Relay :an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. Operating and wiring a relay Relays generally have 4 connectors (if they have 5, you'll ignore one of them). Refer to the schematic that came with your relay, or that may be printed on the side of the relay. The relay's switch connections are usually labelled COM, NC and NO: Connect to COM and NO if you want the switched circuit to be on when the relay coil is on. Connect to COM and NC if you want the switched circuit to be on when the relay coil is off. The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. The circuit must be able to supply the current required by the relay coil. Use Ohm's law to calculate the current: Relay coil current = supply voltage coil resistance For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. Switch ratings (voltage and current) Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. A signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection. A typical schematic of wiring a relay: In the schematic , the #3 connector would be connected to the battery's positive (+12v) terminal, and the #5 connector would power the accessories. The wire leading to the battery should have a fuse in it. Connectors #1 and #2 are used for controlling the relay. Connector #1 would switch the ignition on by providing voltage and current, and no voltage and current when the ignition is off. Connector #2 would go to ground. Solenoid :a transducer devices that convert energy into linear motion. A solenoid is a coil that pulls (or pushes) a metal rod called a plunger when current flows through it. There are many kinds of solenoids: pull and push types, with and without springs (to push back the plunger when current no longer flows), with and without latches, etc. Operating and wiring a solenoid There are two main types of solenoids. In pull type solenoids , the plunger is normally outside the solenoid because the spring naturally forces the plunger out. Yet when energized, the force 'pulls' the plunger into the solenoid. Push type solenoids are the opposite, in that the spring forces the plunger into the solenoid, but when energized the plunger is 'pushed' out. One of the main disadvantages of solenoids and especially Linear Solenoids is that they are "Inductive devices" which convert some of the electrical current into "HEAT", and the longer the time that the power is applied to a solenoid coil, the hotter the coil will become. Also as the coil heats up, its electrical resistance also changes. With a continuous voltage input applied to the coil, the solenoids coil does not have the opportunity to cool down because the input power is always on. In order to reduce this self generated heating effect it is necessary to reduce either the amount of time the coil is energized or reduce the amount of current flowing through it. One method of consuming less current is to apply a suitable high enough voltage to the solenoid coil so as to provide the necessary electromagnetic field to operate and seat the plunger but then once activated to reduce the coils supply voltage to a level sufficient to maintain the plunger in its seated position. One way of achieving this is to connect a suitable "holding" resistor in series with the solenoids coil, for example: However, the power consumed by the resistor will also generate a certain amount of heat, I2R (Ohm's Law) and this also needs to be taken into account. Another more practical way of reducing the heat generated by the solenoids coil is to use an "Intermittent Duty Cycle" . An intermittent duty cycle means that the coil is repeatedly switched "ON" and "OFF" at a suitable frequency so as to activate the plunger mechanism. Intermittent duty cycle switching is a very effective way to reduce the total power consumed by the coil. back to basic electronic circuits for robotics page |
Solenoid and Reley circuits |
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