Monday, November 11, 2013

Simple Transistor Type and Lead Identifier

A bipolar junction transistor (BJT) has three regions, of which the emitter and the collector are made of the same type of semiconductor (‘n’ for npn and ‘p’ for pnp) but the base is of opposite type. If we consider the base and emitter terminals (or the base and collector terminals), we get a p-n junction diode. But if we hold collector and emitter terminals, we encounter two diodes connected back-to-back.

On forward biasing the p-n junction (with a battery and a current-limiting resistor), the diode starts conducting and the drop of voltage across it is very small (0.7 volt for Si). On reverse biasing, the diode stops conducting and the drop of voltage across it is very close to the supply voltage. But, practically no current will flow through the collector and emitter terminals when we make the collector positive with respect to the emitter or vice versa.
This is the basis of the first part of our experiment that helps determine the type of a given transistor (whether npn or pnp) as also its base. But we cannot distinguish between the collector and the emitter in this way. To do so, we have to understand another basic principle.

BJTs have an intrinsic property of current amplification. While operating in the active region, a small current injected into the base can produce a large current flowing between the collector and emitter. The ratio of the collector current to the base current is known as forward current gain βF.

Although the collector and the emitter are made of the same type of semiconductor, we cannot expect a large current gain if we treat the collector as the emitter and the emitter as the collector.

If we forward bias the collector-base junction and reverse bias the base-emitter junction, we call it the inverse mode of operation. Because of the structural difference and doping levels, inverse current gain βI is exceedingly small. Thus while βF can be of the order of hundreds, βI is of the order of a few units only. This principle will be utilised in the second part of our experiment to distinguish between the collector and the emitter leads.

At the heart of the circuit (shown in Fig. 1) is a comparator built around operational amplifier IC 741 (IC1). With the help of a divider network comprising R3 and R4 between the positive supply terminal and ground, the inverting terminal is kept at reference voltage Vref=VssR4/(R3+R4) or –VssR4/(R3+R4) depending upon the selection of contact +Vss (marked ‘npn’) or –Vss (marked ‘npn’) by SPDT switch S1. The voltage at the non-inverting terminal is taken from the bottom end of resistor R2 (1-kilo-ohm) marked V+.
If V+>Vref, the comparator output will be high (positive saturation state ≈ +Vss) and the red LED will glow. On the other hand, if V+<Vref,the comparator output will be low(negative saturation state ≈ –Vss) andthe green LED will glow. The transi-tion from one state to the other is verymuch abrupt and under no circum-stances can both LEDs be ‘on’ or ‘off.’

The first part of the experiment for determination of the transistor type may be carried out as follows:
1. Label the terminals of the transistor under test (TUT) as 1, 2 and 3 arbitrarily.
2. Keeping aside socket C, use only sockets A and B in this part of the experiment.
3. Select the contact marked ‘n-p-n.’ This makes the voltage at socket A positive with respect to socket B (GND).
4. Take any pair of terminals of the transistor and insert one terminal in socket A and the other in socket B. Observe which LED turns on and which remains ‘off’ and record it.
5. Next, interchange the terminals in the two sockets and again observe the status of the LEDs and record it.
6. Repeat steps 4 and 5 for the remaining two pairs and record all the six observations in the form of a table.

From Table I, first find out the pair of terminals for which only the red LED glows even when the terminals are interchanged. These are the collector and emitter terminals. You cannot, however, detect which one of these would be the collector or the emitter from this part of experiment.

Once you have separated the collector and the emitter leads, the remaining lead will obviously be the base of the transistor.

Right up to this point, we do not know whether the transistor is npn- or pnp-type, because the assertion made above will remain valid for the collector and the emitter both being either ‘n’ type or ‘p’ type. The only thing we know is that the base will be of opposite-type semiconductor.

Since we know the polarity of ‘A’ (positive w.r.t. ‘B’), we search from Table I which LED was glowing when the base was put in socket A and any one of the other two leads in socket B.

If the green LED was glowing the base is p-type and the transistor is npn-type, but if the red LED was glowing the base is n-type and the transistor is pnp-type.

A typical record is shown in Table I, from which we find that the transistor is npn-type and lead 2 is the base. But, lead 1 or lead 3 could be the emitter or vice versa.

To detect the collector and the emitter, proceed as follows:
7. If the transistor is npn-type, set the pole of switch S1 towards ‘npn’ marking. If the transistor is pnp-type, set the pole of switch S1 towards ‘pnp’ marking.
8. Next, insert the base lead in socket C, any one of the remaining two leads in socket A and the rest in socket B. Observe as before the status of the two LEDs.

For an npn-type transistor, if the green LED glows the lead inserted in socket A will be collector, but if the red LED glows it will be emitter.

On the other hand, for a pnp-type transistor, if the green LED glows the lead inserted in socket A will be emitter, but if the red LED glows the reverse will be the case, i.e., the lead will be collector.

The status of the LEDs for the above-mentioned npn-type transistor having lead 2 as base is shown in Table II. From this table, we find that lead 1 is collector and lead 3 is emitter of the transistor in the example.

Electronics Lab   created by Muhammad Irfan  
Electronics Lab   created by Muhammad Irfan 

How to use a relay

No comments:
A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.
The relay’s switch connections are usually labeled COM(POLE), NC and NO:
COM/POLE= Common, NC and NO always connect to this, it is the moving part of the switch.
NC = Normally Closed, COM/POLE is connected to this when the relay coil is not magnetized.
NO = Normally Open, COM/POLE is connected to this when the relay coil is MAGNETIZED and vice versa.
A relay shown in the picture is an electromagnetic or mechanical relay.
Fig. Relay and its symbol
There are 5 Pins in a relay. Two pins A and B are two ends of a coil that are kept inside the relay. The coil is wound on a small rod that gets magnetized whenever current passes through it.
COM/POLE is always connected to NC(Normally connected) pin. As current is passed through the coil A, B, the pole gets connected to NO(Normally Open) pin of the relay.
Here is an example,
First of all try the following circuit.
This is a dark sensor circuit.
Output of this circuit: When you block light falling on LDR, the circuit switches on the LED- D1.
Now, replace LED-D1 and R2- 330R with a relay and diode.
Reconfigure the circuit as shown in the figure below:
Note: In R3, you can keep any resistor from 330R to 4.7K, this resistor is for sensitivity of the dark sensor.
The following circuit also works as a dark sensor. When you block light falling on LDR, the relay gets activated and Pole of relay gets connected to NO pin that eventually gives power to LED- D1.
Fig. Dark sensor using two transistors and a relay.

Light sensor using relay and transistors

In this case, the configuration of relay has been changed. Here, NO (Normally open) terminal has been left open. In normal case, the D1-LED remains ON. When light falling on LDR is interrupted, pole of relay gets connected to NO terminal. Hence, NC (Normally connected) terminal does not get power and that switches the D1- LED off.
Fig. Light sensor using two transistors and a relay.
Connect to COM(pole) and NO if you want the switched circuit to be on when the relay coil is on.
Connect to COM(pole) and NC if you want the switched circuit to be on when the relay coil is off.


Fig. Dark sensor circuit for 220V powered lights.
A relay can be used to turn on lights working on 220V, AC. The AC powered light has to be connected to relay as shown in the picture above.
Fig. Connecting wires on relay
The following video shows a soldered/finished prototype.


Fig. Protection diode in the circuit
Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148or 1N4001) is connected ‘backwards’ across the relay coil to provide this protection.
Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.


06VDC- means that the voltage across the relay coil has to be 6V-DC.
50/60Hz- The relay can work under 50/60Hz AC.
7A, 240VAC- The maximum AC current and AC voltage specification that can be passed through NC, NO and pole pins/terminals of relay.

Experiments with 741- Operational amplifier

No comments:
Description: This versatile 741 operational amplifier module can be used for making a dark detector using an LDR, a photo transistor and a photo diode. The amplifier has been configured in inverting mode. It compares the change in voltage at pin 2 with the reference voltage at pin 3 and gives output at pin 6 accordingly. 

Project on mini breadbaord:
dark-sensor-using-741-and-LDR1 title=