Problems with transistor relay driver circuit

[august_nine]

Hi, I am currently experiencing some problem with my transistor relay driver circuit. My led U1 is able to light up after I close the switch S2. However, the led will blow out after 1-2 seconds. Does anyone know what is wrong with my circuit that causes this to happen?

[ejkaiser]

It should normally work. Is the transistor heating up?

[august_nine]

How do I know if it is heating up?

[ejkaiser]

Use a temperature sensor, e.g. your finger tip or a thermocouple. You can also check if the maximum ratings of the transistor are exceeded.

[wd8ivl]

LED's normally have a forward voltage of about 2.1V.  When the relay closes, you are applying 12V across it!  You need to find the forward current of the LED (from the spec sheet) and calculate the value of a dropping resistor to put in series with it to limit the current that flows through it.

[apok]

where is the current limit resistor?

[camerond]

What apok said. For a normal LED, I;d suggest to keep the current down to about 20 mA.

BTW, where did you get a 1 ohm capacitor?

Cameron

[LV_Pro]

He is using a modeling program, I'm pretty sure that is the max resistance of the relay's contacts, which look like a capacitor in this drawing, one reason I use the "curved plate" type of capacitor symbol.

[AnalogKid2DigitalMan]

This works for LED's too 🙂

https://www.sparkfun.com/products/retired/10622

-AK2DM

[august_nine]

Hi guys, thanks for the reply. I have implemented my circuit on a breadboard and I am able to get my relay to work, as I am able to hear it click. However, my led still won't light up even though I put a 2.2kOhm resistor between it and the 12V power supply. Is there something I am doing wrong in the connection at the back of the relay? Please advise. Thanks.