11. We need to choose a transformer to power up a 12 V (DC) LED Strip of 150 LED
ID: 2249919 • Letter: 1
Question
11. We need to choose a transformer to power up a 12 V (DC) LED Strip of 150 LED/meter. The transformer will be operating from the mains power in Lebanon. The available LED strip is two meter long with the following specifications: LED power is 0.08 watt/LED and a block "A" that contains a rectifier circuit with a voltage regulator that convert approximately 8.5 Vrms to 12 VDc. a) Choose the transformers that are able to operate one LED strip from the table below. Justify your answer and show all needed calculations Turns ratio 25.88 10 25.88 10 25.88 10 25.88 10 25.88 10 25.88 10 25.88 10 Transformer type Current ratting (secondary) 1 AMP 1 AMP 1.5 AMP 1.5 AMP 2.5 AMP 2.5 AMP 5 AMP 5 AMP 15 AMP 15 AMP 25 AMP 25 AMP 50 AMP 50 AMP 2 3 4 5 6 7 8 10 12 13 14 b) Which transformer would you buy for the above application? Justify your choice by at least 2 criteria for selection. c) Now, we need to decorate a room of 6 meters long and 4 meters width with the same type of LED strips. All the sides of the room celling should be covered with LEDs i. How many LED strips we need? Justify your answer and show all needed calculations Choose the best transformer to operate ALL LED strips. Justify your answer and show all needed calculations ii.Explanation / Answer
Answer:-a) Labenon uses 220 V mains power supply. So we need a transformer to convert 220 V to 8.5 V, hence the turns ratio would be-
n = 220 / 8.5 = 25.88. Now the power for each LED to glow is 0.08 W and in one strip we have 150 LEDs, so to glow one strip we need power-
P = 150*0.08 W = 12 W. Hence current required for one strip is I = 12/12 A = 1 A. So we need to choose trnsformer having secondary current 1 A and turns ratio of 25.88. So transformer type 1 is suitable..
b) We need to buy type 1 of the transformer. The two points we need to consider are-
i) Transformer should be able to produce the required current,
ii) Tranformer should be able to produce the requied current.
c)i) So we need to decorate at the peripheral of a rectangle having length 6 m and width 4 m, so total peripheral length is-
L = 2*(6 + 4) = 20 m. Hence we need 20 LED strips since one strip is of 1 m.
c)ii) Total power for 20 LED strips is-
P2 = 20*150*0.08 W = 240 W. Current needed for this power with 12 V DC voltage is I = 240/12 = 20 A.
So we need a transformer of turns ratio 25.88 and of secondary current 20 A, hence transformer type 9 or type 11 can be chosen for this purpose. Since type 11 is providing more current than we require so we need to add a resistor in series with LED strip to control the current. If we choose type 9 due to less current LEDs would be less brighter than it should be.
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