Hello, in this project i want to show you how to make a constant-current source which gives 20mA at the output.
Its also known as LEd driver, because many leds need 20 mA to light up. The input of the constant-current source can be between 5V and 15V DC, what is realy usefull for variable voltage sources. The unloaded output will be the same Voltage as the input -0,7V for the circuid, it will regulate itself when you burden it . You can place 1 to 7 LED´s in a row, without a resistor and without destroying the LED´s / the driver.
The LED will try to get the maximum aviable curent, what is 20mA, if the 20mA are reached the voltage will go down to the voltage which the LED need to have a currend of 20mA and stay there --> LED driver
Step 1: What Do You Need?
a circuit board
1x 39Ohm Resistor
1x 10kOhm Resistor
1x BC547B Transistor (Or equal)
2x 1N4007 Diode (Or equal)
Terminals (like the blue ones on the pictures)
LED´s to test/use it
voltage source to test/use it
Step 2: Assembly
Now solder it together, use the schematic. Importand, eighth at the transistor don´t permute the legs or maybe there will be curents around 200mA (i tested it!! ;) ).
The circuit board i used is 7x8 holes small one, you can make it smaller when you dont use the terminals, a cooling element is not necessary, because it will not heat up that mutch.
Step 3: Testing...
If you assembled it all right together you are ready to test it.
Add a Voltage source, and add a LED, the LED should light up :)
You can place 1 to 7 LED´s in a row, without destroying them and the driver, but then the voltages which the LED´s need added together dont have to exceed the input voltage -0,7V for the circuit.
Input voltage = 9V
Circuit = 0,7V
Red LED´s = 1,8V
0,7V 1,8V 1,8V 1,8V 1,8V = 7,9V < 9V OK
0,7V 1,8V 1,8V 1,8V 1,8V 1,8V = 9,7V > 9V Not OK
Thank you for Reading, feel free to Comment.
To test a LED
for bike lights, the dynamo generate different voltages at different speeds
If you are annoyed to calculate a resistor for a LED
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