I never understood what a relay actually does.

Could someone please enlighten me?

Thanks.

A relay generally allows a low power signal to control a high power load. They're usually smaller, enclosed versions of contactors.

Relays are composed of a coil, and a set of magnetic contacts. When energized, the coil will open and/or close some of those contacts, depending on how the relay is designed.

For example, a relay in a heat pump may have a coil rated for a 24 volts at 0.5 an amp, but the relay contacts may be rated for 30 amps at 240 volts. When the 24 volt control signal from the thermostat flows to the relay coil, it closes the contacts which allows 240v to energize the heat pump.

-John

There are many reasons for using relays.

The most common reason I install a relay or contactor is so I can switch a large circuit or many smaller circuits remotely with simple low cost low ampacity switch.

I might install a 200 amp 3 phase 600 volt contactor (relay) to control an entire lighting panel. By using the contactor I can use a small time clock to control the contactors coil and by doing that the entire panel.

Simply put...it's a switch that is controlled by another switch.

Here is an easy example:
You have parking lot lighting supplied by by a 3Ø 480v 40A circuit.
How do you control it?
- You could have the hired help turn it on/off at the desired times with a switch

- You could use a photocell (if you could find one that is rated)

etc

Each of these methods has it limitations:
- The store is closed on the Holidays, but you still want the lights on for security. Do you want to pay the help to turn the lights on/off?

- Still looking for the that rated photocell?

So you get yourself a contactor with a 120v coil and contacts rated at 3Ø 480v 40A.
Now you wire the photocell to the coil. When it gets dark, the photocell does it's thing and completes the circuit to the coil. The coil closes, the lights come on. When dawn comes, the opposite happens.

Here is a little animation of what happens: http://electronics.howstuffworks.com/relay1.htm

The photocell could be a switch or a time clock.

Think about how a sump pump works...how does it know when to start or stop?
On a grander scale, that would apply to a water treatment facility, car wash, etc.

Use a relay to;

Switch high voltage/current with safer/ more economic/ smaller/ lower voltage/ control circuit components, say in power transmission. Or, mixed voltage circuitry; controls= lvdc, power= higher vac.

Switch multiple circuits with no backfeeds.
As for example a heating boiler may be fired by signals from multiple programmable zone thermostats or timers, without backfeeding the thermostats/timers themselves or operating all the solenoid valves controlling water flow or all the circulators in the water circuits.

Switch applied logic circuits in machine controls, say Guard open = machine stops but a keyswitched relay allows running during tool setting. Largely displaced by PLC controllers and ICs.

Mechanically, a relay would just take a switched signal current and use it to energise a coil on a solenoid or a motor to operate another switch.

Alan

All these years I understood what a contactor did. Hell, I've wired more than a few. But I never understood that the relay itself was the guts of the contactor and how it physically worked. Now I know.

Thank You.

You guys are talking about contactors. These are relays https://www.electrical-contractor.net/ubb/Forum8/HTML/000194.html

We did lots of stuff with relays before the transistors got better and they started showing up on silicon chips.

Relay, cintactor...the difference is only in scale!

Well....almost. Most 'contactors' have only "normally open" contacts. A relay typically will also have "normally closed" contacts. These contacts will let the relay operate something when the relay is "off". A common example is your typical alarm system; cut a wire, and you trigger the alarm.

Now...take a contactor, add overloads... and you now have a "starter." Add a disconnect switch, and it's now a "combination starter."

Wow, I feel like I opened a can of worms here. Seems that the ole relay has a million and one applications they can be used in. Cool pictures of those old computers being built way back when.

Relays were really starting to go pretty fast when transistors took over. The basic wire contact relay had a 6ms operating time. The permissive make wire contact relay shaved that to 3ms. The reed relay got that down to about 1ms. You can do about as well with a mercury wetted relay. The slowest transistor gates were running about 1000 times that speed.

For another example, look at telephone exchanges. Before the age of electronic and digital switching, the typical telephone exchange contained literally thousands of relays (not to mention all sorts of other electro-mechanical switching devices).

Some relays could have just a single set of normally-open, normally-closed, or changeover contacts, but others could have a coil operating a dozen sets of contacts.

Other "tricks" were possible, for example by placing suitable magnetic slugs on the core, you could make a relay "slow to operate" or "slow to release." That was often used in complex telephone switching to insure that one relay would always operate before or after another.

And then there were the "unusual" relay configurations, such as latching relays (apply one pulse to the coil to turn it on, another pulse to turn off), dual coil relays (one coil for on, one for off), and stepping relays (multipole switches, with each coil pulse advancing the output position by one "step". They usually had a separate "reset" coil, which would jump the output back to "zero" when pulsed. These were widely used for telephone exchanges, counting pulses from rotary dials to complete calls. Originally called a "Strowger switch" after the inventor.

That's a good question to answer with a question.

What does a relay do? Well, what do you want it to do? Don't forget a whole line of protective relays. I fixed 3, 150Y rate-of-rise relays just last week. They have a power supply, balanced input amp, differentiator, threshold comparator, timing circuit, output relay, and an SCR trip circuit. With all that built in, it is still considered a "relay". Don't forget the time delay relays that might keep your compressors from short cycling or give your fan motors a little time to spin down before reversing. That phase loss/reversal/under/over voltage monitor is a relay. I've used bistable impulse relays to replace Hg switches on an alternator table. I think we used a UHF coaxial relay if we had to switch our transmitter into "emergency multiplexing" mode. And finally, I still have to find a use for those stepping relays I picked up at the electronics surplus. They looked like they would be fun to play with at the time.
Joe

The old IBM accounting machines (402 and 407)could read a card, tabulate up to 20 columns, transfer data from all 80, save data from one card to the next and print out the results at the rate of 120 a minute, all done with relays, brush emitters and ratchet counters. These things were programed with a patch panel that had about 2000 holes you could plug a wire in to select various functions. The wire contact relays in these things could be up to 12 pole double throw. I have a few if someone wants to see one.

The primary purpose of a relay is to relay on a telegraph signal that's getting weak. That would be why it's called a "relay".

Every other use is an afterthought.

[This message has been edited by SolarPowered (edited 05-14-2006).]

... but there was a lot of "afterthinking"

I bet ol' Ben Franklin invented it!

Alan

quote:Every other use is an afterthought

... but there was a lot of "afterthinking"

So does that make any relay not designed for telegraphy, a "time delayed relay"?
Joe

My understanding of any relay is to isolate the Control circuit from the circuit being controlled by magnetic (not electrical) coupling.
Just in the same way that an Opto-isolator does with an LED and a photo-transistor.
Relays can be cascaded to provide any number of contacts (being NO or NC) or the use of higher voltage coils.

Indeed, there was! And is.


I could be wrong, but my recollection is that Samuel F. B. Morse invented the relay, for the purpose I described above.

All of the above as already mentioned by our other members.

Then have a look in your car.
Starter motor relay/solenoid. Quite often the ignition operates a small relay which triggeres the starter motor solenoid and operates the starter motor.

Then all your lights wiring, wipers , blowers, window heaters etc in the newer cars all controlled with relays so relatively short wiring looms can be used from the 12 Volts fuse box to avoid excessive voltage drops in 12 Volts car systems.

The control wires to the dash can be relatively thin and the switches can be light amperage rated.

As Mike mentiones the galvanic separation is a very important one too.
Different voltage levels from the coil voltages and contact voltage levels.

Extreme case in a grid substation a 24 volts scada relay triggers a 110 Volts coil, which operates a 220,000 Volts circuitbraker.

Plenty of uses for relays anyway

Cheers Ray

There is another configuration of relay which was commonly used for remote switching at one time. Instead of the control circuit connecting power to a coil to energize the relay when needed, power is kept applied to the coil all the time, but the flux is designed to be too weak by itself to operate the contacts.

A second coil is then connected completely independently to a control circuit. When the leads from that coil are shorted together, current flows due to the primary winding and core acting like a transformer. The increased flux then provides enough pull to operate the contacts.

Obviously that type of relay can be used only with A.C. on the coil.

Depends on the type of relay, these are standard device numbers and descriptions from IEEE

Device Number Function and Description
1 MASTER ELEMENT is the initiating device, such as a control switch, voltage relay, float switch, etc. which serves either directly or through such permissive devices as protective and time-delay relays to place an equipment in or out of operation.
2 TIME-DELAY STARTING OR CLOSING RELAY is a device that functions to give a desired amount of time delay before or after any point of operation in a switching sequence or protective relay system, except as specifically provided by device functions 48, 62 and 79.
3 CHECKING OR INTERLOCKING RELAY is a relay that operates in response to the position of a number of other devices (or to a number of predetermined conditions) in equipment. It allows an operating sequence to proceed, or to stop, or to provide a check of the position of these devices or of these conditions for any purpose.
4 MASTER CONTRACTOR is a device, generally controlled by device function 1 or the equivalent and the required permissive and protective devices. It serves to make and break the necessary control circuits to place an equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.
5 STOPPING DEVICE is a control device used primarily to shut down equipment and hold it out of operation. The device may be manually or electrically actuated but excludes the function of electrical lockout (see device function 86) on abnormal conditions.
6 STARTING CIRCUIT BREAKER is a device whose principal function is to connect a machine to its source of starting voltage.


Device Number Function and Description
7 ANODE CIRCUIT BREAKER is a device used in the anode circuits of a power rectifier for the primary purpose of interrupting the rectifier circuit if an arcback should occur.
8 CONTROL POWER DISCONNECTING DEVICE is a disconnecting device, such as a knife switch, circuit breaker, or pull-out fuse block, used for the purpose of respectively connecting and disconnecting the source of control power to and from the control bus or equipment. Note: Control power is considered to include auxiliary power that supplies such apparatus as small motors and heaters.
9 REVERSING DEVICE is a device that is used for the purpose of reversing a machine field or for performing any other reversing functions.
10 UNIT SEQUENCE SWITCH is a switch that is used to change the sequence in which units may be placed in and out of service in multiple-unit equipment.
11 RESERVED FOR FUTURE APPLICATION.
12 OVER-SPEED DEVICE is usually a direct connected speed switch, which functions on machine over-speed.
13 SYNCHRONOUS-SPEED DEVICE is a device such as a centrifugal-speed switch, a slip-frequency relay, a voltage relay, an undercurrent relay, or any type of device that operates at approximately the synchronous speed of a machine.
14 UNDER-SPEED DEVICE is a device that functions when the speed of a machine falls below a predetermined value.
15 SPEED OR FREQUENCY MATCHING DEVICE is a device that functions to match and hold the speed or the frequency of a machine or of a system equal to, or approximately equal to, that of another machine, source, or system.
16 RESERVED FOR FUTURE APPLICATION.
17 SHUNTING OR DISCHARGE SWITCH is a switch that @ to open or to close a shunting circuit around any piece of apparatus (except a resistor), such as a machine field, a machine armature, a capacitor, or a reactor. Note: This excludes devices that perform such shunting operations as may be necessary in the process of starting a machine by 6 or 42 or their equivalent, and also excludes device function 73 that serves for the switching of resistors.
18 ACCELERATING OR DECELERATING DEVICE is a device that is used to closes or to cause the closing of circuits that are used to increase or decrease the speed of a machine.
19 STARTING-TO-RUNNING TRANSITION CONTACTOR is a device that operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.
20 VALVE is one used in a vacuum, air, gas, oil, or similar line, when it is electrically operated or has electrical accessories such as auxiliary switches.
21 DISTANCE RELAY is a relay that functions when the circuit admittance, impedance, or reactance increases or decreases beyond predetermined limits.
22 EQUALIZER CIRCUIT BREAKER is a breaker that serves to control or to make and break the equalizer or the current-balancing connection for a machine field, or for regulating equipment, in a multiple-unit installation.


Device Number Function and Description
23 TEMPERATURE CONTROL DEVICE is a device that functions to raise or lower the temperature of a machine or other apparatus, or of any medium, when its temperature falls below, or rises above, a predetermined value.
24 RESERVED FOR FUTURE APPLICATION.
25 SYNCHRONIZING OR SYNCHRONISM-CHECK DEVICE is a device that operates when two a-c circuits arc within the desired limits of frequency, phase angle, or voltage, to permit or to cause the paralleling of these two circuits.
26 APPARATUS THERMAL DEVICE is a device that functions when the temperature of the shunt field or the amortisseur winding of a machine, or that of a load limiting or load shifting resistor or of a liquid or other medium, exceeds a predetermined value. It may also function if the temperature of the protected apparatus, such as a power rectifier, or of any medium decreases below a predetermined value.
27 UNDERVOLTAGE RELAY is a relay that functions on a given value of undervoltage.
28 FLAME DETECTOR is a device that monitors the presence of the pilot or main flame in such apparatus as a gas turbine or a steam boiler.
29 ISOLATING CONTACTOR is a device that is used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or test.
30 ANNUNCIATOR RELAY is a non-automatically reset device that gives a number of separate visual indications upon the functioning of protective devices, and which may also be arranged to perform a lockout function.
31 SEPARATE EXCITATION DEVICE is a device that connects a circuit, such as the shunt field or synchronous converter, to a source of separate excitation during the starting sequence; or one that energizes the excitation and ignition circuits of a power rectifier.
32 DIRECTIONAL POWER RELAY is a device that functions on a desired value of power flow in a given direction or upon reverse power resulting form arcback in the anode or cathode circuits of a power rectifier.
33 POSITION SWITCH is a switch that makes or breaks contact when the main device or piece of apparatus that has no device functions number reaches a given position.
34 MASTER SEQUENCE DEVICE is a device such as a motor-operated multi-contact switch, or the equivalent, or a programming device, such as a computer, that establishes or determines the operating sequence of the major devices in equipment during starting and stopping or during other sequential switching operations.
35 BRUSH-OPERATING OR SLIP-RING SHORT CIRCUITING DEVICE is a device for raising, lowering, or shifting the brushes of a machine, t for short-circuiting its slip rings, or for engaging or disengaging those contacts of a mechanical rectifier.
36 POLARITY OR POLARIZING VOLTAGE DEVICE is a device that operates or permits the operation of, another device on a predetermined polarity only, or verifies that presence of a polarizing voltage in equipment.


Device Number Function and Description
37 UNDERCURRENT OR UNDER POWER RELAY is a relay that functions when the current or power flow decreases below a predetermined value.
38 BEARING PROTECTIVE DEVICE is a device that functions on excessive bearing temperature, or on other abnormal mechanical conditions associated with the bearing, such as undue wear, which may eventually result in excessive bearing temperature or failure.
39 MECHANICAL CONDITION MONITOR is a device that function upon the occurrence of an abnormal mechanical condition (except that associated with bearings as covered under de-vice function 38), such as excessive vibration, eccentricity, expansion, shock, tilting, or sea] failure.
40 FIELD RELAY is a relay that functions on a given or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an a-c machine indicating abnormally low field excitation.
41 FIELD CIRCUIT BREAKER is a device that functions to apply or remove the field excitation of a machine.
42 RUNNING CIRCUIT BREAKER is a device whose principal function is to connect a machine to its source of running or operating voltage. This function may also be used for a device, such as a Contactor, that is used in series with a circuit breaker or other fault protecting means, primarily for frequent opening and closing of the circuit.
43 MANUAL TRANSFER OR SELECTOR DEVICE is a manually operated device that transfers the control circuits in order to modify the plan of operation of the switching equipment or of some of the devices.
44 UNIT SEQUENCE STARTING RELAY is a relay that functions to start the next available unit in a multiple-unit equipment upon the failure or non-availability of the normally preceding unit.
45 ATMOSPHERIC CONDITION MONITOR is a device that functions upon the occurrence of an abnormal atmospheric condition, such as damaging fumes, explosive mixtures, smoke, or fire.
46 REVERSE-PHASE OR PHASE-BALANCE CURRENT RELAY is a relay that functions when the polyphase currents are of reverse-phase sequence, or when the polyphase currents are unbalanced or contain negative phase-sequence components above a given amount.
47 PHASE-SEQUENCE VOLTAGE RELAY is a relay that functions upon a predetermined value of polyphase voltage in the desired phase sequence.
48 INCOMPLETE SEQUENCE RELAY is a relay that generally returns that equipment to the normal, or off, position and locks it out if the normal starting, operating, or stopping sequence is not completed within a predetermined time.
49 MACHINE OR TRANSFORMER THERMAL RELAY is a relay that functions when the temperature of a machine armature or other load carrying winding or element of a machine or the temperature of a power rectifier or power transformer (including a power rectifier transformer) exceeds a predetermined value.
50 INSTANTANEOUS OVERCURRENT OR RATE OF-RISE RELAY is a relay that functions instantaneously on an excessive value of current or on an excessive rate of current rise, thus indicating a fault in the apparatus or circuit being protected.
Device Number Function and Description
51 A-C TIME OVERCURRENT RELAY is a relay with either a definite or inverse time characteristic that functions when the current is an ac circuit exceeds a predetermined value.
52 A-C CIRCUIT BREAKER is a device that is used to close and interrupt an a-c power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.
53 EXCITER OR D-C GENERATOR RELAY is a relay that forces the d-c machine field excitation to guild up during starting or which functions when the machine voltage has been built up to a given value.
54 RESERVED FOR FUTURE APPLICATION.
55 POWER FACTOR RELAY is a relay that operates when the power factor is an a-c circuit rises above or falls below a predetermined value.
56 FIELD APPLICATION RELAY is a relay that automatically controls the application of the field excitation to an a-c motor at some predetermined point in the slip cycle.
57 SHORT-CIRCUITING OR GROUNDING DEVICE is a primary circuit switching device that functions to short-circuit or to ground a circuit in response to automatic or manual means.
58 RECTIFICATION FAILURE RELAY is a device that functions if one or more anodes of a power rectifier fail to fire, or to detect an arc-back or on failure of a diode to conduct or block properly.
59 OVERVOLTAGE RELAY a relay that functions on a given value of overvoltage.
60 VOLTAGE OR CURRENT BALANCE RELAY is a relay that operates on a given difference in voltage, or current input or output, of two circuits.
61 RESERVED FOR FUTURE APPLICATION.
62 TIME-DELAY STOPPING OR OPENING RELAY is a time-delay relay that serves in conjunction with the device that initiates the shutdown, stopping, or opening operation in an automatic sequence or protective relay system.
63 LIQUID OR GAS PRESSURE OR VACUUM RELAY is a relay that operates on given values of liquid or gas pressure or on given rates of change of these values.
64 GROUND PROTECTIVE RELAY is a relay that functions on failure of the insulation of a machine, transformer, Or of other apparatus to ground, or on flashover of a d-c machine to ground. Note: This function is assigned only to a relay that detects the flow of current from the frame of a machine or enclosing case or structure of a piece of apparatus to ground or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in the secondary circuit of a current transformer, or in the secondary neutral of current transformers, connected in the power circuit of a normally grounded system.
65 GOVERNOR is the assembly of fluid, electrical, or mechanical control equipment used for regulating the flow of water, steam, or other medium to the prime mover for such purposes as starting, holding speed or load, or stopping.
66 NOTCHING OR JOGGING DEVICE is a device that functions to allow only a specified number of operations of a given device, or equipment, or a specified number of successive operations within a given time of each other. It is also a device that functions to energize a circuit periodically or for fractions of specified time intervals, or that is used to permit intermittent acceleration or jogging of a machine .


Device Number Function and Description
67 A-C DIRECTIONAL OVERCURRENT RELAY is a relay that functions on a desired value of a-c overcurrent flowing in a predetermined direction.
68 BLOCKING RELAY is a relay that initiates a pilot signal for blocking of tripping on external faults on a transmission line or in other apparatus under predetermined conditions, or cooperates with other devices to block tripping or to block re-closing on an out-of-step condition or on power savings.
69 PERMISSIVE CONTROL DEVICE is generally a two position manually operated switch that, in one position, permits the closing of a circuit breaker, or the placing of an equipment into operation, and in the other position prevents the circuit breaker or the equipment from being operated.
70 RHEOSTAT is a variable resistance device used in an electric circuit, which is electrically operated or has other electrical accessories, such as auxiliary, position, or limit switches.
71 LIQUID OR GAS-LEVEL RELAY is a relay that operates on given values of liquid or gas level c on given rates of change of these values.
72 D-C CIRCUIT BREAKER is a circuit breaker that is used to close and interrupt a d-c power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.
73 LOAD-RESISTOR CONTACTOR is a contactor that is used to shunt or insert a step of load limiting, shifting, or indicating resistance in a power circuit, or to switch a space heater in circuit. It may be used to switch a light or regenerative load resistor of a power rectifier or other machine in and out of circuit.
74 ALARM RELAY is a relay other than an annunciator, as covered under device function 30, that is used to operate, or to operate in correction with, a visual or audible alarm.
75 POSITION CHANGING MECHANISM is a mechanism that is used for moving a main device from one position to another in equipment; for example, shifting a removable circuit breaker unit to and from the connected, disconnected, and test positions.
76 D-C OVERCURRENT RELAY is a relay that functions when the current in a d-c circuit exceeds a given value.
77 PULSE TRANSMITTER is used to generate and transmit pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving device.
78 PHASE-ANGLE MEASURING OR OUT-OF STEP PROTECTIVE RELAY is a relay that functions at a predetermined phase angle between two voltages or between two currents or between voltage and current.
79 A-C RE-CLOSING RELAY is a relay that controls the automatic re-closing and locking out of an a-c circuit interrupter.
80 LIQUID OR GAS FLOW RELAY is a relay that operates on given values of liquid or gas flow or on given rates of change of these values
81 FREQUENCY RELAY is a relay that functions on a predetermined value of frequency (either under or over or on normal system frequency) or rate of change of frequency.
82 D-C RE-CLOSING RELAY is a relay that controls the automatic closing and re-closing of a d-c circuit interrupter, generally in response to load circuit conditions.


Device Number Function and Description
83 AUTOMATIC SELECTIVE CONTROL OR TRANSFER RELAY is a relay that Operates to select automatically between certain sources or conditions in equipment, or performs a transfer operation automatically.
84 OPERATING MECHANISM is the complete electrical mechanism or servomechanism, including the operating motor, solenoids, position switches, etc., for a tap changer, induction regulator, or any similar piece of apparatus which otherwise has no device function number.
85 CARRIER OR PILOT-WIRE RECEIVER RELAY is a relay that is operated or restrained by a signal used in connection with carrier-current or d-c pilot wire fault directional relaying.
86 LOCKING-OUT RELAY is an electrically operated hand, or electrically, reset relay or device that functions to shut down or hold an equipment out of service, or upon the occurrence of abnormal conditions.
87 DIFFERENTIAL PROTECTIVE RELAY is a protective relay that functions on a percentage or phase angle or other quantitative difference of two currents or of some other electrical quantities.
88 AUXILIARY MOTOR OR MOTOR GENERATOR is one used for operating auxiliary equipment, such as pumps, blowers, exciters, rotating magnetic amplifiers, etc.
89 LINE SWITCH is a switch used as a disconnecting, load-interrupter, or isolating switch in an a-c or d-c power circuit, when this device is electrically operated or has electrical accessories, such as an auxiliary switch, magnetic lock, etc.
90 REGULATING DEVICE is a device that functions to regulate a quantity, or quantities, such as voltage, current, power, speed, frequency, temperature, and load, at a certain value or between certain (generally close) limits for machines, tie lines or other apparatus.
91 VOLTAGE DIRECTIONAL RELAY is a relay that operates when the voltage across an open circuit breaker or contactor exceeds a given value in a given direction.
92 VOLTAGE AND POWER DIRECTIONAL RELAY is a relay that permits or causes the connection of two circuits when the voltage difference between them exceeds a given value in a predetermined direction. I can also cause these two circuits to be disconnected from each other when the power flowing between them exceeds a given value in the opposite direction.
93 FIELD-CHANGING CONTACTOR is a contactor that functions to increase or decrease, in one step, the value of field excitation on a machine.
94 TRIPPING OR TRIP-FREE RELAY is a relay that functions to trip a circuit breaker, contactor, or equipment, or to permit immediate tripping by other devices; or to prevent immediate re-closure of a circuit interrupter if it should open automatically even though its closing circuit is maintained closed.
95-99 USED ONLY IN SPECIFIC APPLICATION WHERE NO OTHER NUMBER IS SUITABLE

The biggest difference between relays and contactors is current-carrying capacity.
Feeding power to motors and other equipment is a job for a contactor, relays rarely have ratings more than 20-30 Amps and don't handle inrush currents very well.
Control is another issue. The contactor can be closed by a pushbutton, but then you have to stand there and hold it depressed to keep the contactor closed. The contactor can be closed by a switch, but then how do you stop it remotely? Virtually all control systems use a contact from a small relay to close the contactor.
A pushbutton can easily be wired to close when a pushbutton is depressed, and STAY closed when that button is released. Opening the contactor only requires that the relay be opened by some method, which will usually include current sensors, limit switches on covers, emergcny switches, and a whole raft of other devices.
Also, since the only thing that the contactor does is close when power is applied to (2) wires, it can be in a separate compartment from the rest of the control circuitry; making everything safer for troubleshooting any problems that may arise.

You can also have a dual coil relay that acts like an "exclusive or". If you wire them at opposite polarity the relay will only pick if you have current on one coil and not the other (XO function). This is good as a check circuit to be sure if you have A you also have B.

Here's a typical light-duty, general purpose relay with cover removed. DPDT contact rated at a couple of amps or so. Base is about an inch long:

This is the IBM 12 pole Double Throw relay. I built an unbeatable tic tac toe machine with a few of these.




{ Edited to copy image to ECN server }



[This message has been edited by pauluk (edited 05-16-2006).]

pauluk,

You mentioned the AC remote-control relay that was operated by shorting a low-voltage winding.

Over the years I've needed exactly such a relay, and I remembered them from dozens of 1960's vintage hobbyist electronic magazines, but for the life of me, I couldn't remember what they were called or who made them.

And why don't they make them any more?

One of those in an X-10 size wall-wart with a pair of screw terminals would have hundreds of applications (think magnetic reed switch and fluorescent strip light that turns on when closet door is opened). Isolated, low-voltage, inherently safe, CHEAP!

A contactor and relay are nearly the same thing.

The difference is, per pole/throw, a relay has one fixed contact surface, and one moving contact surface, with the moving contact connected to its terminal with a flexible conductor, whereas a contactor has two fixed contacts per pole, and two conacts per pole on the armature, connected with a solid metal bar.

The starting relay on many cars is a contactor really.

Michael Faraday invented the soleniod, and by proxy the relay. First real use being the telegraph, by Morse, as it was the tapper on the other side. So I don't think from there it was too far of a jump to say the first inventor of the relay as we know it. Although, some dispute the real invetor of the telegraph to be Edward Davy, or Joseph Henry?

Zog,
If you typed all that without cutting and pasting from another source, you are without a doubt the most dedicated contributer to a forum that I have seen in years. Way to go!

Have those fingers put in traction.

[This message has been edited by WFO (edited 05-15-2006).]

John,


I can't remember a specific name or manufacturer either. I do remember picking up one of Tab Books ubiquitous projects books from the early 1970s when I was a kid which was projects almost entirely based around these relays: A remote low-voltage switch for a lamp, TV commercial silencer, a control for a pedestal drill with the secondary coil between the drill ground an insulated plate on the base so that the drill would start automatically when the bit touched the metal piece being drilled, etc.

Unfortunately, that book went on a clear out a good few years ago, so I can't check on the manufacturer.