First time posting on this, so apologies if I mess anything up. I'm having an issue with my circuit that I'm having difficulty figuring out. I am using a TIP120 transistor to turn an LED on/off. The LED has a max operating current of 1A. I'm using a 1A buckpuck LED driver to power the LED, and this is a diagram of what my circuit looks like (wasn't sure what to do with the Arduino, sorry, so I just subbed in a voltage source):
The LED turns on and off when it's supposed to (I'm using an Arduino output pin connected to the TIP120 base through a small resistor). However, the brightness of the LED will sometimes start off very low, and over time (30s to a couple minutes) ramp up in brightness. I'm not sure what is causing this delay- I've tried with multiple LEDs and the behavior is the same, so I don't think it's that. My guess is that somehow I'm messing up the transistors?
I've linked to the product info for the buckpuck I'm using: http://www.luxdrive.com/content/3021-BuckPuck.pdf
And the LEDs: https://www.ledsupply.com/leds/luxeon-rebel-color-leds
I'm also powering the buckpuck with a 12V 4.20 A DC power supply.
Hopefully I didn't miss anything, would appreciate any advice. Thanks!
This is the buckpuck configuration I'm using, with an external pot and a switch to control intensity. Without the transistor, this circuit works as expected. However, as soon as I add a transistor in the configuration posted above, the LED intensity becomes unreliable. The LED- terminal of the buckpuck is grounded, while the LED+ terminal of the buckpuck leads to the anode of the LED, and the cathode of the LED is connected to the collector.
If it helps, this is a screenshot of my Eagle schematic showing some more of the connections. I will try to make a more complete diagram once I have a mouse again. The LED560 label on the base connects to an Arduino GPIO pin via a 330 ohm resistor.
Thanks everyone for the quick answers. Configuring the circuit as in Figure 13 of the buckpuck datasheet works, and I now understand why it was a bad idea to use the TIP120, especially since the buckpuck driver already allows for logic-level control.