Dangers of Blown Diodes
- A shorted diode rectifier in a power supply can present a potential shock hazard to a technician. It allows the full line voltage into the power supply. A technician may expect to find 50 to 60 volts of pulsing DC but instead encounters 115 volts of AC. Other components are likely to fail due to the AC voltage, with electrolytic capacitors the most common failures. Swelled capacitor cases and charring may be evident. Old-time television technicians kept one hand in a pocket to minimize the chance of electrical shock. When working around high voltage, it's a good habit to cultivate.
- Some electronic devices use diodes to block DC voltages. For instance, an analog signal input to an aircraft computer has a diode in line with an op-amp. The op-amp input has a resistor tied to a 28-volt pull-up supply and a capacitor between the resistor and ground. The op-amp input is normally high, but when a low voltage appears on the diode, it conducts and pulls the op-amp low. If the diode shorts, 28 volts goes out on the input line, causing systemwide havoc.
- Glass enclosed diodes will occasionally fail in a way that produces intermittent operation. This can make the unit unreliable. Vibration or thermal stress cracks the glass around the diode, making the mechanical connection with the device subject to intermittent contact. Careful visual inspection under magnification will often reveal a crack in the glass, while a cursory inspection with the naked eye is not sufficient.
- Light emitting diodes (LEDs) are commonly used to indicate a computer's status. In the avionics bay of an airliner, the front panel LEDs may be the only indication of the computer's internal operation. A green LED signals OK while a red one indicates a failure. The problem is that the LEDs are mounted directly into the front panel, making them susceptible to mechanical shock and electrostatic discharge (ESD). Maintenance technicians should always take precautions to prevent ESD damage by wearing proper grounding straps. A failed red LED may convince the technician that a unit is working properly when the opposite is true.
