Relay

Application Note: RELAY By: Isnaraissah Munirah Majilis (HK2006-6598) Date: 23rd March 2009 Abstract This application note explained what is relay, its basic design, how it works and why we used relay in the Electronic Dog Collar circuit. Relay is needed to solve our capacitor for its unable to recharge in a short circuit. Keywords: relay, electronic dog collar, capacitor. Objective: 1. To make an open circuit for capacitor so it will be able to recharge. 2. To show why relay is install to the EDC circuit Introduction A relay is an electrical operated switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier. Basic Design

Simple electromechanical relay 1

Small relay as used in electronics These two pictures above are example of a relay. It consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a moveable iron armature and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energised there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the Printed Circuit Board (PCB) via the yoke, which is soldered to the PCB.

How Does A Relay Operate? When an electric current is passed through the coil, the resulting magnetic field attracts the armature and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was deenergised, then the movement opens the contacts and breaks the connection, and the other way round if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing. 2

Why We Used Relay in Our Circuit? The Electronic Dog Collar (EDC) is design to shock a dog on its neck when the remote control is being pressed. But once the circuit has been built, the shock that we hoped for, occurred only once. This will cause just an immediate pain to the dog that will make our product not effective for dog training. This happened because of the capacitor in the circuit was unable to recharge. Capacitor was unable to recharge in a short circuit. Solution A typical 5V relay is installed at the output to produce open circuit. When it is an open circuit condition, the capacitor is able to recharge. It is able to charge and discharge continuously. When power is supplied to all the components in the circuit, including relay, it will produce magnetic field in the coil inside the relay and force to switch. This will let capacitor to charge. After few milliseconds, the relay will switch to another gate and capacitor discharge. Capacitor will discharge current and this current will flow to the output. This process is a continuous process and fast. Since this is a continuous process, the dog that wears the EDC will feel the pain as long as the trainer keeps on pressing the button. In this case, relay functions as an auto switching.

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Figure b: Electronic Dog Collar circuit. The red circle is the output which will be put at a dog’s neck. The right-side point is where the relay is installed. 4

Conclusion Relay can be used in the EDC circuit to produce open circuit. Capacitor is able to recharge in such condition. Enable a capacitor to recharge and discharge in continuous process, solved our previous problem where shock occurred only once when power is supplied. References 1. www.wikipedia.org/relay 2. Primetech Electronic Dog Collar Circuit

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