What is Relay: Definition and 143 Discussions

A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.
Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. Relays were first used in long-distance telegraph circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations.
The traditional form of a relay uses an electromagnet to close or open the contacts, but other operating principles have been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protective relays.
Latching relays require only a single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling.

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  1. V

    Sum of currents in protection relay

    In a differential protection relay there is a setting of a threshold for the sum of amps (or the squares). In what way would that be of interest? The relay is an Areva P521. Also, the relay can display the ratio between the positive sequence current and the negative. What information does that...
  2. N

    How to Activate Relay using Transistor?

    The title already said the question of this thread. As I searched the internet, transistors were configured in common-emitter configuration. They also show me that in order to activate the relay, the transistor should be operated in saturation and cut-off mode. How to make my BC547 Transistor...
  3. N

    Distance relay - impedance characteristic question

    Dear all! I have a question about the impedance characteristics in the distance relay used for transmission line protection. I understand that in the first quadrant we have a inductive load and in the fourth quadrant we have a capacitive load. I, however, do not understand what we measure in...
  4. R

    What are the functions of sensory, motor and relay neurons?

    What are the functions of sensory,motor and relay neurons?
  5. N

    Small NPN 3904 relay style circuit

    I am currently trying to make a small circuit work, and more than just trying different things until it does work, I wanted to hopefully get an explanation to why it is not working. I am a novice, I only started dabbling in my free time recently and thought I had a decent if only vague...
  6. S

    How Can I Increase Current from a CD4017 IC to Fire a Relay?

    Hi guys I'm new here and need map some help. I have built a simple circuit with a cd4017 IC. The circuit works perfectly and has led connected to it. However I want to connect much larger load to it. A relay is the obvious answer but I'm not getting enough current from the IC to fire the relay...
  7. 462chevelle

    Transistor as Relay Setup: Tips for Quicker Circuit Release

    I'm looking for some help setting up a circuit system. Were currently using a magnetic type trigger relay. I am trying to get a quicker release of the circuit. Its for the trans brakes on our drag cars. Here is what i was thinking, this is of course a very rough and first draft of my thoughts...
  8. C

    Would this relay design be useful?

    Hi I had a neat idea for a relay design, basically its a relay that takes very tiny movements to open/close a circuit, I.e. I can make a 10 micron movement to create/close a 1mm gap very easily and in a small package (3mm by 10mm by 20mm say) and with a fairily large contact area (for that size...
  9. T

    Solving Relay Imaging Problem to Get Image at 370mm

    Hi there, I've been trying to solve this problem regarding relay imaging so that I can modify my experimental setup accordingly. Please refer to the attached jpg file for the lens setup. I am trying to image the object (the purple block in the attached jpg file) at a specific imaging plane...
  10. P

    Relay showing voltage while contacts open

    Hi, I am simulating a switch system on multisim which I have constructed using a normally open contact relay. The problem I am having is that before the coil is energised, I am observing about 15mV on the main circuit side when the contacts are supposedly open. Is this normal? When the coil is...
  11. A

    Satellite relay system and transmission losses

    Hello. Below I have a problem I can't quite understand to the full: http://www.israup.net/images/027d25c3acde86a09bcb9e1ed869560a.png http://www.israup.net/images/6757f5c30cbcfe051295b1b1e4614489.png And that is the suggested solution...
  12. S

    Relay Control System (Normally Open,Held close)

    Hi guys, currently i am stuck with the limit switches in the sequential control circuit. the Normally open,heldclosed, Normally closed,held open , normally closed and normnally open. Let's say if bo is NO,HC. so the switch will be normally open then once bo is actuated then the switch will...
  13. A

    120-240VAC Digital Logic Power Relay Timer

    Design Goal: Remove power from 4 washing machines and 4 dryers using relays and a digital logic timer. (9 Hour cycle of no-power from 22:00 to 07:00 the next day) Design Problem: What relays can I use that can be actuated by logic voltages? Should I use 2 relays, a logic relay that actuates a...
  14. A

    Creating a Digital Logic Timer for a 120-240VAC Relay System: Tips and Solutions

    Hi Guys, I'm tired of waiting for my tenants to finish their midnight laundry loads. I have a hydraulic sliding door assembly ready but I am having trouble getting parts to create the digital system to tie it all together. I want: 120-240VAC Power Relays (for the washing/dryer machines...
  15. R

    Trigger a relay via resistive load in AC circuit

    How does one go about triggering a switch (i.e. relay) via a resistive load in an AC circuit? For instance, if I have an AC relay (high impedance) and put the coil in series with the resistive load, it won't work: the resistive load will stop working. How does one go about triggering a relay...
  16. A

    SPST Relay Ratings and Operation Confusion

    I'm having trouble understanding the ratings for an SPST Relay. I need one with the following specs: Coil Rating: 12Vdc, Contact Rating: 20Amps @ 15Vdc If I use a relay with a 24Vdc Coil rating, does that mean that it would require 24Vdc to operate, or is that the maximum amount of...
  17. B

    How Can a 4-Input Controlled Relay Be Implemented?

    I am trying to implement the following device: It takes a 4-bit input string, and a control input. If the control input is 1, the output is a 4-bit string. If the control input is 0, the output is 0000. What sort of device might make this possible? Obviously, brute force logic with AND gates...
  18. I

    Selection of relay for driving a immersion rod

    i am designing a heating element control system. i am looking for a relay(with 5V input voltage if possible or 12V) which can control an immersion rod with specification 16-A & 250 V AC. The following are my questions. 1. the heater has to be driven with power sockets(type M sockets -16A)...
  19. F

    Controlling line power with a 6V circuit using a relay.

    I need a sort of relay which takes only about 6V in the electromagnet, but can handle line current in the connector? It would also be preferable, not necessary, that it's inverted, so when there is power to the coil, the connector is disconnected, and when no power is in the coil, the connector...
  20. G

    Can a relay handle both AC and DC power sources?

    Hello! I found a cool site to buy odds and ends electronics. It sells RF wireless remote systems equipped with relays. With halloween coming up this will be really fun to play with }:) . My question is, the description states the relay output is 125V VAC but if can't I run VDC through it...
  21. J

    Solid State Relay: Odd Failure

    I'm not that familiar with the inner workings of solid state relays, and I'm seeing an unusual anomaly in my circuit that I'm not quite sure how to explain. For my job, I'm running tests on some sensors. One of the requirements for the test is to turn the power to the parts on and off...
  22. G

    Why is WR for 4x100m relay less than 4 times WR for 100m sprint.

    Hello, The 2012 London Olympics concluded a day or two ago. We saw that a few records were broken in athletics. In particular, the 4x100m men's relay record. Currently the WR are as follows :- 1) 100m Men's sprint - 9.58 s 2) 200m Men's sprint - 19.19s 3) 4x100m Men's relay - 36.84s...
  23. V

    Relay switch to detect power source

    I need to find a switch that can detect whether it should go from 9v source or 5v Source. I am making a tool that has a battery power of 9v and it can also get powered from a USB device, which is 5V. If the customer need to connect the tool to a computer while its running from the battery...
  24. Femme_physics

    Max and min pickup current of the relay

    I hope I'm translating this correctly from Hebrew. http://img716.imageshack.us/img716/7621/mamser.jpg I have a relay here and I'm told that the min pickup current is 6mA, and the max is 10mA... does it mean that the relay only works in this range? What happens if there is more than 10mA?
  25. F

    Figuring out how a Relay circuit functions

    I'm trying to figure out how a relay functions in a circuit and I'm having trouble figuring why it has the effect it does in a circuit. I set up a simple circuit (see picture) using a 5 pin [ouaz-ss 105D] relay where the LED on the bottom lights up while the push button is not pressed. When...
  26. S

    Momentary relay or transistor?

    I am working with a kit circuit that I ordered online. The circuit triggers a relay when it "hears" a loud noise. I am going to hook the relay to a strobe to capture the moment of impact. with my camera. The circuit by design triggers and "holds" the signal until it hears it again. This will...
  27. A

    Solving Relay Problem in Thermistor Controlled Comparator Circuit

    Hello everyone, I've made a thermistor controlled comparator circuit which has got a relay connected to its output, when the temperature of the thermistor rises, the relay switches and the fan on its output is turned on the coll the thermistor down. Now, my circuit is working perfectly, but...
  28. H

    Low voltage relay and AC motor questions. Heeeelp. Please

    Hi, I have a central vacuum system that has blown it's circuit board up. Basically a hoover with a low voltage switching system to fire the motors up. Finding a new board will be difficult so I'm going to rebuild the control system. I can solder, can rewire a house, but the finer points of...
  29. C

    Need help building an IR receiver that will trigger a relay to turn a light on

    I am designing a game where you would shoot an IR gun at an object that would turn a light on and stay on until manually reset. I don't know much about electronics and any help would be appreciated.
  30. E

    Connecting two heaters of 240V to a relay rated at 240V

    Ok this might be a really stupid question but I would appreciate some help. I have a controller with a relay output rated at 240 V 3A. I was wondering what happens if I connect 2 heater elements of 240 V each in series to this relay. Will it ruin the relay over time? Will the output of the...
  31. B

    Understanding the Functionality of Relays: Exploring Input and Output Control

    Hello all! I just had a quick question to ask about relays. I understand that you can use a relay as an input voltage-controlled on-off switch but it can also be used to control the output voltage to the load controlled by an input voltage, correct? If I am understanding everything correctly...
  32. B

    Solid State Relay power source

    Hello, I am not an electrical engineer but I do have a slight background in some electrical engineering concepts. My question is a general one about relays (more specifically solid state relays but I don't think that really changes my question). I understand that a relay gets a small input...
  33. K

    Overcurrent-Undervoltage relay?

    Is there such a relay that can act as overcurrent as well as undervoltage? I might have mistaken it for something else so I just want to make sure. Thanks in advance.
  34. J

    Can a 12V Relay Handle a 36V Application?

    Hello All, I'm working on a project to design and build an experimental motor controller that operates on a 36V supply. As part of my test setup, I would like to include a mechanical relay to disconnect the battery supply from the H-Bridge output stage in case of a failure during testing...
  35. C

    Radio Relay System: Hardware, RC Car/Plane, Cheap, Durable

    I am looking for a strictly hardware radio signal system. Something along the lines of what you would find in a RC car or plane. Any suggestions? Hopefully something relativily cheap, and fairly durable. Thanks for any response.
  36. B

    Solid State Relay for Fast Switching Electromagnet DC/AC 24-380V 25A

    So I want to use this solid state relay, to fast switch an electromagnet. About 2 times a second. Weight: 112g Size:5.9 x 4.5 x 2.3cm Color : Grey Fast switching Reduced electromagnetic interference Output voltage: AC 24-380V Output current: 25A Input voltage: DC 3-32V Control Method: DC to...
  37. N

    What Are Common Problems with Inductive Loop Relays?

    Hi all Well I tried googling this but could not find a satisfactory answer , so here I am seeking help from all you experts :biggrin: So my question is regarding inductive loop sensors used on the road to give presence, vehicle classification etc information. These inductive loops are...
  38. K

    Axis Camera with I/O and relay to turn on lights

    Hi, I do not know anything about relays. I been trying for days to hook up a relay to turn on a light using an Axis camera with an I/O switch. This is the relay http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=105585863&uq=634536768231996536&DPU=submit This is the manual...
  39. S

    DURKOOOL DM84-2012-35-5024 Power Relay: Contact DC Voltage & Coil Activation

    considering this relay http://my.element14.com/durakool/dm84-2012-35-5024/power-relay-dpdt-2co-24vac-8a-pc/dp/1557207" what does contact DC voltage meant? and to energize / activate the coil i need a 24v current to do the job?
  40. 1

    Asking on how to choose proper relay

    Dear All, I have a task at hand to design a controller circuit to modulate ON/OFF a high voltage function generator (sinewave 500 Vpp, f 100 kHz) via a relay. I have never handled a relay before, and I am positng this question here to ask for your kind help to give me some suggestions...
  41. J

    Can a 24V Relay be Energized Safely with a 6A Power Supply?

    I would like to energize the coil of a 24V relay with switch contacts rated at 2 amps and a coil draw of 17ma with a power supply rated at 24V & 6 amps. Will this be fine and not fry the power supply or coil or do I need a resistor. Any help would be very appreciated. Joe
  42. J

    Is there a transistor that acts like a relay?

    I need some kind of four pin transistor that is neither common drain nor common source. Both positives and grounds need to isolated. Does that exist?
  43. L

    DC to DC circuit for automotive relay - 5v to 12v

    Greetings E.E. Gurus! I'm a noobie in need of your guidance! Here's the situation: I have a 5V DC, 0.25 amp (1.25W ?) power source from an oil pressure switch wire on an old truck that is activated only when the engine is running. My goal is to activate a 12V DC, 2.5 amp, 30/40W...
  44. I

    Relay creates false triggering

    I was creating a touch switch using CMOS IC 4017. The input pin is 14 and the output pins are 3 and 2. When pulses are given to the input, the IC makes the outputs high alternatively. Thus if a pulse make the pin 3 high the next pulse make pin 2 high. From the diagram it can be seen when pin 3...
  45. K

    Average velocity with a relay race

    Homework Statement A relay race is run along a straight-line track of length 300.0 m running south to north. The first runner starts at the south end of the track and passes the baton to a teammate at the north end of the track. The second runner races back to the start line and passes the...
  46. Z

    Looking for a low/no current alternative to a SPST relay

    I have a circuit with a 13.8v DC output that powers a solenoid. I'm not sure what the current draw of the solenoid is, but it's probably not much. I would like to have a 3W light come on when the solenoid is energized. The problem is that the circuit is very old and is known to have very low...
  47. Spinnor

    Could cell phones act as cell phone relay towers in an emergancy?

    In an emergency could scattered cell phones form a communications network if properly programed without need for relay towers? I'm guessing such a system might have severe limitations but if the software was built in it might be an inexpensive way to keep communications going during an...
  48. Y

    What distinguishes ELCB from Earth Fault Relay?

    Hi all, what is different between ELCB and Earth Fault Relay? why there is a need to change from eLCB to Earth Fault Relay? thanks
  49. T

    Relay for Infrared controlled switch

    hi,, i m building this IR receiver circuit, to work as a switch for electrical appliances such as a lamp,, so when i send an IR beam to the circuit, the lamp should turn ON and stay ON even the beam is no longer present, but it should turn OFF when i send IR beam again. Which relay...
  50. M

    Control Relay Grounding Issue

    I'm trying to refurbish a Heliostat and utilize a computer to control the three phase motors that came with the device. However, I'm having an issue with my control circiut for the three phase relays I've purchased. I'm using two Struthers-Dunn 645RT series SS motor reversing relays which...
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