Surge protector
From Wikipedia, the free encyclopedia
A surge protector is an appliance designed to protect electrical devices from power surges and voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or shorting to ground voltage above a safe threshold.
Surge protectors can be built with one or more of the following electronic components:
- A fuse burns out or circuit breaker trips when excessive power is being consumed and fed to a device within the designed time lag and current rating. These can protect both from surges caused by the device itself and from external surges that last long enough.
- An iron-poor transformer can transmit AC power similar to a normal iron core transformer (although less efficiently), but will be unable to transmit sudden surges that saturate the small iron core.
- A metal oxide varistor (MOV) is a small device that will short to ground when presented with a voltage above its rated "clamping voltage", so that the surge current bypasses the protected device, provided the current rating (joule) is not exceeded. If the surge exceeds the joule rating, the MOV will be destroyed. Even if not destroyed, MOVs may degrade each time they receive a voltage spike, and thus may stop working after some time. Some surge protectors have a light indicating when this stage has been reached.
- A selenium voltage suppressor is a "clamping" semiconductor similar to a MOV, but it can handle longer voltage transients. It is used mostly in high-energy DC circuits, like the exciter field of an alternator. It can dissipate power continuously, and it retains its clamping characteristics throughout the surge event, if properly sized.
- A zener diode is a small diode designed to protect against normal spikes in a circuit, especially motor controller circuits. These are sometimes paired as a transient voltage suppression diode.
- A gas discharge tube or gas arrestor is used much like a MOV, except that it relies on a trapped gas to become ionized to pass the voltage. This has the advantage of being able to pass much more power without self destructing, but with the disadvantage of reacting to the high voltage more slowly. Gas arrestors are commonly used in telecommunication equipment to protect against lightning strikes. Gas arrestors may only be used around 10 times before replacement is needed.
- A quarter-wave coaxial surge arrestor is used in RF signal transmission routes. It features a tuned quarter-wavelength short-circuit stub that makes it pass a relatively narrow band of characteristic radio frequencies, but presents a short to any other signals, especially down towards DC. Quarter-wave coax surge arrestors have coaxial terminals, compatible with common coax cable connectors (N, BNC or F types). They provide the strongest available protection for narrow-band RF signals, much better than gas discharge cells typically used in the universal/broadband coax surge arrestors. Very useful for WiFi base band at 2.4 or 5 GHz, not much use for TV/SAT/CaTV, nor for systems sending phantom power for an LNB up the coax downlink.
- An inductor resists sudden changes in current flow. Surge protectors using inductors are sometimes called "series mode" surge protectors, because the inductor is connected in series with the load, rather than diverting current to neutral or ground as with MOVs and diodes.
- A capacitor resists sudden changes in voltage, and acts as a low pass filter, reducing spikes and noise.
- Devices having several outlets with surge protectors described may have all or some outlets protected. The better ones have higher ratings and offer more modes of protection against surges between the phase and neutral and grounding conductors; maybe for telephone and coax cable connections. (The best have insurance policies paying for damages from surges.)
- A UPS of the flywheel type seldom pass surges. The battery variety may absorb spikes much like a capacitor acts as a low pass filter. The battery ones also have protectors described above. "On line" UPSs provide the best protection.
- A polyfuse or resettable fuse is commonly used in low voltage circuits. Polyfuses have very slow turn-on times of a few seconds depending on the overcurrent and operate on high temperatures of around 100C. However, when the fault is removed, the polyfuse will return to normal after a few minutes when it cools back down to room temperature.
- A crowbar (circuit) using a zener diode driving the gate of a SCR (silicon controlled rectifier) and a polyfuse are used to latch any faults until power is turned off. The zener diode sets the trip voltage, and when exceeded, the SCR will latch on and force the polyfuse to open circuit until power is removed.
