Problem with PNP switching circuit

eblc1388

Joined Nov 28, 2008
1,542
Without a current limiting resistor to limit the current to the LED, the LED and possibly the BD140 PNP transistor are both killed by overcurrent.

Learns how to calculate the resistor for a LED with different supply voltage so this won't happen again.

You can find out voltage across the LED(it depends on LED color) from the Internet, then apply simple Ohm's law. Subtracts this voltage from the supply voltage then divides the result by the current(usually 10~20mA) you want to use will give you the required resistor value.
 

Audioguru

Joined Dec 20, 2007
11,248
An LED that is burned out does not glow when the proper current-limiting resistor is added in series with it.
Get a new LED and transistor and don't forget to limit the current.
 

soda

Joined Dec 7, 2008
177
I bought new LED , New BD140 and tried the same, still not working..
Did you connect it the way i showed you? Make sure the LED anode is connected to the collector and the cathode to the 0v rail. On one side of the led is a flat spot which mark the cathode, the other side is the anode.
 

Thread Starter

shivaaa

Joined Jan 5, 2011
55
Thanks, its working now, i have added a 2K in base, and replaced the BD140.

Before this, I added 1K in base and closed the switch as per the circuit diagram, it was started glowing and stopped. again I tried but dint work. so now replaced the BD140 and working thanks for all your help.
 
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