Common Cathode Led being Switched By a transistor

Status
Not open for further replies.

RodneyB

Member
I am trying to switch on a common anode LED as per the inserted diagram. Having no luck and cant see what I am doing wrong




 
Your schematic shows common-anode LEDs. Which do you have?

If they are actually common-cathode then you need to reverse the supply voltage polarity and use PNP transistors.
 
Most ordinary 5mm LEDs are spec'd with a current of 20mA. The current-limiting resistor value for the RED LED is far too low:

1) A red LED is about 1.9V so when the transistor turns on and saturates with a voltage loss of 0.1V then the 270 ohm resistor limits the current to (12V - 1.9V - 0.1V)/270= 37mA which will melt a normal 5mm LED.

2) With 510 ohms the current is 19.6mA.

3) A new bright green or blue LED is about 3.5V. Then the 430 ohm resistors limit the current to 19.5mA.

Little transistors saturate well when their base current is 1/10th the collector current.
The collector current will be 19.5mA so the base current should be at least 1.95mA. (12V - 0.7V)/1.95mA= 5795 ohms. Use 5600 ohms for each base resistor.
 
Status
Not open for further replies.
Cookies are required to use this site. You must accept them to continue using the site. Learn more…