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Relays

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We have a lot of power around us.. A lot of electrical power.. This form of energy is dangerous to us in equal aspects as it is useful and necessary for us...
Just imagine that you are driving on a road in a wonderful weather and with you kids and wife, with a beautiful song playing in your car. Suddenly you find a transmission line along the road gets broken and it falls on yur car... Then what happens....

Scary?
Don't even want to imagine this situation?

Engineers have come up with an awesome solution to protect your car and your family... and the device is called a RELAY...

RELAYS are basically switches that controls the flow of current from one point to another..
Just like your normal switches, MCB's( Miniature Circuit Breakers) or any switch they perform the same function. They control the flow of current.
They are smart and can even detect any kind of faults like Over current, Over Voltage, Transient Current or voltages or any kind of fault in the path of the transmission line. They gets switched OFF automatically and stops the flow of current further..
All countries have these kinds of protection in their transmission lines now a days, contrary to those built around 20 years back....

So next time when you go on an outing, dont worry of the broken transmission lines..
BUT

BE CAREFUL... PREVENTION IS BETTER THAN CURE..

Published in: Business, Technology
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Relays

  1. 1. RELAYS
  2. 2. • A relay is an electrically operated switch.Current flowing through the coil of the relaycreates a magnetic field which attracts a leverand changes the switch contacts. The coilcurrent can be on or off so relays have twoswitch positions and most have double throw(changeover) switch contacts as shown in thediagram.
  3. 3. • The relays switch connections are usuallylabelled COM, NC and NO:• COM = Common, always connect to this, itis the moving part of the switch.• NC = Normally Closed, COM is connected tothis when the relay coil is off.• NO = Normally Open, COM is connected tothis when the relay coil is on.
  4. 4. Choosing a relay• Physical size and pin arrangementIf you are choosing a relay for an existing PCB you will need to ensurethat its dimensions and pin arrangement are suitable. You should findthis information in the suppliers catalogue.• Coil voltageThe relays coil voltage rating and resistance must suit the circuitpowering the relay coil. Many relays have a coil rated for a 12V supplybut 5V and 24V relays are also readily available. Some relays operateperfectly well with a supply voltage which is a little lower than theirrated value.• Coil resistanceThe circuit must be able to supply the current required by the relaycoil. You can use Ohms law to calculate the current: Relay coilcurrent = supply voltage coil resistance For example: A 12V supplyrelay with a coil resistance of 400 passes a current of 30mA. This is OKfor a 555 timer IC (maximum output current 200mA), but it is too muchfor most ICs and they will require a transistor to amplify the current.
  5. 5. • Switch ratings (voltage and current)The relays switch contacts must be suitable forthe circuit they are to control. You will need tocheck the voltage and current ratings. Note thatthe voltage rating is usually higher for AC, forexample: "5A at 24V DC or 125V AC".• Switch contact arrangement (SPDT, DPDT etc)Most relays are SPDT or DPDT which are oftendescribed as "single pole changeover" (SPCO) or"double pole changeover" (DPCO). For furtherinformation please see the page on switches.
  6. 6. Protection diodes for relays• Transistors and ICs must be protected fromthe brief high voltage produced when a relaycoil is switched off. The diagram shows how asignal diode (eg 1N4148) is connectedbackwards across the relay coil to providethis protection.• Current flowing through a relay coil creates amagnetic field which collapses suddenlywhen the current is switched off. The suddencollapse of the magnetic field induces a briefhigh voltage across the relay coil which is verylikely to damage transistors and ICs. Theprotection diode allows the induced voltageto drive a brief current through the coil (anddiode) so the magnetic field dies away quicklyrather than instantly. This prevents theinduced voltage becoming high enough tocause damage to transistors and ICs.
  7. 7. Relays and transistors compared• Like relays, transistors can be used as anelectrically operated switch. For switching smallDC currents (< 1A) at low voltage they are usuallya better choice than a relay. However, transistorscannot switch AC (such as mains electricity) andin simple circuits they are not usually a goodchoice for switching large currents (> 5A). Inthese cases a relay will be needed, but note thata low power transistor may still be needed toswitch the current for the relays coil! The mainadvantages and disadvantages of relays are listedbelow:
  8. 8. Advantages of relays:• Relays can switch AC and DC, transistors canonly switch DC.• Relays can switch higher voltages thanstandard transistors.• Relays are often a better choice for switchinglarge currents (> 5A).• Relays can switch many contacts at once.
  9. 9. Disadvantages of relays:• Relays are bulkier than transistors for switchingsmall currents.• Relays cannot switch rapidly (except reed relays),transistors can switch many times per second.• Relays use more power due to the currentflowing through their coil.• Relays require more current than many ICs canprovide, so a low power transistor may beneeded to switch the current for the relays coil.

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