A friend and I - both electronics hobbyists - figured replacing the blown LEDs in his home exterior lighting fixtures couldn't be THAT hard... (pause for all to say 'yeah, it's not straightforward you two idiots!'). I'm seeing some behavior that I can't really explain or understand, so thought it was time to give in and seek some help.
Here's the basic setup, what we've tried, and what seems strange. Thank you all in advance for your help and insight.
The lighting fixture gets installed into standard 120V AC and its first component is a variable output power supply with the following specs:
- Vin: 108-135 VAC 50/60Hz
- Iin: .10 A Max
- Vout: 14-20 VDC
- Iout: 350 mA C.C.
- Max Pout: 7W
- PF: >0.92
The output leads from the power supply then feed an LED cluster - all surface mounted LEDs with some circuitry buried under the surface foil which I cannot see nor peel back without destroying the copper layer underneath - so I have no idea how the LEDs are wired. But that doesn't really matter, except I was hoping it would shine some light on why our replacement attempts aren't working...
Strange thing #1
I have two of these blown fixtures from my friend - call them A and B. I've tested the power supply on each, feeding 120V straight from the wall and hooking up a multimeter to the output leads from the power supply.
- A reads 27V
- B reads 29V
...both obviously higher than the rated 20V max output. Something seems wrong here... but we can't tell if the power supplies are both malfunctioning, or just rated incorrectly (as the power output is relatively stable, fluctuating +-.2V while read continuously, which seemed reasonable given possible 120V line fluctuations and this being a variable output transformer). We have plans to pull one of the working fixture and test that power supply, but have not yet.
Strange thing #2
My friend bought a 12V LED bank to wire in as a replacement a while back - obviously this will blow given the power output of the supply. But he noted that when he hooked it up to power supply A, it just blinked on and off. When I hook the same 12V LED bank up to power supply B, it lights continuously (although it will blow, and already smoked once, so I disconnect it pretty quickly - I know we're killing it... it's a burner at this point anyway). If I add any resistors in series with the LED bank to try to regulate the power/voltage down, the LEDs just blink, even if the resistor is too small to really have any significant effect on the current.
For my personal tests, I assembled 10 LEDs (2.8-3.2V) in series on a breadboard to handle the 29V output. When I connect my LEDs to power supply B, they just blink as well. It's actually probably more accurate to say they flash bright, then fade down to dark... flash bright, then fade down to dark... repeatedly. Adding any resistors into the series only makes the LED flash pattern dimmer, but the flashing pattern continues.
My research would suggest that the power supply is going into safety mode and cycling between attempting to power, and shutting down. I've measured the Amps draw for my LED cluster on a bench power supply and it appears to be well within spec for the fixture power supply:
- 10x LED Cluster @ 30V : 5 mA
I've also tried to test the Amps draw on the 12V LED bank when hooked up to the fixture power supply, but can't get a reading. This is what happens:
- 10A Setting: LEDs do not illuminate. 0.00 reading on meter.
- 200mA Setting: LEDs illuminate. 0L reading on meter.
- 20mA Setting: LEDs flash. 0L on meter.
- 2000µA Setting: LEDs flash. 0L on meter.
Much appreciated if you're still with me! The resulting questions we have are:
When we try to hook up new LEDs to the existing power supply, why do they blink and not illuminate continuously?
Is there a way to utilize this power supply and repair this lighting fixture with new LEDs?
Is there something we should be wary of with the power output on these power supplies being higher than the max spec? (I have to assume this is why the stock LEDs have been burning out)
Thanks again for any insight!