The objective of this construction project is to build a simple and easy to use Opamp Tester. This test gadget can be used to test the popular general purpose opamp type 741 and other such opamps that are pin to pin compatible to 741.These include the opamp type numbers much as LM 709, LM 201, MC 1439, LM 748, OP-02, LM 318, LM 356 etc. As an opamp finds its existence in almost every project of interest due to a large number of application circuits that can be built around opamps, a test gadget like the one hitheto described is certainly going to be an asset to an electronics experimenter or a hobbyist. Building this project will not only be another construction or learning exercise for them, it will become a very useful tool for them in the other projects they intend to construct. With this gadget available with them, they don’t have to randomly replace the opamps in case the circuit they have built with so much of enthusiasm happens to use a few opamps and is not working. They will be able to testate opamp before declaring it fit for use. One important point: Since the pray has an educational valuator, it could be a very good choice as a project for those doing diploma or an equivalent course in electronics.

 

CIRCUIT DESCRIPTION

A careful look at the circuit diagram will tell you that the opamp under test can be wired either as an INVERTING AMPLIFIER (with GAIN equal to R3/R2) or as a NON-INVERTING AQPLIFIER(with GAIN equal to 1+R4/R2) depending upon (With the position of the DPDT switch. The input to the amplifier in the two cases is a 3VDC. The circuit has its own regulated power supply to generate (+12V) and (-12V) for the opamp from the mains. The power supply circuit consists of a transformer with a center tapped secondary winding followed by a conventional full wave rectifier circuit for both positive as well as negative supplies .Rectifiers are followed by simple single element capacitor filters. Diodes D1 and D4 constitute the full wave rectifier for the positive supply while the filtering action is provided by capacitor Cl. Diodes D2 and D3with the same center tapped secondary

winding provide rectification for generating the negative supply and capacitor C2 does the filtering. lC-1 is a+12volt three terminal regulator of 78-series (7812 istle IC number) while lC-2 is a -12 volt three terminal regulator of 79series (7912 is to: IC number). Capacitors (C3, C4, C5 and C6 are decoupling capacitors. These are used to decouple DC power supplies for high frequency noise, if any, present on the DC power supply line. Resistor R1 and Zener diode VZI generate 3VDC used as a test input to the opamp.

CONSTRUCTION GUIDELINES

The components layout and the PCB layout are respectively shown in Figs. 6.2 and 6.3.

PARTS LIST

Resistors

R1 of 83 of 470 ohms, 1/2 Watt

1 K, 1/4 Watt

IK, 1/4 Watt

IK, 1/4 Watt .

R5 2.2 K, 1/4 Watt

R6 2.2 K, 1/4 Watt

Capacitors

C1:   (electrolytic)

C2:   (electrolytic)

C3:   (ceramic disc)

C4:   (ceramic disc)

C5:   (ceramic disc)

C6:   (ceramic disc)

Semiconductor Devices

IC-1: Three terminal voltage regulator type 7812.

IC-2: Three terminal boldest regulator type 7912.

Diodes

D1 to do IN 4001 or equivalent

LEDS

LED-1 and LED-2: The two LEDS should preferably be of different colours.

V                 :ZI 3V, 400mW zener diode

Transformers

T1               :Mains transformer (15-0-15,250mA)

Miscellaneous

1. ON/OFF toggle switch (SW1)

2. Double Pole Double Throw (DPDT) switch (SW2).

3. Fuse, 500mA 4. Spin IC Socket (Dual-lo-Line type)

TESTING GUIDELINES

1. Switch on the test gadget. Measure the regulated +12V, -12V and +3V DC.         Ensur e that all the DC voltages are available.
2. Keep the DPDT switch (SW2) in position l-l .
3. Switch off the power supply.
4. Insert the opamp (Take a good opamp) in the IC socket meant for the purpose.
5. Switch on the power supply. LED-1 glows indicating that the opamp is functioning properly in the inverting amplifier configuration. LEDQ should not glow while the switch SW2 is in position l-l .
6. Change the DPDT switch to position 2-2.LED-) extinguishes and LED-Z starts glowing. This confirms the operation of the opamp in the non-inverting mode.
7. Switch off the power supply.
8. Remove the opamp from the test socket.
9. Remember that a bad opamp will not work in either of these configurations. But even if the opamp exhibits a behavior where it works in one and not both of these configurations, it should still be rated as a bad IC.

IDENTIFICATION OF PIN CONNECTIONS

The pin connection diagrams of three terminal regulators are shown in  Figs.6.4 (for 7812) and 6.5 (for 7912).

If you enjoyed this post, make sure you subscribe to my RSS feed!