Identifying Components

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle before we start. Four tiny parts lie mixed up on your lab bench, and each has just two legs. One is a resistor, one is a diode, one is an LED, and one is a capacitor. You have only a multimeter. Can you tell which is which? And look at the resistor. There is no number printed on it, only four coloured stripes: green, brown, red and gold. Yet those stripes tell you its value, and even how accurate it is. Questions like these do come up in experimental skills in JEE, so let's begin.

0:37Method one: tell them apart by appearance

Look at the figure: four parts, side by side. On the far left is the resistor, a small body with coloured bands, and two similar leads. Okay? Next is the diode, a small dark cylinder. See the light band near one end? That band marks the cathode. In the figure, the capital A on the left stands for the anode, and the capital K on the right stands for the cathode. Right? Third is the LED, short for light emitting diode. Look at its two legs in the figure. They are not equal, and the longer leg is the anode. Its body is a clear or coloured dome, the rounded cap you see on top. Clear? On the right are two capacitors: a can, and a round disc. The figure calls the can electrolytic, and the disc ceramic. Each one has two leads, and its capacitance is usually marked on it. With me? Now suppose you pick up a device with three legs. Is it one of these four parts? No, it is none of them. With three legs, it is a transistor. Got it?

1:42Method two: the multimeter, probes both ways

Now the second method. Set the multimeter on its resistance range. Touch the probes to the two leads, then swap them over. Both ways round, okay? Now, the two leads of a resistor are just alike. So when you swap the probes, what will it show? Yes, it shows the same steady reading both ways. Same steady reading, both ways round, means a resistor. Right? Now the diode. It conducts for one connection of the probes, and not for the other. So it conducts one way only, and it gives no light. Okay? But wait. The LED is also a diode, so it also conducts one way only. Then how will you tell these two apart? You watch for light, yes, exactly. When it conducts, the LED glows. So one way only, with a glow, tells you it is an LED, got it? Now the trickiest one, the capacitor. You touch the probes, and the needle gives a kick. So it conducts, like a resistor, right? No, not like a resistor, and this is the trap, so be careful. The kick is brief, and it dies away to no current. And it happens both ways round. That is the capacitor. With me?

2:56Reading a resistor: the colour code

Now back to our stripes. On a resistor, every colour stands for a digit. Black is zero, brown one, red two, orange three, and yellow four. Okay? Then green five, blue six, violet seven, grey eight, and white nine. Ten colours, ten digits, in order. Right? Now, how do we read the bands? The first two bands are digits, and the third band is the power of ten. Here is a simple trick: for a colour from black to white, the third band tells you how many zeros to write after the two digits. Clear? But gold and silver have no digit in our list. So what if the third band is gold? Then it means times zero point one, and the cross on screen is the times sign. And a silver third band means times zero point zero one. Got it? Finally, the last band is the tolerance band. Gold means five per cent, silver means ten per cent, and no band at all means twenty per cent. The little sign after each number is the per cent sign. So notice, gold means two different things, depending on where it sits. Okay?

4:06Example 1: read green, brown, red, gold and its limits

Now, remember the stripes from the start? Let's read them. The resistor has bands green, brown, red and gold. What is its value, and between what limits does its resistance lie? Pause the video and try it first. Done? Okay, let's check. Green is five, and brown is one. So the first two digits give fifty one. Clear so far? Now the third band is red, and red means two. So the value is five, one, two, which is five hundred and twelve ohms, right? No, it is not, and that is the classic trap. The third band is not a digit; it is the power of ten. So red means times ten to the power two, the small raised two. Okay? So fifty one times ten to the power two gives five thousand one hundred ohms. The Greek letter omega is the ohm sign, and the small k in front of it means kilo, a thousand. So that is five point one kilo-ohms. With me? And the fourth band is gold, so the tolerance is five per cent. The sign on screen, a plus over a minus, is read as plus or minus. So the answer is five point one kilo-ohms, plus or minus five per cent. Clear? Now the limits. Some students write five thousand one hundred, plus or minus five ohms. Is that right? No, it is not. Five per cent is a part of the value itself. Five per cent of five thousand one hundred ohms is two hundred and fifty five ohms. Got it? So subtract it, and add it. Five thousand one hundred minus two hundred and fifty five gives four thousand eight hundred and forty five ohms. And five thousand one hundred plus two hundred and fifty five gives five thousand three hundred and fifty five ohms. The resistance lies between these two, okay?

6:02Example 2: name P, Q, R, S from multimeter tests

And now, the puzzle from the start. Four components, each with two leads, are tested on the resistance range, probes both ways. Component P gives the same steady reading both ways. Component Q conducts one way only, with no light. For component R, the needle kicks, then returns to show an open circuit, either way. And component S conducts one way only, and emits light. Name all four, and say why R kicks. Pause the video and try it first. Done? Okay, let's check. Component P gives the same steady reading both ways round, so it is a resistor. The double arrow on screen is read as implies. Right? Component Q conducts one way only, and gives no light, so it is a diode. Now look at component R: it moved the needle, so is it another resistor? No, it is not, and that is the same trap again. The kick dies away to an open circuit, and it does so both ways round. So component R is the capacitor. Clear? But a capacitor does not let a steady current through. So why does the needle kick at all? Because the meter has a small battery inside it. When you touch the probes, a momentary charging current flows from that battery, and it stops once the capacitor is charged. Okay? And the last one, component S, conducts one way only, and it glows. So it is the LED, the light emitting diode. All four done, in under a minute. See how fast that was?

7:41Quick revision: multimeter tests and the colour code

So, quick revision. On the multimeter, the same reading both ways means a resistor, and one way only means a diode, or an LED if it glows. Then, a brief kick that dies away, both ways round, means a capacitor. And for the colour code: two digits, then the power of ten, then the tolerance. Now go and try the practice problems on this page. You will find them easy. All the best!