Shielding Circuits: Guard Against Surges

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As design engineers, we focus on adequately operating our designs under optimal conditions. While low-voltage electronic designs pose little danger to end users, surges, spikes, electrostatic discharge (ESD), and power supply faults can damage equipment. With higher voltage, such as 120- and 220-volt AC appliances and devices, these electrical faults can be lethal to the end user.

Fortunately, circuit protection devices and design practices limit these dangers and help protect equipment and users. In this blog, we will examine several types of protection devices and methods that design engineers can consider.

Several sources and types of threats to equipment, appliances, and people are associated with overvoltage and current faults, with ESD being the most common. ESD can occur simply from walking on a carpet or floor, and we have all experienced the electrical arc as our finger approaches a grounded or metallic object.

ESD fast transients are usually very narrow in duration and very high in amplitude, like an impulse function. ESD can wreak havoc on electronics, especially modern low-voltage systems and processors. ESD can disrupt static and dynamic RAM contents, corrupting programs and data. If ESD is severe enough, it can also damage integrated circuits and discrete components.

Overcurrent and overvoltage faults usually occur simultaneously, but not necessarily. A power supply or line power level can expose devices to higher voltages that may cause higher currents if outside their regulation range. There can also be internal faults that cause overcurrent conditions like a failing (short) resistor or rectifier.

Usually, the most dramatic faults come from lightning. A powerful enough bolt hitting in close enough proximity can destroy virtually any equipment. Even if all the breakers are open during a lightning strike, the ground still runs everywhere in a house or a building and can cause safety hazards to people and devices. Fortunately, some protection schemes can help safeguard against ESD, transients, overvoltage, and overcurrent faults, and can even help protect against lighting in many cases.

The simplest form of protection is a fuse. We are all familiar with fuses in one way or another. Older homes still use fuses at the service box to protect all downstream equipment and people. Many electronic devices still use fuses as a last means of protection when upstream surge protectors or circuit breakers fail.

A fuse simply breaks the path of current flow and must create a gap large enough for a voltage not to arc across. This is particularly true in higher voltage devices, which can arc across smaller gaps, especially if exposed to environmental factors like high humidity or conductive dust.

Fuses come in a variety of form factors, from tiny micro glass tubes typically used in digital multimeters to small- and medium-sized automotive fuses and large, slide-on screw or stud terminals of high voltage and high current types. High voltage and high current types like the Littelfuse SPFJ Series are typically used in telecommunications applications and solar installations (Figure 1). While it is true that solar installations usually won't see surges since the sun doesn't vary that much (except for catastrophic coronal mass ejections or solar flares), equipment connecting solar energy to the grid, like inverters, still causes faults and fires, so large current and voltage fuses are required.

 

Figure 1: High voltage and high current fuses react quickly to overvoltage, which can cause overcurrent faults. These slides in the Class J series from Littelfuse, which also offers fuse holders, can handle up to 1000 volts at 450 Amps. (Source: Mouser Electronics)

 

 

Fuses can be expensive and complicated to access. Many circuit board-mounted fuses require equipment disassembly to change and replace the fuse. Furthermore, if the fault that caused the blown fuse isn't addressed, it will blow again. Circuit breakers can help solve this issue.

Circuit breakers are typically more complex and expensive than fuses but are resettable, making them easier and safer to use. Switching an insulated lever or pushing an insulated button is simpler and more secure than putting your hands into a potentially dangerous box with high voltage. With electronic equipment, it is easy to remove power. But, with AC line power, the fuse is always connected to power—unless a safety cutoff mechanical switch is installed.

Like fuses, many types of circuit breakers use different form factors. We all have experience using breakers in our homes and facilities, and more than likely have used circuit breakers on our equipment (Figure 2). Home and building circuit breakers are well-insulated, easy to use, and have straightforward installation. Small equipment circuit breakers, usually on a rear panel, are also easy to use, without the need to disassemble equipment racks or cabling.

 

Figure 2: Buildings and facilities can use the safe single, dual, and triple pole breakers with high voltages and currents. (Source: Mouser Electronics)

 

The PolySwitch resettable devices  are a valuable technology for overcurrent protection. These resettable fuses heat up to perform a thermally triggered open circuit. Once cooled down, they close the circuit to allow the equipment to operate again automatically.

Like all protection technologies, PolySwitch resettable devices have overvoltage and overcurrent ratings, maximum limits, and reaction times (Figure 3). They work by increasing resistance as temperature increases, and are also available in various packaging, including surface mount and thru-hole.

Figure 3: Littelfuse PolySwitch Resettable Devices increase resistance as temperature increases due to increased flow. Designed to limit unsafe currents while allowing constant safe current levels, resistance will "reset" automatically when the fault is removed and temperature returns to a safe level. (Source: Mouser Electronics)

Lower power circuits that operate on low voltages need protection against transients and surges, and fortunately, design engineers have options when it comes to this. Transient voltage suppressor (TVS) diodes and arrays, transorbs, and varistors are the most common devices used to protect electronic systems from transients. 

TVS diodes and transorbs are designed to have particular breakdown voltages. This lets them clamp the voltage on that line at a safe and protected level. Single and multichannel versions allow designers to select the clamp voltages and number of channels. Multichannel devices are ideal for protecting data and address lines, for example, on a microprocessor bus. 

Overshoot and undershoot reverse polarity protections, but unidirectional and bidirectional options allow overshoot only. The various package choices can be selected to absorb up to 30KW surges.

A varistor is a similar device. It is also a diode, but its resistance varies with applied reverse bias voltage. Like TVS devices, varistors are available in many form factors and power ratings. Multilayer suppressors and metal oxide technologies absorb energy above a certain limit. Varistors react very quickly and, as such, are ideal for ESD and even lightning-speed surges. Remember that varistors can only absorb a specific amount of energy, and once absorbed, they become open circuits and should be replaced.

For high-voltage designs, a gas discharge tube is a good technology to consider. Here, tubes of ceramically enclosed gas are used to spark over the circuit lines being protected. Like an enclosed spark gap, it limits voltage by allowing the excess electrical energy to arc. The gas type and space-enabled device range from 75V to 7.5KV spark over points.

Now more than ever, circuit protections are needed to assure reliability in modern products, with many industries not quite mastering these safeguards yet. Modern appliances like ovens, washers, dryers, dishwashers, air conditioners, and refrigerators have replaced older, rugged, and reliable mechanical controllers with electronic controllers. The newer electronic controllers fail at very high rates, primarily because of weak transients and overvoltage or undervoltage surges. Littelfuse's protection technologies offer many great benefits to design engineers looking to ensure safety and dependability.