Purpose
The engineering systems often require control and maintenance. Therefore, the role of the electrical engineer cannot be underestimated. A qualified engineer has to possess knowledge of the electrical equipment operation, repair and facilitation, as well as perfect understanding of physics beyond the practical process. The report presents the description and functioning details of the relay systems and aims to assist an HR representative with non-engineering background in assessing the qualification of the candidates. The interview questions are included in the end of the report.
Discussion
A relay is an electrical switch that allows automatically turn on or off the equipment using the certain mechanism to move the switch. Fig. 1 presents the illustration of the Double pole double throw (DPDT) relay (Learning about Electronics).
The relay is useful when it is connected to the devices with wires. The connection is performed with terminals. A terminal refers to a plug that allows conection of the relay to the electrical circuit (the sequence of the electrical devices, joined with wires and fed by electricity). The DPDT relay uses four types of terminals, that are listed below.
1) NC refers to Normally Closed switch. NC terminal is connected to the device that has to be turned on when the relay is not supplied with power; on the other hand, when the relay is supplied with power, the device turns off.
2) COIL terminal are applied voltage to power the coils, that close the electrical circuit. One COIL terminal gets positively charged, and the other gets negatively charged, and the polarity does not make difference, unless diode is used.
3) NO, Normally Open switch, is the terminal responsible for powering the device. The device is turned on when the relay is powered, and turned off when the relay is turned off. 4) COM means common. COM and NO terminals are characerized with continuity, when the relay is powered; when it is not, COM and NC have continuity. COM is the first terminal that gets connected to the circuit (Learning about Electronics). The diagram of relay connection to the circuit is presented on Fig. 2
The assignment for a candidate might be about connecting the relay into the simple scheme to determine the type of the terminal (fig. 3).
The terminals have to be tested for intactness (physical damage).The relay gets connected to the electrical circuit. The voltage necessary to run the circuit is 12 V. As the testing device, the green LED lamp is used. The presented scheme has some limitations. The LED current is 20 mA (data obtained from the manufacturer, a characteristics of the device usually found in the instruction). The forward current is 2 V, and the voltage in the circuit is 10 V. The system requires additional resistance to prevent overload (UNLV Department of Physics & Astronomy). When the resistance is absent, the LED lamp gets overload and breaks down. The system requires resistance, which value is calculated as: (12 – 2) V / 20 mA = 500 Ohm (indicated in Fig. 2 as 500 Ω).
The relay is tested, particularly the terminals. The device and other equipment are inspected for physical damages. It should be noted, that not all the damages can be identified visually. Therefore, if there is a suspicion, more detailed technical interview has to be run.
The current of the device is obtained from the technical documentation or device labelling.
Then, the voltage test is run to check if current in the circuit is appropriate for the device. The resistance is calculated, if necessary. This step is important since the mistake in current or resistance calculation results in device damage.
The electrical system is assembled by connection of the device, resistance and relay according to the scheme as displayed in fig. 3.
The operation of the system is tested, namely if the relay operates properly. The relay has to turn the device on and off (UNLV Department of Physics & Astronomy).
Action
A candidate for the electrical engineer position has to be able to understand the theory of the relay application, as well as possess the skills for testing the equipment and assembling the circuits. The following questions are meant to help the manager to assess the qualification of the candidate.
Q1. What is the relay intended purpose?
A1. The relay automatically switches on or off a devices connected to them.
Q2. What are NO and NC terminals responsible for?
A2. NO and NC stands for Normally Opened and Normally Closed. When the relay is supplied with power, NO turns on the device, while NC turns on the device when the relay is not supplied with power.
Q3. What voltage has to be supplied to the circuit for relay to operate according to the circuit in the example (fig. 3)?
A3. The voltage is 12 V.
Q4. What is the role of resistance?
A4. It consumes the excess power and prevents the device from breaking down.
Q5. Can the circuit work without a resistance?
A5. Yes, if the device current corresponds the circuit current, or if the voltage provides the necessary current.
Q6. How is it possible to get to know current supply for the certain device?
A6. The current supply is the technical characteristics established by the manufacturer. It is available among other electrical characteristics on a label or a box, and it is certainly stated in the technical documentation or online.
Q7. Please list the meaning of the relay flowchart operations.
A7. The testing of the relay, device, and other equipment is necessary to identify the physical damage. The voltage supply is tested, current characteristics of the device are checked, the appropriate resistance is calculated and installed. Then, the electrical circuit is assembled, and the device of interest is connected. The relay is then installed into the electrical circuit, with all the terminals have to be correctly connected. Then, the power supply can be turned on and the operation of the circuit tested.
Q8. Please comment on the voltage test. Why is it necessary?
A8. The voltage test is meant to prevent the device breakdown. Often the current in the system is higher than allowable by the device, and therefore the additional device, the resistance, has to be installed to consume the excess power.
This report provides concise information on relay application with minimum complicated theoretical issues. The information should be enough to run an effective interview for an electrical engineer position. The qualified candidate should be able to answer these questions easily.
Works Cited
Learning about Electronics. “How to Connect a DPDT Relay in a Circuit.” Learning about Electronics, n.d. Web. 15 Feb. 2016.
UNLV Department of Physics & Astronomy. “Relay Basics.” UNLV Department of Physics & Astronomy, n.d. Web. 15 Feb. 2016.
