Experiment no 6
Aim:-Identify the terminals of following components:Diode,Zener
Diode,Varacter diode,LED,Photo diode,BJT,PHOTO
TRANSISTOR,fet,ldr,solar cell,photocell,Opto-coupler,7 segment
display,relays.
Theory:-Terminals of Diode
Turn the multimeter to the diode setting (usually indicated by a diode symbol), and touch each
probe to one of the LED terminals. If the LED lights up, the positive probe is touching the
anode, and the negative probe is touching the cathode.
Terminals of zener diode
Zeners are tested the same way you would a regular diode. To recall, diodes behave like a switch
that is open in one direction but closed in the other. Before testing, make sure that the multimeter
is placed on the diode setting.
Measure the forward-biased voltage of the diode by placing the positive or red lead of
the multimeter on the diode's anode part. This is the side of the zener that is unmarked.
The negative or black lead of the multimeter should be on the cathode or marked side of
the diode. A forward-biased silicon diode should read 0.5 to 0.7 volts, so this is the
reading you should see for the zener.
To test the reverse-biased voltage, switch the multimeter leads. The multimeter should indicate
an overload or no voltage drop, indicating no current flow or infinite resistance.
Terminals of varacter diode
The symbol of the Varactor diode looks like a common PN- junction diode that includes two
terminals namely the cathode and the anode. And at one end this diode is inbuilt with two lines
that specifies the capacitor symbol.
Terminals of LED Diode
The larger end inside the led is -ve and the shorter one is +ve,
that is how we find out the polarity of a LED
Terminals of Photo Diode
The larger end inside thephoto diode is -ve and the shorter one is
+ve, that is how we find out the polarity of a photodiode
Terminals of BJT
We know that the Bipolar junction transistor has three terminals namely
1. Emitter (E)
2. Base (B)
3. Collector(C)
Identifying BJT Types:
Transistors may be NPN or PNP which are available in Plastic casing or Metal Can package. In plastic
casing, one side of the transistor is Flat which is the front side and the pins are arranged serially. To
identify the pins, keep the front flat side facing you and count the pins as one, two etc. In most
NPN transistors it will be 1 (Collector), 2 (Base) and 3 ( Emitter ). Thus CBE. But in PNP transistors,
the condition will be just reversed. That is EBC.
PNP NPN
In Metal can types, the pins are arranged circularly. Just see a Tab in the rim. In NPN type, the pin
close to the Tab is Emitter, the opposite one ,the Collector and the middle one, base. In PNP type
the pins are reversed. Pin close to the Tab is Collector.
TERMINALS OF Phototransistor
The longer of the two pins indicates the phototransistor’s collector terminal. The shorter pin
indicates the emitter, and it connects closer to a flat spot on the phototransistor’s clear plastic
case.
Terminals of FET
In PNP type the pins are reversed. Pin close to the Tab is Collector. To identify a Field Effect
Transistor, one should keep the curved portion facing him/her and start counting in anti clockwise
direction. The 1st one is the source, then the gate and then the drain.
Terminals of LDR
LDR is also a two-terminal electrical which can be connected either way, that means it have no
positive and negative terminal.
Terminals of Photocell
The convention is the red is the positive, black is the negative
Terminals of opto-coupler
The purpose of an optocoupler is to transfer signals from one circuit to another yet keep them
galvanically isolated.
I want to show you how to check if an optocoupler is working. So I’ve chosen one of the most
commonly used optocouplers ( PC123 – 4 pins) for the demonstration, but you can use the same
principle for all optocouplers ( note: check the datasheet first ).
Step 1
Using the diagram in the right identify the pins; first the anode and cathode of the LED ( in this
case pins 1 and 2 ), and then using an ohmmeter set on the ‘X1 Ohm’ domain, measure between
pins 1 and 2, and you should get one reading measuring one way and no reading the opposite
way (just like you check a diode). If you get a value either way or no value at all, then certainly
there is a problem with the LED, and you should find another optocoupler.
Step 2
If the LED is good then we should check the phototransistor, you could measure it with the
ohmmeter just like the LED between pins 3 and 4 ( the emitter and collector ), and you should
get a high resistance value both ways if the phototransistor is good. If you’ll get no reading at all,
is probably because most phototransistors have such high resistance between emitter and
collector that the ohmmeter can’t measure; if this is the case you could connect two ohmmeters
in series thus increasing the measuring domain; …although i think most don’t have two meters
so i recommend the ’empirical’ method, presuming you have a variable DC regulated power
supply.
“Empirical” method
Connect the ohmmeter ( X1K Ohm or X10K Ohm ) between emitter and collector ( 3 and 4 )
like this: red probe to collector and black probe to emitter. Now connect a resistor of a few
hundred ohms ( ~300 ohms ) in series with the LED anode, after this turn on the power supply
and start increasing the voltage from 0 to 2…3 volts, and you should be able to see on the
ohmmeter how the output resistance decreases as the input voltage increases and viceversa.
Terminals of 7 segment Display
For checking the display you require a simple tool that is your multi-meter. Steps follow while
testing seven segment displays-
1. Hold the display in your hand and identify the pin 1.
2. Now take multi-meter (Assumption followed red lead for positive and black lead for negative).
Set the multi-meter in continuity range.
3. Check for sound test (touch both the leads together sound will produce). Sometimes it may
possible, battery of your multi-meter become weak and we will be not being able to get the
display.
4. Put the Black lead of multi-meter on pin 3 or 8 both are common pin as they are internally
connected.
5. Now put Red lead of multi-meter on any other pin may be 1, 5.
6. If any of the segment glows then your display is common cathode.
7. If none of the segment glows than interchange the leads of multi-meter.
8. Connect the Red lead of multi-meter on pin 3 or pin 8 as both are common pin and internally
connected to each other.
9. Now put the black lead of the multi-meter on other remaining pin. If any of the segment glow
than your display is common anode, as in common anode positive pin is common and rest are
supplied with negative supply.
10. Check all segments of both common cathode and anode to ensure your display is working
properly.
Terminals of Relays
The typical usage of relay is to allow a low DC voltage circuit (circuit #1) to switch on or
off a high voltage (DC or AC) circuit (circuit #2) without direct electrical connection
between them. This means circuit #1 and circuit #2 are magnetically and mechanically
linked but not electrically connected.
Part 1: 5 pins relay (SRD-05VDC-SL-C)
1. Let’s start with determining the relay coil pins using a multimeter. Set the multimeter
to resistance measuring mode with a scale of 1000 ohm since the coil resistance
normally ranges between 50 ohm and 1000 ohm.
2. Turn back the relay to see pins located at its bottom part. Try to touch the probes of
multimeter on a pair of pins of the relay until the touched pair shows a resistance value
(but not zero).
**Only the relay coil pin pairs will show non-zero resistance value.
**Other pin pairs will show either zero resistance or infinite resistance.
**There is only 1 pair of coil pins found in this case
1)Top View 2)Bottom View
Figure 3: Coil pins of the relay is determined (in this case, the coil resistance is 70 ohm)
3. Since the coil pin pair is found. There is only 3 remaining pins to determine which are
the NC pin, NO pin and common pin. Set your multimeter to continuity test mode. Try
to touch the probes of multimeter on remaining pins to determine which are the pins
actually connected with each other.These pins actually consist of normally closed (NC)
pin/pins and common pin/pins. Based on this finding, the remaining pins must be
normally open (NO) pins.
**In this case, there is only 1 pair of pins found connected. Hence, the 3rd pin must be
the normally open (NO) pin.
**Since there is only 1 NO pin, there will be only 1 NC pin in the connected pair. The
other 1 remaining pin in the connected pair must be a common pin.
4. To differentiate NC pin and common pin from the connected pins, there is a need to
apply voltage across the relay coil so as to activate it. The required activation voltage
for the relay can be determined from its product code in the section of code enclosed
with a dash which contains a number suffixed with VDC as shown below.
Figure 4: The part of product code indicates relay activation voltage
Figure 5: Zero-ing the output voltage of power supply
5. Turn on the DC power supply. Without connecting its output terminal to coil pins of
the relay, please turn the voltage adjusting knobs until the voltage reading reaches the
zero value.
6. Then, connect the output terminal of DC power supply across the coil pins of the
relay using crocodile clips
7. Next, increase the output voltage by slowly turning the voltage adjusting coarse knob
only in clockwise direction until a “click” sound emitted by the relay is heard ( this
means that the relay is activated). In this case, although the activation voltage stated is
DC 5V, the activation voltage recorded is about 3.3V which is lower.
Figure 6: Connect the output terminal of power supply across the coil pins of relay
using crocodile clips
a) Connection of crocodile clips at the relay coil pins b) Connection of crocodile clips at
the DC power supply
Figure 7: Activation voltage of the relay during this demonstration
8. Then, with the multimeter set to continuity test mode, check which pin is now connected to
the normally open (NO) pin in this moment. That pin would be the common pin.
Figure 8: Determine the common pin of the relay when the relay is activated
9. Since the common pin is determined, thus another one remaining pin in the
connected pins detected in step 3 must be the normally closed (NC) pin.
10. Now, all the pins of relay have been determined as in Figure 9 below.
