This circuit helps you easily identify a blown fuse without manually checking the fuse itself.
When the fuse is intact a green LED lights up signifying normal operation.
But if the fuse blows the circuit path opens and a red LED illuminates, indicating a blown fuse that needs replacing.
Circuit Working:
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistor | ||
| 1k 1/4 watt | 1 | |
| Semiconductors | ||
| Diode | 1N4148 | 1 |
| LEDs | ||
| Red | 5mm, 20mA | 1 |
| Green | 5mm,20mA | 1 |
| Fuse | 1 |
This circuit uses two LEDs one red, one green and a diode to indicate a blown fuse.
Here is how the circuit works:
When the fuse is intact and conducting current there is minimal voltage drop across it.
Most of the voltage appears across the 1N4148 diode forward voltage drop of around 0.7V.
This keeps the red LED off because the voltage at its anode is lower than the forward voltage needed for it to light up.
The green LED connected directly to the positive supply on the load side of the fuse receives full voltage and illuminates.
If the fuse blows the circuit opens and current flow stops.
With no current through the fuse voltage appears across it.
The voltage drop across the blown fuse is now significant compared to the 0.7V drop of the 1N4148 diode.
This voltage at the red LEDs anode is now enough to overcome its forward voltage causing it to light up indicating a blown fuse.
Since the circuit is open the green LED receives no voltage and turns off.
Formulas:
From the above circuit diagram the LED (green or red) glows is determined by the voltage drop across the resistor and LED in the circuit.
Selecting Resistors
You will need to select a resistor value that is suitable for the green LED.
This entails taking into account the intended current and forward voltage of the LED.
Ohms law may be utilized to determine the suitable resistance:
R = (Vsupply – VfLED) / ILED
where,
- VfLED forward voltage of the green LED
- R resistor value Ω
- Vsupply supply voltage V
- ILED desired current through the LED A
General recommendations for choosing resistors:
To get the forward voltage VfLED of your particular green LED, consult its datasheet.
Select a current ILED that is within the LEDs advised working range.
Find the resistance value that permits the green LED to light up brightly without going over its current rating by experimenting or using an online LED resistor calculator tool.
How to Build:
To build a Simplest Blown Fuse Indicator Circuit you need to follow the below mentioned connections steps:
Set up the PCB:
- On your PCB, identify the positive and negative supply.
Power Supply Connection:
- Connect the positive terminal of your power source to a row on the positive supply of the PCB.
- Do the same for the negative terminal of the power source to a row on the negative supply.
LED and Resistor Placement:
- Place the red LED and green LED on the PCB with their legs spread out in separate rows.
- Make sure the shorter leg cathode of each LED is on the same side.
Connecting the Green LED:
- Using a soldering wire connect the positive leg anode of the green LED to an empty hole on the same row as the positive power supply connection.
Resistor Connection:
- Place the 1k resistor on the PCB with one leg in a row adjacent to the common cathodes short legs of the LEDs.
- Connect the other leg of the resistor to an empty hole on the same row as the negative power supply connection.
Connecting the Red LED Cathode:
- Connect the short leg cathode of the red LED to the same row where the resistor leg is connected.
1N4148 Diode Placement:
- Place the 1N4148 diode on the PCB with the banded side cathode towards the positive power supply connection.
Connecting the Diode:
- Connect the banded leg cathode of the 1N4148 diode to the same row where the positive power supply is connected.
Connecting the Red LED Anode:
- Connect the long leg anode of the red LED to the same row where the unbanded side anode of the 1N4148 diode is located.
Fuse Connection: Important Safety Note:
- Ensure your power source is off before connecting the circuit to the fuse.
- Identify the terminals of the fuse you want to monitor.
- Connect one side of the fuse to the same row where the positive power supply is connected.
- Connect the other side of the fuse the load side.
Testing Double check everything is off before turning on:
- Turn on your power source.
- If the fuse is intact the green LED should light up indicating normal operation.
- If the fuse is blown the red LED should illuminate.
Safety Precautions:
- Always double check your connections before turning on the power source.
- Use a power source with a voltage appropriate for your LEDs typically 2 to 3V for small LEDs.
- Never work on live circuits.
- If you are unsure about any steps, consult with someone knowledgeable about electronics.
Conclusion:
The blown fuse indicator circuit described uses a red LED to indicate a blown fuse and a green LED to indicate a functional fuse.
This simple circuit can provide a visual indication of the fuses status making it useful for troubleshooting and maintenance in electronic circuits.