Everything you need to know about picking the right circuit protection

Wed, 09/11/2013 - 3:33pm
M. Simon, Technical Contributor

When it comes to circuit protection, the first line of resistance is resistance. Resistance limits current flow and resistance, in conjunction with capacitance, slows the rate of increase of voltage, giving other protection measures in the circuit time to act. But, resistance has its drawbacks. If the current through the resistance is significant, it wastes power, and power waste generates heat. Resistance also causes voltage drops, and voltage drops where you don't want them can adversely affect circuit behavior. 

Fuses and fuse ratings
The best known alternative—because it has been around for so long— is the fuse. A fuse is a piece of meltable metal that melts or vaporizes when the current running through it exceeds the fuse's rating. There are two main types of fuses including: the fast blow fuse which, as its name implies, responds quickly to an overload and the slow blow fuses, which take longer to melt when the overload is moderate. This allows for startup surges and temporary - if allowable - device overloads.

Fuses typically do not respond to currents less than 1.5 times their rated value, but that can vary according to the standards with which the fuse is rated. The two most common are IEC and UL. IEC rated fuses can carry one hundred percent of their rated current continuously. UL rated fuses can carry seventy five percent of their rated current continuously. Above the rated current fuses respond in what is known as an I2t fashion. That means the time it takes for a fuse to blow is related to the square of the current or the time to blow is a function of the power the fuse dissipates. Even fuses dissipate power, but they are designed to dissipate very little in comparison with the circuit they are protecting. Fuses also have a breaking current rating. This is the maximum short circuit current that the fuses are guaranteed to break. Along with that, fuses have a voltage rating that insure they can break the current in the circuit without creating a continuous arc, which would defeat their purpose. When specifying a fuse, you need to look at its rated current, the rating system, the delay (slow - fast), the breaking current, the breaking voltage, and the "I2t" rating.

Exploding fuses
Exploding fuses are something electronic designers don't run into frequently, but there are fuses designed to explode. They are called "sand fuses" in the trade and are designed to be used on power poles. When the fuse blows, it ignites a small explosive charge which propels a nail out of the end of the fuse a little ways so a lineman can tell from the ground if a fuse on a power pole has blown.

Circuit breakers
Circuit breakers operate in a fashion similar to fuses. They have continuous current ratings and trip ratings. They are also available in thermal trip and magnetic trip types – sometimes the two types are combined. Typically, they need to be manually reset. Low voltage thermal trip breakers are the circuit breakers most commonly found in electronic circuits.

Another necessary type of protection is overvoltage protection. Zener diodes can perform that function in circuits where the rate of rise of the voltage transient is not too high. For inductive kickback from a DC powered inductor or coil a high speed diode of the proper speed, current, and voltage rating will work. For applications where the coil or inductor is de-energized infrequently, the most important diode rating is the surge current rating. Transient Voltage Suppression (TVS) diodes, sometimes also referred to as Tranzorbs, are a kind of zener diode that respond in nanoseconds to overvoltage. They are rated for operating voltage, breakdown voltage, and surge current. Recently low capacitance types of TVS diodes have become available for protecting high speed signaling circuits.

MOVs and Tranzorbs for power protection
For input AC power protection (surge protection) AC rated TVS diodes and Metal Oxide Varistors (MOV) are used. These are relatively high capacitance devices, but at the frequencies they are used for (50 or 60 Hz) that is not a significant consideration. They are rated for working voltage, breakdown voltage, and surge current. Be sure to look at the peak voltage in your circuit and not the RMS voltage or you will have a continuously conducting device. Something they are not designed for.

Electrostatic discharge (ESD) protection
Electrostatic discharge (ESD) can be a real circuit killer if the circuit is not designed to deal with it. The most common electrostatic discharge effect can be seen indoors in winter in environments with low humidity. For a real life example of ESD: rub your shoes on the carpet and touch for a metal door knob. The small spark that jumps to the door knob (from the voltage stored in your body's capacitance) is ESD and it can range into the thousands of volts. Rotating machinery can produce ESD and it can be more severe because generally capacitance is higher and resistance is lower. ICs are rated for the human body model (HBM), and the machine model (MM). The rating ICs have for these types of events is a voltage rating. The rating for the machine model is usually much lower than for the human body model. To protect circuits from ESD an input resistor in conjunction with a capacitor is used to slow the rate of rise of the voltage. In severe cases a TVS diode can be used for additional protection. In high current circuits a TVS diode may be your only choice. They also may be your only choice in high speed signaling circuits.

The why of circuit protection
Fuses and circuit breakers are used to prevent fires from overheated equipment. Your safety agency (UL, CSA, etc.) will require them. ESD protection and power overvoltage protection insures that your circuits can take a licking from their environment and keep on ticking. That prevents product returns or field service calls. And most of all it prevents unhappy customers.


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