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Basic Electromagnetic Relays
11 questions By Tony R. Kuphaldt
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Question 10 of 11
Design an experimental circuit for determining the pull-in and drop-out current values for an electromagnetic relay coil. Why would these statistics matter to us, when working with relay circuits?
Reveal answerThe circuit for measuring these values is quite simple. I leave the task of designing it in your capable hands!
Notes:The real challenge of this question, of course, is for the students to determine what in the world “pull-in” and “drop-out” current values are. Once these definitions have been found, it is a very simple matter to experimentally determine them for any given relay.
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Question 11 of 11
Find one or two relays and bring them with you to class for discussion. Identify as much information as you can about your relays prior to discussion:
- • Coil voltage
- • Coil resistance
- • Number of switch poles
- • Number of switch throws
- • Voltage rating of contacts
- • Current rating of contacts
Also, calculate the amount of current that will be drawn by the coil when energized.
Reveal answerIf possible, find a manufacturer’s datasheet for your components (or at least a datasheet for a similar component) to discuss with your classmates.
If your relay’s coil is designed for AC voltage rather than DC voltage, the amount of current drawn by it will be far less than what you have calculated by using Ohm’s Law, due to a property of coils known as inductive reactance.
Notes:The purpose of this question is to get students to kinesthetically interact with the subject matter. It may seem silly to have students engage in a “show and tell” exercise, but I have found that activities such as this greatly help some students. For those learners who are kinesthetic in nature, it is a great help to actually touch real components while they’re learning about their function. Of course, this question also provides an excellent opportunity for them to practice interpreting component markings, use a multimeter, access datasheets, etc.
