, "I wish I knew more about basic electricity." This prompted me to do some thinking. Could it be that there are technicians in the field who are not proficient in the ways and means of those electrical greats such as George Simon Ohm, Michael Faraday (Farad), Gustav Kirchoff or even James Watt? Once my mind came back to the real world, I asked the group, "What is Ohm’s Law?" Almost everyone responded knowingly, "E equals I times R." I then asked, "Well, what does that mean?" The answer: "Voltage equals Current times Resistance." Next question: "What does that really mean in terms of something I can apply to an electrical problem?" This time, there was no response — only silence. During these few seconds of quiet, it occurred to me that possessing the knowledge to operate even the most advanced computers is essentially worthless with respect to troubleshooting without understanding the basic ins and outs. All electronic equipment requires operating power and an interruption of electrical flow or a significant reduction in voltage may cause a loss of data or even a complete shutdown.
Start circuit card. Metal rectangular boxes are relays energized by computer drives.
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.
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