Transistors are really important parts of electronic devices.
They help make signals stronger, switch things on and off and do a lot of other tasks.
To make sure they are working properly it is important to test them.
This helps fix problems in circuits and find any broken parts.
In this article, we will look at an easy way to test transistors using the 555 timer IC which is a well known component.
This tester can check both NPN and PNP transistors and shows if they are working by lighting up an LED.
Circuit Working:
Parts List:
| Component | Specification | Quantity | Remarks |
|---|---|---|---|
| Resistors | 1k | 2 | 1/4 watt unless specified |
| 100Ω | 1 | ||
| 2k | 1 | ||
| Preset 10k | 1 | ||
| Capacitors | Ceramic 0.01μF | 1 | |
| Electrolytic 100μF 16V | 1 | ||
| Semiconductors | IC 555 | 1 | |
| LED (any 5mm, 20 mA) | 1 | ||
| Power Source | Battery 9V | 1 |
In this article the 555 timer is set up as an astable multivibrator, which means it creates a square wave output signal.
This signal goes to the base of the transistor we are testing.
The square wave makes sure the transistor turns on and off repeatedly while it is working.
For NPN transistors, you should connect the collector to point T1 the base to point T2 and the emitter to point T3.
For PNP transistors connect the emitter to point T1 the base to point T2 and the collector to point T3.
If the LED blinks, it means the transistor is working correctly.
The blinking shows that the transistor is switching between being fully on and fully off, matching the timers square wave.
If the LED blinks, the transistor is good and is functional.
And if the LED is off, then the transistor might be faulty or not connected right.
Formulas with Calculations:
Following are the formulas with calculations for 555 Timer Based Transistor Tester Circuit
Frequency of the 555 Timer (Astable Mode):
The 555 timer is configured in astable mode, and its frequency is determined using the formula:
f = 1.44 / ((R1 + 2 * R2) * C1)
where,
- R1 is 1kΩ
- R2 is 100kΩ (adjustable via VR1)
- C1 is 100μF
For example, if VR1 is 10kΩ:
f = 1.44 / ((1,000 + 2 * 10,000) * 0.0001)
The output frequency is approximately 0.686 Hz.
LED Current Limiting Resistor (R4):
The resistor R4 limits the current through the LED to prevent damage, the formula is:
R4 = (Vcc – V_LED) / I_LED
where,
- Vcc is 9V supply voltage
- V_LED is 2V forward voltage of LED
- I_LED is 20 mA current through LED
R4 = (9 – 2) / 0.02
R4 = 7 / 0.02
R4 = 350Ω
A standard 1kΩ resistor is used to ensure further safety for the LED.
Capacitor Charge and Discharge Times:
Charge time (t_charge): t_charge = 0.693 * (R1 + R2) * C1
Discharge time (t_discharge): t_discharge = 0.693 * R2 * C1
For R1 = 1kΩ, R2 = 10kΩ, and C1 = 100μF:
t_charge = 0.693 * (1,000 + 10,000) * 0.0001
t_charge = 0.763 seconds
t_discharge = 0.693 * 10,000 * 0.0001
t_discharge = 0.693 seconds
These times correspond to the high and low states of the square wave output.
How to Build:
To build a Simple 555 Timer Based Transistor Tester Circuit follow the below mentioned steps for connections:
- Assemble all the components as mentioned in the above circuit diagram.
- Connect pin 1 of IC1 555 to GND.
- Connect one terminal of VR1 to the junction of R1 and pin 2 of the IC1 and connect the other terminal of VR1 to the positive terminal of capacitor C1 and the negative of C1 to GND.
- Connect pin 3 of IC1 555 to the T2 base of transistor under test which can be a NPN BJT or PNP BJT
- Connect pin 4 and pin 8 of IC1 555 to positive supply of 9V.
- Connect pin 5 of IC1 555 to GND through capacitor C2.
- Connect pin 6 of IC1 555 to pin 2 of IC1 555.
- Connect pin 7 of IC1 555 between the junction of resistor R1 and R2 in series .
- Connect T1 collector of transistor under test to positive supply through LED1 and resistor R4.
- Connect T3 emitter of transistor under test to negative supply of 9V battery
Conclusion:
This Simple 555 Timer Based Transistor Tester Circuit is a great way to check if NPN and PNP transistors are working properly.
It uses the IC 555 timer to create a steady square wave signal, which makes testing reliable.
With just a few parts and simple assembly, it is a perfect project for anyone interested in electronics and a useful tool to have in your testing kit.