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Description.

Here is a very simple circuit that can b e used to check the hfe of transistors. Both PNP and NPN transistors can be checked using this circuit. Hfe as high as 1000 can be measured by using this circuit.The circuit is based on two constant current sources build around transistors Q1 and Q2.The Q1 is a PNP transistor and the constant current flows in the emitter lead. The value of constant current can be given by the equation; (V D1 -0.6)/ (R2+R4).The POT R4 can be adjusted to get a constant current of 10uA.

The Q2 is an NPN transistor and the constant current flows into the collector lead. The value of this constant current can be given by the equation; (VD2-0.6)/(R3+R5).The POT R5 can be adjusted to get a constant current of 10uA.This constant current provided by the Q1 circuit if the transistor under test is an NPN transistor and by Q2 circuit if the transistor under test is a PNP transistor is fed to the base of transistor under test. This current multiplied by the hfe flows in the collector of the transistor and it will be indicated by the meter. The meter can be directly calibrated to read the hfe of the transistor.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from a general purpose PCB.
• J1 and J2 are transistor sockets.
• The Zener diodes must be rated at least 400mW.

Description.
This is a simple automatic emergency light which remains OFF when the mains power is available and switches ON when there is a power failure. When mains power is available, it is stepped down by transformer T1, rectified by diodes D1&D2, filtered by the capacitor C2 and regulated by the 9V regulator IC 7809 to produce 9V DC. This 9V DC is used for charging the battery. While the battery is charging the diode D3 is forward biased and this makes the emitter potential of transistor Q1 lower than the base potential and this makes the transistor OFF. The lamp will not glow.

When AC mains is not available, the diode D3 will be reverse biased and this makes the emitter potential of transistor Q1 higher than base potential and Q1 becomes ON. The lamp glows and continues to glow as long as there is no mains supply.The capacitors C1 and C3 acts as decoupling capacitors.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on general purpose PCB.
• The transformer T1 can be a 230V AC primary, 12-0-12V seconadary, 500mA transformer.
• Battery B1 can be a 6V lead acid battery or five 1.2V NiCd cells in series.
• The fuse F1 can be a 0.5A one.

Testing SCR using a multimeter.
A multimeter can be used to test SCRs quite effectively. The first procedure is to check the diode action between the gate and cathode terminals of the SCR. This test is just like what you have done in the case of testing a silicon diode (see testing a silicon diode).

Now put the multimeter selector switch in a high resistance position. Connect the positive lead of multimeter to the anode of SCR and negative lead to the cathode. The multimeter will show an open circuit. Now reverse the connections and the multimeter will again show an open circuit.

Then connect the anode and gate terminals of the SCR to the positive lead of multimeter and cathode to the negative lead. The multimeter will show a low resistance indicating the switch ON of SCR. Now carefully remove the gate terminal from the anode and again the multimeter will show a low resistance reading indicating the latching condition. Here the multimeter battery supplies the holding current for the triac. If all of the above tests are positive we can assume the SCR to be working fine.

Circuit for testing SCR.

This is another method for testing an SCR. Almost all types of SCR can be checked using this circuit. The circuit is just a simple arrangement for demonstrating the basic switching action of an SCR. Connect the SCR to the circuit as shown in diagram and switch S2 ON. The lamp must not glow. Now press the push button switch S1 ON and you can see the lamp glowing indicating the switch ON of SCR. The lamp will remain ON even if the push button S1 is released (indicates the latching).If the above checks are positive then we can conclude that the SCR is fine.

Testing triac using a multimeter.
A multimeter can be used to test the health of a triac. First put the multimeter selector switch in a high resistance mode (say 100K), then connect the positive lead of multimeter to the MT1 terminal of triac and negative lead to the MT2 terminal of triac (there is no problem if you reverse the connection).The multimeter will show a high resistance reading (open circuit).Now put the selector switch to a low resistance mode, connect the MT1 and gate to positive lead and MT2 to negative lead. The multimeter will now show a low resistance reading (indicating the switch ON).If the above tests are positive then we can assume that the triac is healthy. Anyway this test is not applicable triacs that require high gate voltage and current for triggering.

Circuit for testing a triac.
This is another approach for testing a triac. Almost all type of triacs can be tested using this circuit. This circuit is nothing but a simple arrangement to demonstrate the elementary action of a triac. Connect triac to the circuit as shown in circuit diagram and switch S2 ON. The lamp must not glow. Now press the push button switch S1.The lamp must glow indicating the switching ON of triac. When you release the push button, you can see the lamp extinguishing. If the above tests are positive you can assume that the triac is healthy.