Even though a two-cycle power outage can create havoc with electronic equipment, a three-cycle outage sometimes can cause no problems at all.
To many of us, the utility power grid is a vast system of unknowns. Its performance can make unprotected electronic equipment useless. Why? Because grid voltage values, higher or lower than guaranteed nominal values, have an effect on electronic intelligence processing equipment. Incoming grid power “sees” the equipment’s DC power supply, which bears the brunt of any AC grid voltage variation.
The impact of voltage sag. We can hardly assume our electronic hardware operates from a distribution network with zero internal impedance, receives a pure undistorted sine wave, and never “sees” line voltage variations of 55% from nominal. Yet that’s exactly what electronic system manufacturers think when they design their power supplies.
The combination of utility- and locally generated disturbances results in no such modest limits. Utilities are permitted line voltage reductions (brownouts) to cope with seasonal demands. Large motors accelerating high inertia loads, spot welding, and other loads act to further drop the voltage level delivered to our power supplies.
Computer shutdowns and sag-induced logic errors aren’t the only problems. Damage to the DC power supply is a greater danger. Reduced input voltage can cause excessive power supply heat dissipation, resulting in short equipment life.
Why this overheating? While trying to maintain constant DC output as the line voltage declines, the DC-to-DC converter circuit has to draw from the reservoir capacitor. With line voltage reduced, this capacitor experiences deep discharges between the twice-per-cycle charging periods.
Now, electrolytic capacitors aren’t designed for deep discharge. And, they’re not designed for the resulting large terminal variations. So, the excessive capacitor charge and discharge currents cause internal heat dissipation, which produces dielectric stress. This condition results in reduced mean time between failure (MTBF).
Also, rectifiers and DC-to-DC converter switching transistors draw high-peak currents, which raise their junction temperatures. These temperature excursions take a toll on semiconductor longevity.
The impact of overvoltage, surges, RFI, and harmonics. Short-term voltage surges (10% beyond nominal) aren’t usually harmful. However, higher input voltages can overwhelm the voltage regulating ability. The result is damaging voltage levels fed to the electronic circuits.
High input voltage can also puncture a power supply’s rectifier and switching transistor junctions, causing MTBF reduction and eventual breakdown. High-voltage transients lasting microseconds can permanently wreck the power supply and its electronic equipment load.
Digital logic circuits that define 0s by voltages in the OV-to-0.5V range and 1s by 4.5V-to-5V levels are highly susceptible to inductive “kicks” directly impressed on their 5VDC power supply. The power supply’s reservoir capacitors don’t absorb transient energy, because their wiring inductance (negligible at 60 Hz) introduces isolating impedance at the MHz-equivalent frequencies of fast-rise transients. As a result, transient energy follows the line of least resistance, which is to the power supply’s output terminal.
Line-borne noise (RFI and low-voltage transients created by high-current logic circuits) will unlikely damage a power supply. But, relatively few power supply designs have careful component shielding and placement. So, line noise can couple (by stray capacitance) to the DC output, where it can disrupt communications and computer circuits. Because this noise may be intermittent and beyond the frequency range of many measuring instruments, you may have trouble diagnosing the source of the malfunction.
Harmonic voltages of the 60 Hz line frequency impressed on the AC power line are unlikely to damage a power supply. However, higher harmonics of the 60 Hz power supply can fool control circuits. The more numerous zero crossings of higher harmonic frequencies can falsely trigger timing operations the sine wave’s zero crossings initiates.§
