Its about transistor

1. 1. Transistor Test Type and Polarity
You will need the following:-
 1 multi-meter that has a diode test function [Fig.2].
 1 9V battery
 1 1K resistor
 1 LED [Fig.3]
 Some hookup wire.
Now for the first part - determining whether the transistor is NPN or PNP. Grab your
multimeter, and set it to diode test. Now test all the different combinations of legs that the
transistor has:
Fig.1 Fig.2
If you label the three legs [Fig.1] leg 1, leg 2 and leg 3, then you test the continuity of:
1 to 2
2 to 1
1 to 3
3 to 1
2 to 3
3 to 2
If the transistor is working, then you will have gotten a value of about 0.5 - 0.7 volts for two of
the six tests. The rest of the tests should have said that there is infinite resistance between the
legs tested. If you get more than two positive results, or you get less than 2 positive results, then
the transistor is fried.
What's more, the two tests that return a positive result will have a common leg. ie: if you test a
BC550 transistor, you will get a result for testing legs 2 to 1, and also 2 to 3.
The common leg is the base of the transistor.

2. If the common leg conducts with the positive lead of the multimeter connected to it, then it is
a NPN variety.
If the common leg conducts with the negative lead of the multimeter connected to it, then it is
a PNP variety.
So for our BC550, the common leg is leg 2, and since the red probe was on Leg 2 when it
conducted, then it is an NPN transistor.
So how do you figure out which legs are the collector and emitter? This is a bit trickier, but still
not that difficult. You will need the rest of the components mentioned earlier. You need to build
this circuit:
Fig.3
What we have here is a simple continuity tester - a 9V battery, a resistor, a LED, and two
terminals, all in series. Short out the + and - terminals, and the LED will light up.
Before we go much further, I just thought you might like to know that transistors DO work
backwards - that is, if you mix up the collector and emitter of a transistor, then they will work.
Not very well, maybe, but they will work.
The way we figure 3 out which is the collector and emitter goes like this:
For the NPN transistor:
Connect the + terminal to one of the 2 mystery legs on our transistor, and connect the - terminal
to the other mystery leg.
Now connect your multimeter so that the black probe touches the 0 volt rail, and the Red probe

3. touches the base. You should see the LED light up. Note how bright it is.
Now take the transistor out of the circuit, and change the connections to the + and - terminals on
our circuit, so that both the mystery legs get a turn at being the one connected to the 0 volt rail.
Again, touch the black probe of the multimeter to the 0 volt rail, and the red probe to the base of
the transistor. Again, note how bright the LED is.
The emitter is the leg that, when connected to the zero volt rail, makes the LED glow the
brightest. And, since we only have three legs, the collector must be the one left over!
For the PNP transistor:
Connect the + terminal to one of the 2 mystery legs on our transistor, and connect the- terminal
to the other mystery leg.
Now connect your multimeter so that the red probe touches the + terminal of our circuit, and the
black probe touches the base. You should see the LED light up. Note how bright it is.
Now take the transistor out of the circuit, and change the connections to the + and - terminals on
our circuit, so that both the mystery legs get a turn at being the one connected to the 0 volt rail.
Again, touch the red probe of the multimeter to the + terminal of our circuit, and the black probe
to the base of the transistor. Again, note how bright the LED is.
The collector is the leg that, when connected to the zero volt rail, makes the LED glow the
brightest. And, since we only have three legs, the emitter must be the one left over!
Note the differences between the two testing procedures. Just remember that (in drawings) the
emitter is the side with the arrow on it, and when the transistor is in the right way, the current
follows the direction of the arrow.
Sometimes, with small signal high gain transistors it is difficult to figure out which way round
makes the LED the brightest. You could add a resistor between the multimeter and the base leg,
or set your multimeter to Ohms instead of diode test, or add another LED in parallel with the first
one, or by changing the resistor value.

4. Transistor as aSwitch
Objective: To understand the working of a transistor as a switch.
Materials Required:Bread board, Resistors (330 ohms to 1k ohms), 3v to 9v DC supply,
connecting wires, NPN and PNP transistors (BC548/ BC557).
Procedure:-
Wire the circuit as shown in diagram. Check the terminals of the transistor (T) and the LED (D) to
make sure no wrong insertion ismade. Connect the base of the transistor to the positive voltage by
connecting the red wire (W) to the base. D will illuminate now since electric current is allowed to
pass through the transistor.
Fig. 4 Transistor as a switch
Activity:
Based on the above observations and on your understanding of capacitors, analyze the working
of the circuit shown in Fig. 5 and Fig. 6. The components mentioned in the circuit are:
For NPN For PNP
D1 Safety diode Safety diode
PB Push-button Push-button
C1 10uF 10uF
R1 22k ohms 100k ohms
R2 470 ohms 22k ohms
R3 - 470 ohms
Q1 NPN 3904 PNP 3906
LED 5mm round 5mm round

5. Fig. 5 Transistor circuit to be analyzed
Fig. 6 Circuit using PNP transistor
Record the time trials for how long the LED stays on with different values of C1 in the table
listed below:
Capacitance Time 1 (sec.) Time 2 (sec.) Time 3 (sec.) Average time
(sec.)
10uF
100uF
1000uF