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Electrical Relays

Electrical Relays. Prepared by Ben Oman, ECE 5320, March 2012 . Outline. Reference List What is a relay? Major Applications How each type works Specifications Limitations Pricing Where to purchase . References. "Relay - Wikipedia", http://en.wikipedia.org/wiki/Relay

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Electrical Relays

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  1. Electrical Relays Prepared by Ben Oman, ECE 5320, March 2012 

  2. Outline • Reference List • What is a relay? • Major Applications • How each type works • Specifications • Limitations • Pricing • Where to purchase 

  3. References • "Relay - Wikipedia", http://en.wikipedia.org/wiki/Relay • "Smarthome", http://smarthome.com • "HowStuffWorks.com 'How Relays Work'", http://www.howstuffworks.com/relay.htm

  4. What is a relay?     A relay is an electronically controlled switch enabling one circuit to control a completely independent circuit.  Relays are used everywhere and extend a similar concept to Transistors and MOSFETs to many different applications.

  5. Major Applications     Relays have applications in residential, commercial, and industrial electrical control.  In homes relays are often used to control lighting electronically.  X10 and Insteon are two popular distributors of home lighting control equipment that always utilize relays.     Commercial applications include most classrooms at Utah State University where the lights are controlled by a touch panel at the front of the classroom.  When the lights are turned on or off a small "click" can be heard in the ceiling.     Industrial applications include the control of large electronic components such as motors that require great amounts of power to run but need to be controlled by a lower voltage circuit.

  6. Basic types of relays • Solid State • DC (MOSFETs and Transistors) • AC (Triacs) • Electromechanical • Basic • Latching • Reed

  7. DC Relay (Solid State)     DC Relays utilize transistors and MOSFETS to switch larger circuits.  When a voltage is applied to the Gate on one of these components, current is allowed to pass through them.     Pictured on the left is a MOSFET that tolerates up to 60 volts at 30 amps.     Solid state relays are advantageous to mechanical relays because they have no moving parts and thus never wear out.  Mechanical relays have a finite number of switches. Image Source:http://www.sparkfun.com/products/10213

  8. AC Relay     AC Relays are another solid state relay that are built primarily from a Triac circuit.  A Triac is a component made of two "Thyristors" which are voltage controlled unidirectional switches.     When a voltage is applied to a thyristor, the switch will remain on until there is no more current passing through.  By placing two thyristors in opposite directions, AC current can be controlled as each thyristor turns on and off due to the change in current direction.     As seen in the picture above, the two thyristors are placed in opposite directions.  AC Relays with Triacs are often used in lighting applications where dimming is involved. Image Source: http://www.allaboutcircuits.com/vol_3/chpt_7/6.html

  9. Electromechanical     Basic Electromechanical relays are essentially a switch leg actuated by a electrical motor.  When a voltage is applied to the motor (coil) the switch apparatus rotates causing the contacts to close the circuit.     There are also electromechanical relays that are by default in the "ON" state and require a voltage to turn off.     Electromechanical relays are preferred  to solid state in applications with large inductances and applications sensitive to static charge. Image Source: http://en.wikipedia.org/wiki/Relay

  10. Latching     Latching Relays utilize magnets to keep the relay in the ON or OFF positions causing the control pulse to simply change the state of the switch without the need to maintain it.  These relays conserve power in applications where the relay remains at the same state for long periods of time. Image Source: http://en.wikipedia.org/wiki/Relay#Latching_relay

  11. Reed     Reed relays are composed of a vacuum tube crossed with wires adjacent to a electrical coil.  When the coil is charged, a magnetic field causes the wires within the tube to touch.     Reed relays are not used often due to their tendency to "stick" over time.   Image Source: http://en.wikipedia.org/wiki/File:Reedrelay.jpg

  12. Poles and throws     Depending on application, different types of relays are available.  "Poles" are the number of different circuits are switched by a relay.  "Throws" are the number of lines going out from the switch.  For instance, a 3 way relay circuit in home lighting (two switches controlling one light) use Double Throw switches to allow control of the same light.  The image to the left shows the circuits of 4 common types of relays and switches. Img source: http://upload.wikimedia.org/wikipedia/commons/thumb/6/67/Relay_symbols.svg/200px-Relay_symbols.svg.png

  13. Specifications Solid State -   The most common solid state relays are found in the range of 30 volts to 240 volts and with control voltages in the range of 3 to 32 volts. Mechanical - The range of mechanical relays varies greatly from small relays used for control within cars to large relays used at electrical substations.  Typically the power required to control the relay is proportional to the power being switched.

  14. Limitations Solid State - Due to the nature of the semiconductors within solid state relays, they typically aren't used to switch in high power applications > 10 kW.  Semiconductors are very sensitive to large voltage spikes such as lightening and static charge which make them very vulnerable in unprotected applications. Mechanical - These relays are limited to the electrical properties of the conductors used.  The huge switches in power substations are essentially mechanical relays that switch up to 2 MV per electrical phase.  Larger power switches wear out faster as the arcing during switching destroys the contacts of the switch.  As with anything mechanical, components weaken over time.

  15. Pricing Solid state - Typical relay packages are in the range of $4 to $7 each on eBay decreasing with volume.  Mechanical - Starting at $5 and increasing with size and capacity.

  16. Where to purchase     Relays can be purchased reliably at the following locations: • www.ebay.com • www.digikey.com • Radio Shack • www.smarthome.com (you'll find relays packaged in with home automation systems . • Ford motors (as well as any other auto manufacturer) • Almost any other electrical distributor 

  17. How to choose a relay     Since the price of a relay increases proportionally to the amount of power it can switch, a relay should be chosen with a rating greater than the maximum power required in the switching application.  Especially in solid state relays, a relay with a current rating twice the maximum need should be used to prevent overheating.  If the maximum current rating is reached, a heat sink will need to be used to dissipate heat.     How often the relay will be switched and whether the relay will be exposed to large electrical impulses also affect whether mechanical or solid state relays should be used.

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