Switches and Protection Devices

This page covers electromechanical components like switches, relays, etc, as well as protection devices like fuses, thermistors, and metal oxide varistors.

Switches

The switches you are probably most familiar with are toggle, rocker, rotary, and push button switches. This type of switch is commonly used where only a simple on-off function is required as in power switches. Such switches have a voltage and current rating, it's important to adhere to the voltage rating, and extremely important to observe the current rating, since using an undersized switch can result in a fire. Most common power switches are rated for 15A at 250VAC, which is more than enough for any domestic appliance. There are various different types of contacts used in these and other switches, and it's important to know the difference.

Single Throw

Common power switches are usually single pole/single throw or SPST switches. This means that the switch only has one set of contacts and can only turn on and off. When using a switch of this type as a power switch, the hot or phase wire should be switched and the neutral/ground wire should be connected directly, which is the same way that the light switches used in building wiring are installed. Double pole/single throw or DPST switches have two sets of contacts that are switched on or off  together. This type of switch can be used where two single ended signals or one balanced signal needs to be switched at the same time, or as a power switch where the component needs to be completely isolated from the building wiring when it's turned off.

Double Throw

Double throw switches are the second common type of switch, and are also available in single pole (SPDT) and double pole (DPDT). An SPDT switch will have three terminals. Flipping an SPDT switch one way connects the common terminal to one terminal, while turning it the other way connects the common terminal to the other terminal. A DPDT switch is two SPDT switches ganged together, and will have six terminals. These types of switches are used where it's necessary to switch between two sources, for example, a double throw switch can be used to select between AC and battery power, or between power supplies with different voltages. Some double throw switches have a third position, where the common terminal isn't connected to anything, but it's not typical. Double throw switches can be used in place of single throw switches by leaving the unused contact unconnected.

Switches having more than two positions are usually referred to as selector switches. Two common forms are available, sliding and rotary. This type of switch can have a common terminal, where changing the position of the switch connects the common terminal to one and only one of the other terminals. Another variety will have an array of switches with their own independent contacts, and changing the position of the switch closes one switch while opening all of the others. Like other switches, they are available in single pole and double pole varieties.

Pushbuttons

There are two types of pushbutton switches, normally open (NO) and normally closed (NC). An NO switch is held open by a spring, and only remains closed as long as someone is pressing the button. An NC switch is held closed by a spring, and goes open when the button is pressed. Common examples of these types of switches are the buttons on remote controls, the light switches in refrigerators, and reset buttons. Latching push buttons use a mechanism similar to the one in a ball-point pen to lock in one or the other position when pressed.

Digital Encoders

Rotary encoders are a type of rotary switch usually only used with digital electronics, specifically microcontrollers. A rotary encoder has three leads. One lead is for power, the other are brought high (1) or low (0) in a pattern called gray code. Performing an XOR on the inputs from the encoder will give 1 for a turn in one direction and 0 for a turn in the other direction. Common examples of this are in digital volume controls where they're combined with a microcontroller and digital potentiometer, and in digital speed sensors. Encoders are availble with and without detents.

Magnetic Switches

Reed switches are composed of one or more magnetic "reeds" sealed in a glass envelope. Moving a magnet near the switch causes the reeds to come together, closing the circuit. Hall-effect sensors also close in proximity to a magnet, but have no moving parts and can be configured to pass a voltage proportional to how close the magnet is. These types of switches are used in proximity sensors, and some types of speed sensors and rotation counters. A common example of a reed switch are the proximity switches used on door and window alarms. Two or more hall-effect sensors are used in joysticks to tell the computer where the magnet in the joystick is relative to the sensors.

Relays

This type of switch is composed of an electromagnet and one or more magnetic contacts. Applying current to the electromagnet actuates the switch. Like toggle switches, relays are available in SPST, SPDT, DPST, and DPDT. Like pushbutton switches, a relay can be NO or NC, with "normally" meaning without voltage applied to the electromagnet. To further complicate things, relays are sometimes refered to by "form". The common forms are Form A, which means normally open, Form B, which means normally closed, and Form C, which means double throw. The number of poles is usually specified before the form, for example, a "2 Form C" relay is the same thing as a DPDT relay. A fourth form, Form D, is a make-before-break double throw relay, where the actuating the switch leaves the terminal connected until a split second after it connects to the opposite terminal. This is useful for signals, where using a break-before-make style contact would cause a loud POP when the switch is actuated. It can be dangerous to use this type of switch when the contacts are at dramatically different voltages, especially where high current is involved. There are two common types of relays; reed relays and cube or "sugar cube" relays.

Reed Relays

Reed relays are made from a reed switch surrounded by an electromagnet. These tend to work best on low power circuits, and require very little current to enegize their coil. These are commonly used in phone modems, and is the source of the click heard when a modem picks up the line.

Cube Relays

Cube type relays come in all shapes and sizes, from tiny small signal relays with maximum currents under an amp, up to large power relays capable of passing hundreds of amps. A cube relay contains a lever made of copper plated with silver or gold with an iron backing. The relay is held in its "normal" position by a spring. Applying voltage to the coil of an electromagnet actuates the switch. This type of relay is usually housed in a gas tight plastic cube. A cube type relay is responsible for the click heard when turning on a TV.

Usage

When driving a relay with a switching transistor, a fast diode of type 1N4148 should be placed backwards across the relay coil to absorb the voltage spike caused by de-energizing the relay coil. The coil of a relay will have a maximum operating voltage, usually 5VDC, but 6V and 12V relays are also commonly available. Using a dropping resistor as half of a voltage divider with the relay's coil as the other half allows the use of a lower voltage relay with a higher voltage supply. Using AC or unfiltered DC will cause a relay to rapidly switch on and off, possibly destroying it in the process.

Fuses

The fuses used in home electronics are usually made from a glass or ceramic envelope with metal end caps, enclosing a thin, slightly resistive wire. The wire is designed to stay cool and provide a closed circuit under normal conditions. When the current flowing through a fuse excedes the maximum value, the wire melts at a speed proportional to the amount of excess current, opening the circuit. All electronic devices should be fused, as a safety precaution. In the event of a catastrophic failure, such as a short in a transformer or capacitor, the fuse blows, limiting the potential for damage. Unfused appliances constitute a serious fire risk.

A fuse should be large enough to handle normal operating currents, but small enough that it will open quickly if the device exceeds its normal current draw. If you're unable to calculate a device's current usage mathematically, you can use a multimeter with current measurement capabilities, or a number of commercially available watt meters. The fuse should have a capacity of about 10% more than the maximum normal operating current. Devices with heating elements (like tubes) and power supplies with large filter capacitors draw significantly more current when they're first switched on. Some heating elements are almost a dead short when cold. This type of device should use a "slow blow" type of fuse.

Under normal operating conditions, a fuse should never have to be replaced, since they don't really wear out. If a fuse blows, it is a signal that something is wrong and needs to be fixed. Resist the temptation to use a larger fuse or non-fusible jumper to deal with a blown fuse.

Metal Oxide Varistors

A metal oxide varistor or MOV is a type of surge supression device. The way that MOVs are manufactured results in a random array of crystals that act as diodes with a defined breakdown voltage above normal line voltage. In use, a MOV is placed between the hot/phase line and the neutral/ground line close to where the power cord enters the device, after the fuse. At normal line voltage, the MOV barely conducts at all, when the line voltage rises above the microscopic diodes' breakdown voltage, it shunts the excess voltage to ground. Each surge heats the MOV, and repeated surges will degrade or even destroy the MOV. I recommend adding MOVs to any project, especially solid state devices and anything running directly from mains voltage without a power transformer.

Thermistors

A thermistor is a resistor whose resistance is affected by temperature. The direction and amount which the resistance changes is called the temperature coefficient. Thermistors with a negative temperature coefficient (NTC) have a high resistance when cold and a trivial resistance when hot, while positive temperature coefficient (PTC) thermistors are exactly the opposite. An NTC thermistor placed in series with the AC line will limit the flow of current on startup, which has the effect of prolonging the life of some components. In this application it is referred to as an inrush limiter. The thermistor will become hot during normal use, so it should be placed away from heat sensitive components. Excessive heat may eventually damage the thermistor, and it may fail after some years even if it is operated within its specified environment.

Line Filter Capacitors

A line filter capacitor is the first line of defense against high frequency noise superimposed on the AC mains voltage. Sources of such noise include fluorescent lamp ballasts, flat screen TVs and light dimmers. In this application a small value ceramic or film capacitor, around 0.1-0.22μF is placed between the hot and neutral legs of the AC line, after the fuse. This has the effect of shunting unwanted audio frequency and radio frequency energy to ground, while only passing a trivial amount of 60Hz AC current. Since this capacitor takes a lot of abuse, it should be a fail-safe "Type X" safety capacitor, which is designed so that in the event of a failure it will go open circuit rather than shorting and possibly starting a fire. Line filter capacitors should be rated at or above 1kV. For an extra margin of saety, wire two capacitors in series.