Transistor Driving a Lamp - Problem

[idreamz]

Hi everyone,

I'm working on an electronics project and here's the problem:

I'm using an Arduino microcontroller and I want to modulate the brightness of a bright 12V 50W Halogen Lamp.

I used a TIP120, which is a 5A Darlington Transistor and connected it up to the microcontroller as such. The specs say I need 20mA at 4V to the base to switch 5A, which is sufficient for my halogen lamp. The circuit diagram as I have wired it up is attached in the image below. The ground of the microcontroller is connected to - of the bridge rectified 12VAC supply.

So I modulate the PWM from brightness 0 to maximum, in a very slow frequency, like 0.2Hz. The halogen bulb does indeed vary from dim to bright as expected. However, it still remains fairly bright even when the PWM signal is 0 (i.e. no current to the base), and turns very bright when PWM is at maximum.

My question is why does the bulb still light up when no current is applied to the base? Did I wire something up wrong, miss out something, or is the TIP120 darlington just not suitable for this and I should use a MOSFET instead? If so, does anyone have any suggestions for other transistors or MOSFETs I can use capable of at least 5A at 12V?

Thanks for the help.

 

[MikeMl]

Do you have a scope? If so, look at the voltage at the u-controller pin when you think the lamp should be off. I suspect that the Pulse width is not going to zero.

What happens if you disconnect the 50Ω resistor from the pin?

Your choice of 50Ω is suspect. I would have used ~470Ω, and then put a 10K from base to emitter to guarantee that the Darlington turns off.

 

[BrownOut]

Most likely, the "0" output from the uP isn't quite 0V. Try connecting a 10K ohm resistor from the transistor base to ground.