Reed Relays – What Are Reed Relays and How Do They Work?


Reed Relays – What Are Reed Relays and How Do They Work?

A relay is an electro-mechanical switch. A coil wire energizes an electro-magnet and this pulls a movable contact. This contacts a second contact and electrically connects the circuits together.

The current in the coil is regulated by a coil resistance and a diode. This reduces the peak relay voltage to safe levels for human exposure.

Reed Relays

Reed relays are similar to other electromagnet controlled relays except that they have no armature. Instead they use a reed switch with a magnetically sensitive based material. The reed is sealed in a glass envelope that’s filled with dry inert gas to avoid contact oxidation. The reed switches have a normally closed and a normally open contact. The contacts are connected to an electromagnetic coil and when the switch is energized a magnetic field passes through the reed blade. The magnetic field that’s created by the coil must be confined to avoid interaction with a closely spaced neighboring relay.

To do this Pickering uses a former-less coil that’s wound directly around the glass envelope. This approach improves the coil efficiency by concentrating gate driver the magnetic fields closer to the reed blade. In addition, the former-less design allows for a much denser relay layout on the PCB.

Reed switches are used in a wide variety of applications including high voltage switching, instrumentation and automatic test equipment (ATE). Some of the key specifications that must be met include carry current, lifetime, minimum switch capacity, hot switching and operating speed. They are also rated for an extended temperature range and can handle high thermal EMF.

Reed Switches

Reed switches are used in a variety of applications from burglar alarms for doors and windows to fluid level sensors/indicators. They work by using a pair of ferromagnetic reeds that are in close proximity to each other to complete an electrical circuit. When a magnet is placed in close proximity to the switch the reeds are drawn together and the circuit is completed, when the magnet is removed the reeds return to their original position.

The reeds are a soft iron-nickel alloy typically sputtered with Rhodium or Ruthenium to prevent contact bounce and increase lifespan. The switch contacts are also sputtered with hard materials such as tungsten, gold or low-resistivity silver for superior current handling. The reeds and the switch contacts are housed in a glass envelope that is sealed with inert gases or for high voltage switches a vacuum to keep out contamination and allow for a longer mechanical life.

The operating voltage is supplied by an operating coil fitted over one or more of the reed switches. The current passing through the coil creates a magnetic field that in turn closes the switches. This allows designers to use reed switches in environments where other, more delicate devices could be damaged by physical stress. However, it is still necessary to employ a de-bounce circuit in order to avoid unnecessary make/break cycles.

Switching Capacitors

In circuits that process signals quickly, capacitors can be useful to filter out low-frequency or DC signal components while allowing higher frequencies through. They’re like bouncers at a club for high-frequency signals!

Capacitors do this by accepting a total charge and changing their voltage proportionally (inversely) to the difference between system and supply voltages. Switching them on and off rapidly like this, however, can produce a very sharp current pulse that creates what’s known as an “electronic noise” waveform. To prevent this from happening, a pre-insertion of a properly sized resistor for a short period during the capacitor’s energization is often used.

The pre-insertion of a resistance also helps reduce the inrush current required to get a switch or switching device to close at the correct zero voltage level, reducing the magnitudes of energization transients. However, it is important to note that this approach still requires a current limiting reactor.

A switched capacitor circuit is made up of two switches that are alternately opened and closed by non-overlapping pulse generators VSA and VSB. This creates a “resistor” that has a very flat response to a changing signal, and it saves space by using actual capacitance values instead of large resistors. In addition, it can be controlled by a clock, which sets the frequency of the switches opening and closing.

Switching Diodes

Diodes are commonly used as Quadrant Analog Multiplier simple on/off switches. They are also useful in circuits that need to operate at high frequencies and have low current leakage.

In general, a diode has two regions, called the n-type region and the p-type region. At the boundary between these two regions is a point known as the p-n junction. When a higher electrical potential is applied to the p side (the anode) of the diode than to the n side (the cathode), electrons flow through this depletion region from the n-type to the p-type side, creating a unidirectional current path.

But if the applied potential is reversed, the p-n junction becomes forward biased and allows a small amount of reverse current to flow for an instance. This is a process called RINGING. During this time, the diode may create oscillations which cause loss of power.

Switching diodes have been designed to reduce the ringing effect & provide lower leakage currents. They have faster switching characteristics, less junction capacitance of the PN junction, and smaller internal resistance during conduction. The time it takes for a switching diode to turn on from the cut-off state is called the turn-on time. The shorter this is, the better.

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