A switch that can be operated electrically is known as a relay. It consists of several operating contact terminals as well as a group of input terminals for multiple or single control signals. The switch might come with a different number of contacts in several contact forms, like break contacts, make contacts, etc.

Relays have found its uses at places where several circuits need to be controlled by a particular signal or where it is important to control the circuit by a low power signal which is independent. Firstly relays were utilized as signal repeaters in long-distance telegraph circuits. Here they used to revive the signal rolling in from one circuit by broadcasting it on another circuit. Relays were utilized excessively in early computers as well as telephone exchanges to carry out logical operations.

The basic version of relays takes the help of an electromagnet to open or close the contacts, however, other working standards have been concocted like the solid-state relays which utilize the semiconductor properties for controlling. Relays that comes with multiple operating coils and adjusted working attributes are utilized in protecting electrical circuits from faults and overload. In current electric power frameworks, these capabilities are taken care of by digital instruments known as protective relays.

Time Delay Relay

The arrangement of timing relays is made in such a way that it causes a delay in operating their contacts. To cause a very small delay, a copper disk is placed in between the moving blade assembly and the armature. The electric current which flows in the disk keeps up the magnetic field for a brief timeframe, protracting discharge time. For a bit longer delay, a dashpot is utilized. A piston loaded with fluid that can escape slowly is referred to as a dashpot. In this case, both oil-filled, as well as air-filled dashpots, are taken help of. To vary the time period, one can either increase or decrease the flow rate. The installation of a mechanical clockwork timer is done for long time periods. Relays may either be field adjusted or organized for a fixed time span or set remotely from a control panel. Presently we have relays that come with a microprocessor that offers accuracy timing over an extraordinary range.

Today we also have relays that are made with a “shock absorber” mechanism to prohibit full, immediate motion at the point when the coil is either stimulated or de-invigorated. Thus it provides the relay the option of time-delay actuation. This kind of relays can be manufactured to result in a delay armature motion on coil Energization or De-Energization or even both.

The contacts of time delay relays should be specified not simply as closed or open, but also whether the delay gets operated in the opening direction or the closing direction.

This was all we have regarding the time delay and timing relays. Hope you liked the article. Brands that manufacture them are TE Connectivity, Littelfuse and Crouzet. Make sure to go through them using the links as provided. Thank you for reading.

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A Semiconductor Controlled Rectifier or an SCR or a Silicon Controlled Rectifier is a 4 layered solid-state current controlling electrical equipment. This technique of 4 P-N-P-N switchings was established in 1956 by Tanenbaum, Holonyak, and Goldey. In January 1958, Dr. Ian M. Mackintosh of Bell Laboratories presented the practical explanation of SCR. The name Silicon Controlled Rectifier is the trade name for General Electric for a kind of thyristor. The Silicon Controlled Rectifier was made by a group of power engineers which was led by Gordon Hall, furthermore, marketed by Frank W. "Bill" Gutzwiller in 1957.

A few sources characterize thyristors and Silicon Controlled Rectifiers as almost the same thing but some say that they are an appropriate subset of the arrangement of thyristors and it comes with at least 4 layers of alternating P and N-type materials. Bill Gutzwiller says that the word “Controlled Rectifier” and “SCR” were used earlier and the word “thyristor” came into existence when the equipment started to get spread internationally.

SCRs are devices that can make the current flow in only one direction (i.e. unidirectional). It is completely different from TRIACs (Triode for Alternating Current) where current can flow in both the directions (i.e. bidirectional). SCRs can be activated regularly just by the flow of current into the gate rather than TRIACs, which can be activated ordinarily by either a negative or a positive current applied to its gate electrode.

Modes of Operation

Depending on the biasing given to an SCR, it can be classified into 3 modes.

Forward Blocking Mode

In this category, the cathode is provided with a negative potential and the anode gets a positive potential and the gate is kept disconnected i.e. at Zero potential.

As you can see here, the junction J2 is reverse biased and J1 and J3 are biased forward. Thus, permitting the flow of only small amount of leakage current from another to the cathode. An avalanche breakdown is initiated when the applied potential gets to the break-over value for J2. J2 starts to conduct at this break over but below that the current used to face high resistance from it and the SCR is known to be in off state.

Forward Conduction Mode

An SCR can be converted to Conduction mode from Blocking mode in 2 different ways.

• At the gate offer a positive pulse.
• Uplifting the potential difference between the cathode and the anode over the break-over voltage. As soon as the SCR starts conducting, to maintain it in the ON state no gate voltage is needed.

Reverse Blocking Mode

The SCR is said to be in reverse blocking mode when a positive potential is provided to the cathode and a negative potential at the anode. Thus, J2 is biased forward and J1 and J3 are reverse-biased. The equipment acts as 2 reverse-biased diodes associated in series. A small amount of leakage current flows through it.

So this was all we have about SCRs. Hope you enjoyed reading it. Brands that make SCRs are Littelfuse, STMicroelectronics, and NTE. Refer to them through the links as provided.

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A thermal cutoff is a safety device for electrical equipment. It ceases the flow of current in a device when heated to a specific temperature. These devices can be reset either automatically or even manually.

Thermal fuse

Thermal fuse is a cutoff that utilizes a fusible link (applicable for only one time). The thermal fuse is not like the thermal switch that gets reset automatically whenever there are temperature drops. On the contrary, it's like an electrical fuse – a one-time-use device that cannot reset itself and needs to be replaced when it is triggered. Thermal fuse is generally used in those situations where overheating is a rare occurrence.

One component is a little meltable pellet that holds down a spring. The spring gets released when the pellet melts completely. As a consequence of this, the circuit breaks up. The NEC Sefuse SF series, Tamura LE series, Hosho Elmwood D series and Microtemp G4A series may manufacture the alloy pellets containing silver, copper, and beryllium to melt at a precise temperature.

Thermal fuses have found its use in heating electrical appliances like hairdryers and coffeemakers. It serves as a safety equipment to cease the flow of current to the device in case of any malfunction that would result in the temperature to ascend to risky levels, potentially lighting a fire. Thermal fuses react only in case of excessive temperature and not during the flow of excess current. This is one of the main differences of the device with a normal electrical fuse. 

Thermal Switch

A thermal switch popularly known as a Thermal Cutout (TCO) or thermal reset is an electrical equipment that opens with a "plink" sound at a higher temperature and closes itself automatically when there is a drop in temperature. The thermal switch is a bimetallic strip that is often encased in a cylindrical glass bulb to shield it from short circuits or dust particles. Another basic configuration utilizes a bimetallic shallow dome-shaped top which "clicks" to an inside out inverted cap when warmed, for example, the "Klixon" brand of thermal cutouts.

The thermal switch can be reused. Thus it is used in temporary places that are user-correctable. Thermal switches are used as thermostats in some cooling and heating system. It is also used in power supplies in case of any malfunction (overload). Another category of thermal switch is the Positive Temperature Coefficient thermistor (PTC). These kinds of thermistors have a “switch” temperature. At this temperature, the resistance rises rapidly and limits the current in the circuit. It is used in motor overheat protection.

Thermal switch is also used in some luminaire, particularly with downlight, where unnecessary heating is well on the way to happen. This might sometime result in "cycling". It is the process where the lights turn on and off after every few minutes. Glimmering radiant Christmas lights take advantage of this impact.

This was all we have about Thermal Cutoffs. Brands that manufacture this equipment are TE Connectivity, Littelfuse and Bourns. Make sure to refer them using the links as provided. Hope you liked the article and Thank you for reading.

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A resettable fuse is a PPTC (Polymeric Positive Temperature Coefficient) device that is a piece of passive electronic equipment utilized in defending against overcurrent faults in electrical circuits. It is also popularly known as polyswitch or a multifuse or a polyfuse. In certain situations, it is noted that this device is much similar to PTC thermistors but instead of charge carrier effects, they are operated on mechanical changes in semiconductors. A PPTC device consists of a voltage rating and a current rating. This electronic device was invented by Gerald Pearson in 1939 at Bell Labs. It was described in US patent #2,258,958.

A PPTC device is manufactured using a crystalline organic polymer matrix which is non-conductive in nature. To make it conductive it is added up with black carbon particle. Several conductive chains of carbons are formed between the crystals when it is cooled. Now as it is good at conducting electricity, the current will pass easily. Passage of a large amount of current through the device will make it heated up. The polymer will expand due to the heating up of the device. As a result, the polymer will change to the amorphous state from its crystalline form. Now as the heat is causing the expansion in the polymer, the carbon particles will start to get separated and the conductive pathways will start to break. As a consequence of this, the device will heat at a much faster rate, expand more and the resistance will be increased. Due to this increase in the resistance the circuital current will decrease. The device still consists of a small amount of current (leakage current) flowing through it which is enough to keep the temperature at a level such that the equipment is at a high resistance state. The range of leakage current is from a few hundred mA at lower voltages to less than a hundred mA at rated voltage. It can be said that latching functionality is one of the characteristic features of the device. 

As soon as the power is removed from the device, the heating caused by the leakage current will cease and the resettable fuse (PPTC) will start to cool down. With the cooling of the device, the original crystalline structure is regained and it gets back to its low resistance state that can hold current. The cooling process of the device generally takes a couple of seconds. 

Resettable fuses are most commonly used in computer power supplies (generally of PC 97 standard), nuclear applications as well as aerospace applications. This device is also used in protecting audio loudspeakers (tweeters). A circuit designed with a PPTC device connected in parallel to the light bulb or a resistor helps in limiting the current through the tweeter to a safer limit. Thus the tweeter will not get damaged if the amplifier is conveying more power that is endurable by it.

This was all we had about Resettable fuses. Brands that make them are TE Connectivity, Littelfuse and Bourns. Make sure to follow them by the links as provided. Thank you for reading up the blog.

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A Gas Discharge Tube or a Gas Plasma Arrestor is an arrangement of gas in an electrode inside a temperature-resistant and an insulating envelope. Gas Discharge Tubes results in the phenomena linked with the electric release in gases and gets initiated by converting the gas into ions with an applied voltage adequate to cause electrical conduction by the basic principle of the Townsend discharge. A gas discharge lamp is an electric light that takes the help of Gas Discharge Tubes; these consist of neon lights, metal-halide lamps, fluorescent lamps, and sodium-vapor lamps. Some specific gas discharge tubes like thyratrons, ignitrons and krytrons support as switching devices.

The required voltage for initiating and sustaining the discharge completely depends on the pressure, the amount of gas-filled in the tube as well as its geometry. Though the envelope is made up of glass, military tubes use glass-lined metal most often while power tubes use ceramics frequently.

As said earlier, the principle mechanism is the Townsend discharge. It is defined as the sustained increase of electron stream by ion impact when a critical estimation of the strength of the electric field for the thickness of the gas achieved. With the increase in the electric field, various discharge phases are encountered. The gas utilized significantly impacts the parameters of the cylinder. The breakdown potential difference relies on the distance between the electrodes and the composition of gas inside the tube.

Gas Used

• Hydrogen – It is used in tubes that require fast switching.
• Deuterium – It is used in neutron generator as well as UV lamps for UltraViolet spectroscopy.
• Noble Gases – It is used for several purposes, from switching to lightning.
• Elemental Vapors – Mercy vapors is used in ignitors, mercury-arc valves etc; Sodium vapors in sodium-vapor lamps; Sulfur vapor in sulfur lamps.

Gas Pressure

The gas pressure is kept between 0.001 and 1,000 Torr. But generally, it is kept between 1 to 10 torr. Factors that affect the gas pressure are as follows:

• Sputtering of cathode – With an increase in pressure, it gets reduced.
• Ignition Voltage or the Breakdown Voltage
• Backfire Voltage
• Operating Voltage
• Current Density
• Lifetime of the tube – Tubes having lower pressure have a short duration of life because of the gas consumption.

Gas Purity

To maintain the desired properties it is important that the gas present inside the tube needs to be kept pure; even a small quantity of impurities can significantly change the values of the tube; the presence of gases that are non-inert results in the increment of burning as well as the breakdown voltage. One can get to know the presence of impurities inside the tube by experiencing the change in the color of the gas glowing. Leakage of air inside the tube results in the introduction of oxygen, which is exceptionally electronegative and represses the generation of electron avalanches. This leads the discharge to look milky, reddish or pale. Traces of mercury vapors gleam pale blue, darkening the original gas shading. To forestall outgassing of the components of the tube during the process, a bake-out is required before loading up the tube with gas and fixing it.

This was a brief idea about the Gas Discharge Tube. We hope that you liked it. Brands that make Gas Discharge Tubes are Bourns, Littelfuse, and EPCOS. Make sure to check them out.

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