Summary:First a bit of history about the remarkable development of this 'relay' device. Joseph Henry (1797-1878), a brilliant US scientist who invented and used the electro-magnetic relay in his university laboratory. His low-power electro magnet c
First a bit of history about the remarkable
development of this 'relay' device. Joseph Henry (1797-1878), a brilliant US scientist who invented and used the
electro-magnetic relay in his university laboratory. His low-power electro magnet could control a make-and-break switch
in a high-power circuit. Henry believed the potential was in the relay's use as a control system in manufacture, but
he was only really interested in the science of the electricity. The relay was a laboratory trick to entertain
students. Samuel Morse later used Henry's relay device, after re-designing it using thinner wire, to carry morse-code
signals over long kilometers of wire.
In 1846, Joseph Henry was professor of natural philosophy (physics) at the College of New Jersey (now known as Princeton University). He had published scientific articles on a wide variety of subjects, including electro magnetism, optics, acoustics, astrophysics, molecular forces, and terrestrial magnetism, but his reputation was built primarily on his work in basic and applied electro magnetism. Among his discoveries in electro magnetism were mutual induction, self-induction, the electro magnetic relay--enabling him to devise the first electro magnetic telegraph that could be used over long distances--and the concept of the electric transformer. He also invented the first electric motor. Henry was often referred to as the scientific successor to Benjamin Franklin. Today, it is the general opinion that Joseph Henry was the inventor of the telegraph and not Samuel Morse, who did not have a technical background to begin with. Samuel Morse adapted the ideas and inventions of Henry (and Vail) into his own and patented it, making him the owner.
It is certain that Joseph Henry was important to the history of the telegraph in two ways. First, he was responsible for major discoveries in electro magnetism, most significantly the means of constructing electromagnets that were powerful enough to transform electrical energy into useful mechanical work at a distance. Much of Morse's telegraph did indeed rest upon Henry's discovery of the principles underlying the operation of such electromagnets.
Below are a couple other thoughts collected from the Smithsonian Institute (of which Henry was First Secretary), and Harvard University:
"... Secondly, Henry became an unwilling participant in the protracted litigation over the scope and validity of Samuel Morse's patents. ..."
"... Joseph Henry began his research into electro magnetism in 1827, while he was an instructor at the Albany Academy in New York. By 1830, he achieved two major breakthroughs .... His first crucial innovation, which he demonstrated in June 1828, was to combine Schweigger's multiplier with Sturgeon's electro magnet to obtain an extremely powerful magnet. While Sturgeon loosely wrapped a few feet of uninsulated wire around a horseshoe magnet, Henry tightly wound his horseshoe with several layers of insulated wire. In March 1829 he demonstrated an electro magnet with 400 turns, or about 35 feet, of insulated wire. This magnet, Henry remarked later, "possessed magnetic power superior to that of any before known."
"... Henry did set out to demonstrate the practicality of an electro magnetic telegraph immediately after his paper appeared. His prototype consisted of a small battery and an "intensity" magnet connected through a mile of copper bell-wire strung throughout a lecture hall. In between the poles of this horseshoe electro magnetic he placed a permanent magnet. When the electro magnetic was energized, the permanent magnet was repelled from one pole and attracted to the other; upon reversing battery polarity, the permanent magnet returned to its original position. ... Henry caused the permanent magnet to tap a small office bell. He consistently demonstrated this arrangement to his classes at Albany during 1831 and 1832."
In short, this man, Joseph Henry, was a scientist and physicist by heart and a true teacher to his students. As a reward, they (not his students) stole his ideas and scientific experiments. For that reason I'm convinced that Morse was a fraud, preying on the ideas and inventions (like the telegraph) of others and putting his patent on it (obtained on October 3, 1837). I do credit Samuel Morse, however, for the morse-code, which is still used to this date and no person without a 5-wpm morse-code qualification exam can call him/herself a Advanced Radio Amateur! (see Fig. a).
Since this important day in 1836, when Joseph Henry transmitted a small electrical current down a wire to energize a remote coil and the completed circuit ringing a bell some 150 years ago, the relay has undergone steady evolution and today's relays are far removed from the crude and clumsy relays of Henry's days. In our millennium, solid-state has replaced the actual relay in many circuits, especially in AC applications. To sum it all up, the relay (and solenoids) have survived the tube, transistor, and integrated circuit (IC) eras.
Basically, a relay is an electrically operated switch, and actually the predecessor of the transistor. Solenoids are relays also but the very large types which carry huge amounts of current. Relays are the smaller types. Relays come in three types: electro mechanical, solid-state, and so-called hybrids which are a combination of the first two. There are also some specialized types that fall into neither category but I will deal with them later in this tutorial. Lets take electro-mechanical types first, they are available in three main models; armature, plunger, and reed. The Armature Relays are the oldest (see Fig. 1) but elegant. Plenty turns of very fine magnet-wire are wound around an iron core to form an electro-magnet. The movable metal armature has an electrical contact that is positioned over a fixed contact attached to the relay frame. A spring holds the armature up so that the movable and fixed contacts are normally separated (open). When the coil is energized, it attracts the pivoting armature and pulls it down, closing (make) the SPST contacts and completes the power circuit. Vice-versa, this relay can be made to open the contacts instead of closing them, or can do both either way. The armature relay is pretty old and no longer used in new applications, they do still exist however and are being used still at the time of writing this document.
The Plunger-Relay type have a plunger instead of a pivoting armature, as shown in Fig. 2 and uses a solenoid action to close the contacts. The electro magnetic core is hollow and a metal rod or plunger extends halfway through it when the relay is not energized.
When energized, the coil draws the plunger in and a shorting-bar, attached to the end of the plunger, closes the contacts. When the coil is de-energized, the spring (which is mounted over a section of the plunger) retracts the plunger and positions the assembly in the idle position breaking the contacts.
The coil plunger design allows much greater contact travel than the pivoting armature design, thus allowing wide contact separation. The increased space allows plunger systems to be used with higher voltages than armatures. The higher closing force of the solenoid permits the use of larger contacts and provides greater current handling capability. One application of a plunger-type relay is an electronic lock or door opener.
Since their inception, electro-magnetic relays have improved in sensitivity, switching complexity, current handling, response time, and reliability. It allowed for miniaturization in radios, cassette recorders, Radio Control(R/C), NASA, cameras, and other home electronics applications. Being relatively small, Reed Relays fill the demand for all sorts of 'small' stuff. The use of flexible reeds and self-attraction distinguishes the reed relay, see Fig. 3, from other electro magnetic relays. Note that the drawing shows the reed-relay without the actuator coil. The contacts are mounted on thin metal strips (reeds) and hermetically sealed in a glass tube. This tube is surrounded by a magnetic coil which, when activated, magnetizes the reeds and causing them to attract each other which closes the contacts. When the coil is de-energized, the spring tension in the reeds causes them to separate again. This type of design has the advantages of high speed operation, long life, and very low price. One of the great advantages reed relays share with other electro magnetic relays is the relative ease with which they can be fitted with multiple contacts. As in the armature and plunger designs, the contact mechanisms can be stacked to provide multiple circuit-close or open designs, or even a combination of both, all activated by a single coil. In reed relays, multiple contact pairs, but in individual tubes, can be stacked and used with a single coil in a very small space. This is not a feature of solid-state relays.
Some Relay-driver Examples using regular components:
Fig. 4 Sound-Activated type. The relay remains dormant until the op-amp activates upon sound via the electret-microphone. The input stage is a regular off-the-shelf 741 operational amplifier and connected as a non-inverting follower audio amplifier. Gain is approximately 100 which you can raise by increasing the value of R2. The amplified signal is rectified and filtered via C3, D1/D2, and R4 to an acceptable DC level. Potentiometer R5 is used to set the audio level to a desired sensitivity value to activate the relay via transistor Q1. Diode D3 is mounted over the relay coil to absorb sparks. The op-amp configuration in this particular drawing needs a dual voltage power supply which can be made from two 9-volt batteries. The circuit in Fig. 4a is similar but with less component.
Fig. 5 This relay driver boosts the input impedance with a regular 2N3904 transistor. Very common driver. It can drive a variety of relays, including a reed-relay. Transistor Q1 is a simple common-emitter amplifier that increases the effective sensitivity of the 12 volt relay coil about a 100 times, or in other words, the current gain for this circuit is 100. Using this setup reduces the relay sensitivity to a few volts. R1 restricts the input current to Q1 to a safe limit (the impedance is equal to the value of R1 plus 1K). Diodes D1 and D2 are EMF dampers and filter off any sparking when the relay de-energizes.
Fig. 6-1 is a delayed turn-on relay driver and can produce time delays for up to several minutes with reasonable accuracy.
The 14001 (or 4001) CMOS gate here is configured as a simple digital inverter. Its output is fed to the base of a regular 2N3906 (PNP) transistor, Q1, at the junction of resistor R5 and capacitor C2. The input to IC1 is taken from the junction of the time-controlled potential divider formed by R2 and C1. Before power is applied to the circuit, C1 is fully discharged. Therefore, the inverter input is grounded, and its output equals the positive supply rail; Q1 and RY1 are both off under this circuit condition. When power is applied to the circuit, C1 charges through R2, and the exponentially rising voltage is applied to the input of the CMOS inverter gate. After a time delay determined by the RC time constant values of C1 and R2, this voltage rises to the threshold value of the CMOS inverter gate. The gate's output then falls toward zero volts and drives Q1 and relay RY1 'ON'. The relay then remains on until power is removed from the circuit. When that occurs, capacitor C1 discharges rapidly through diode D1 and R1, completing the sequence. The time delay can be controlled by different values for C1 and R2. The delay is approximately 0.5 seconds for every µF as value for C1. The delay can further be made variable by replacing R2 with a fixed and a variable resistor equal to that of the value of R2. Taken the value for R2 of 680K, it would be a combination of 180K for the fixed resistor in series with a 500K variable trim pot. The fixed resistor is necessary.
Fig. 6-2 is configured in an automatic turn-off mode. It shows how the circuit function of Fig. 6-1 can be reversed so that the relay turns on when power is applied but turns off again automatically after a preset delay. This response is obtained by modifying the relay-driving stage for an NPN transistor like the 2N3904. It is worth noting again that the circuits in Fig. 6-1 and Fig. 6-2 each provide a time delay of about 0.5 seconds for every micro-Farad in the value of capacitor C1. This permits delays of up to several minutes. If desired, the delay periods can be made variable by replacing resistor R2 with a fixed and variable resistor in series whose nominal values are approximately equal of the total value of R2 (680K).