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Need help with the working of this circuit

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Topic starter

Hello everyone,

I found this circuit in the "100 transistor circuits" ebook. Can somebody explain the working? 

Thank you

mertozkan 13/05/2024 4:47 am
This post was modified 2 months ago by mertozkan

@nathan This circuit diagram is for a simple LED flashlight controlled by a push button switch. Here's how it works:

 

1. Power Supply: The circuit is powered by two 3V batteries connected in series, providing a total of 6V.

 

2. Switch: When you press the button ("Push ON"), it completes the circuit, allowing current to flow.

 

3. Capacitor and Diode: - The 100u capacitor is used to smooth out any voltage fluctuations, acting as a buffer to provide a steady voltage to the LED. - The 1N4148 diode prevents current from flowing back into the capacitor, ensuring that the capacitor discharges only towards the LED.

 

4. Resistor (100k): This resistor limits the base current going into the transistor. It ensures that the transistor is not damaged by excessive current and controls the transistor's switching action.

 

5. Transistor (BC547): This acts as a switch. When current flows through the 100k resistor to the transistor's base, it allows a larger current to flow from the collector to the emitter of the transistor, powering the LED.

 

6. LED: The "Super-bright White LED" lights up when the transistor conducts. LEDs are diodes that emit light when current flows through them in the correct direction, as indicated by the arrow in the symbol.

 

7. Resistor (15R): This 15-ohm resistor is connected in series with the LED to limit the current flowing through the LED, protecting it from burning out by excessive current.

 

In summary, when the push button is pressed, it completes the circuit allowing current to flow. The current passes through the 100k resistor to the transistor's base, turning it on. This allows a larger current to flow through the transistor to the LED, lighting it up. The resistor in series with the LED ensures that only a safe amount of current flows through the LED. The diode and capacitor work together to manage the voltage and current supplied to the LED for steady operation.

nathan Topic starter 13/05/2024 6:07 am

Thank you for the detailed analysis of the circuit and each component used. But can you explain this where you said, "The 1N4148 diode prevents current from flowing back into the capacitor, ensuring that the capacitor discharges only towards the LED." Based on this the polarity of the diode should be in reverse because in its current state when it discharges, the current can easily flow through the diode.
Also, why use a capacitor here? Even without it, the circuit will work just fine, right?

Sebastian 14/05/2024 5:05 am

@nathan If you look closely- when the button is pressed, the capacitor is actually connected across the battery. This ensures that there are no voltage fluctuations across the load(LED + transistor). Diodes are there just for discharging the capacitor without damaging the transistor.

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