1
\$\begingroup\$

I have a "GE 7.5 foot" pre-lit Christmas tree that was purchased from Costco some years ago. This tree has a small hard-wired controller box with four settings: white, white/colored (fades back and forth), colored, and off. Last year, the lights failed, and I replaced the 29v DC power supply with one from Amazon (TS-18WL29V) which was somewhat higher wattage. The new power supply worked for a short time (tens of minutes) and then the string failed again. I have determined that the power supply still works, but the controller box has failed. Connecting the DC power supply directly to the strings (of which there are approximately 8 on the tree) lights them up in the white color.

The controller box takes in the 29VDC and outputs... two wires. Opening the controller box shows what appears to be a set of MOSFETs in an H-bridge configuration. I am guessing that the LED "bulbs" on the tree actually contain two LEDs, one white wired in one polarity, and the other colored and wired in the reverse polarity. I have theorized that the controller box simply swaps the polarity of the power to the LED strings to choose between colored and white.

Is this accurate? Is this a "standard" configuration? Could I replace the controller with something easily (and cheaply) obtained? Is it safe to drive the LEDs directly from the DC power supply, or do I risk damage to them by driving them at too high a current, in the event that the old power supply's lower current limit was serving as a limit to the current in the LED string? If this is the case, could I set up a simple MOSFET driven via PWM to limit the current to something safer?

\$\endgroup\$
5
  • \$\begingroup\$ if there's only two wires along the string, it's how you describe. Use voltage to limit current: run them steady with a bench supply and ramp up until reasonable and note the voltage, then match that before the hbridge. The wire resistance will keep current roughly in check to where it was setup at first. \$\endgroup\$
    – dandavis
    Commented Nov 19, 2023 at 4:06
  • \$\begingroup\$ @dandavis given the power supply is/was 29v, do I need to limit the voltage to something lower than that? Were they being driven by the controller using some fancy scheme, or was the controller simply dumping the 29v directly into the strings? \$\endgroup\$
    – Mark
    Commented Nov 19, 2023 at 4:45
  • 3
    \$\begingroup\$ @Mark I think that's what dandavis is trying to get you to find out. By testing. Just pull the string out as a separate 'device', removed from the controller. Then run it using a lab supply where you can limit current and voltage and take it slowly. You can from this work out what voltage and current "looks right" to you. And you can also verify the polarity question, too. All is good then. If the voltages work out to about 29 V to operate them, you have your answer about that. If it works out to a lot less, then something else is also going on. Experiment. Find out. \$\endgroup\$
    – periblepsis
    Commented Nov 19, 2023 at 5:16
  • \$\begingroup\$ @dandavis Ok, I understand. Unfortunately, I had to take a break, but from experimentation, I have found that the strings light up to "acceptable" brightness at, say, 24VDC. At this point, I can simply drive the lights with a non-current-limited 24VDC supply and be safe? \$\endgroup\$
    – Mark
    Commented Nov 20, 2023 at 19:01
  • \$\begingroup\$ sounds about right. I would leave them on at 24v for a few hours while your around and if they aren't concerningly hot, it's fine. \$\endgroup\$
    – dandavis
    Commented Nov 20, 2023 at 19:13

1 Answer 1

Reset to default
0
\$\begingroup\$

I was thinking that these coloured LED strips were RGB and give white light when all the colours are on, but with two wires it's impossible. Do the LED strips have only one colour (when not white)? Unfortunately your power supply doesn't allow voltage adjustment. The LED control has probably a PWM control to adjust the current with a maximum equivalent to the current used with 29VDC directly. My recommendation is to use an adjustable SMPS and reduce the voltage to +- 27V. That will have the same effect as PWM regulation.

You can test the reverse current theory by adding a 22K resistor in series, for a very short time, and see if there is some very little coloured light (try in the dark). Try to see how the LED's are interconnected...

\$\endgroup\$
2
  • \$\begingroup\$ Yes, each LED is white when powered with one polarity, and some specific color when powered with reverse polarity. I have verified that at ~24v they light up to acceptable brightness. \$\endgroup\$
    – Mark
    Commented Nov 21, 2023 at 13:19
  • \$\begingroup\$ So use 24V and it should be ok. \$\endgroup\$
    – Fredled
    Commented Nov 21, 2023 at 14:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.