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):

enter image description here

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.

enter image description here

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.

enter image description here

Edit 2:

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.

  • \$\begingroup\$ Which variant of the BuckPuck are you using? \$\endgroup\$
    – awjlogan
    Feb 11, 2018 at 22:16
  • \$\begingroup\$ Buckpuck is 3021, 1000 mA \$\endgroup\$
    – cyin
    Feb 12, 2018 at 3:38
  • 1
    \$\begingroup\$ See figure 13 in the datasheet- that's what you should be doing. \$\endgroup\$
    – τεκ
    Feb 12, 2018 at 3:43

2 Answers 2


You should either use a driver that has a control pin (the preferred solution) or if you must use a driver that has no control pin you'll have to switch the power to the module.

The LED wires should not be connected to anything other than the LEDs. You're interfering with the current sensing most likely, and opening up the connection to a current source will (at best) result in triggering protection circuitry designed to prevent overvoltage.

  • \$\begingroup\$ Oh, so the transistor could be messing with the driver's ability to regulate its current output? \$\endgroup\$
    – cyin
    Feb 12, 2018 at 3:40
  • 1
    \$\begingroup\$ Yes, but it's likely that what you are observing is due to the driver protecting itself. If you open-circuit an ideal current source the voltage increases without limit. You can't short it either (which would be okay for an ideal current source) because the driver has a minimum output voltage specification. \$\endgroup\$ Feb 12, 2018 at 9:58

Your schematic does not match any of the suggested schematics in the datasheet.

The buck regulator must have Vcc and Gnd connected all the time and use the control pin in any way that it is suggested for proportional or on-off control.

READ the datasheet until you fully understand each pin and terminate with the required connections. You don't need a TIP120 to control it, only 0 to 4.2V on the control pin.

  • \$\begingroup\$ Sorry, I lumped the buckpuck into a current source. I'm using configuration 12 from the datasheet, I'll update my question w/ the schematic. I just wasn't sure how to get a 6-terminal device in PSpice. Re using a TIP120- I want external control of the current intensity, so I've got a pot wired across the REF and CTRL pins. I see that I could program the Arduino to provide this control voltage instead, but it'd be preferable on my end to get this setup working, and I'm not sure why it wouldn't be. \$\endgroup\$
    – cyin
    Feb 12, 2018 at 3:16
  • \$\begingroup\$ add details to question and dont use and switch \$\endgroup\$ Feb 12, 2018 at 3:18
  • \$\begingroup\$ Report all input and outputs \$\endgroup\$ Feb 12, 2018 at 3:20
  • \$\begingroup\$ This is the answer. Most likely what the OP wants is figure 13 as suggested by @τεκ. \$\endgroup\$
    – user57037
    Feb 12, 2018 at 5:31

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