Now that the music festivals are getting closer this summer, I thought I would build some low-power dancefloor lights for my camp. The circuit I've found is really simple, but unfortunately I am not an expert on the field. enter image description here

The driver will accept 12V as the main current for the circuit, except the control input will be 5V from a microcontroller. The FET/BJT will make the circuit always run at a specific amount of current, depending on the value of R1.

Once i'm building the shown circuit on a breadboard, it seems to work pretty okay, however when I solder it to the PCB's I have strange behaviour:

At first, the FET is open for current flowing through it, even if there's 0V on the gate. How can that even be possible? Also, the current is not limited at all!

After a while (~an hour with too much current! I suspect the FET has been very hot) it starts to work as intended, and limits the current to what I calculated. However if I put the control signal to ground, it will not be able to completely shutdown the current flow, resulting in weak but clearly visible light from the LED's.

I really cannot figure this out. Temperature (afterwards) doesn't seem to affect anything. I've built 4 of these units now, since I though i had a loose connection somewhere, but I start thinking that isn't the issue anymore.

Any thoughts?

(I'm running 25mA through each chain of LED's, i.e. 100mA in total. The voltage drop across each LED is ~3.2V (green LED's). R1 is 5R6 and R2 is 10k)

  • \$\begingroup\$ Could the current source be having parasitic oscillations? Can you scope the base connections of the transistors to check? \$\endgroup\$
    – MattyZ
    Commented Jun 25, 2013 at 20:00
  • \$\begingroup\$ I dont have access to a scope, unfortunately \$\endgroup\$
    – Tagger
    Commented Jun 25, 2013 at 20:02
  • 1
    \$\begingroup\$ Try inserting some small (~1k) resistors in the lines going to the base/gate connections of the transistors and see if that makes a difference. \$\endgroup\$
    – MattyZ
    Commented Jun 25, 2013 at 20:06
  • 1
    \$\begingroup\$ @Bitrex 's suggestion re parasitic oscillation seems possible. | Anti oscillation diode: Place a Schottky diode gate to source reversed biased mounted as electrically close to FET as possible. | BS170 is marginal at Vgs = 5V. Much better FETS available. | Rdson rises as FET gets hot but should not be main factor. | FET dissipation will be ~= (Vsupply - 3 x VLED) x ILED =~ 3V x 100 mA = 300 mW.| TO92 version Datasheet for both allows 800 mW but SOT23 is 300 mW so you would be at limit. BUT thermal resistance TO92 is 150 c/w or about 50C at 300 mW. \$\endgroup\$
    – Russell McMahon
    Commented Jun 25, 2013 at 20:43
  • \$\begingroup\$ What happens if you lift the gate drive on JP1 to the 12v rail - does that work? If it does then it's the Vgs(threshold) that's too high of the FET as RM says. As temperature rises the Vgs(th) gets lower and the circuit works as expected but prior to that the FET isn't quite turning on properly and will get a little hotter than normal I suspect. \$\endgroup\$
    – Andy aka
    Commented Jun 25, 2013 at 20:50

2 Answers 2


Few things to check:

  • Are the grounds of the 12V and microcontroller supplies tied together? (they should be)
  • Is the MOSFET the right way round? (also the BJT)
  • Is the MOSFET gate floating at any point? (check wiring with multimeter for continuity between gate and uC pin)
  • As suggested by Bitrex, is there any oscillation?
  • Check the rest of wiring and also node voltages with multimeter to make sure things are as they should be - also make sure resistor values are correct

If you still can't find the issue, can you post an image of your board layout (pics/gerber) and measured voltages if possible.

  • \$\begingroup\$ Accepting your answer because it is a good checklist when debugging. They are all working now, after I changed to a stronger mosfet - maybe I made them all go into smoke and the behaviour was undefined, i'm not sure. \$\endgroup\$
    – Tagger
    Commented Jun 27, 2013 at 8:06

Place a 1m resistor between the gate and the source. This will ensure that the FET completely shuts off - it is also called a TERMINATOR. Once on, a FET would rather stay on then work at stopping the flow off current.


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