Re: Voltage Regulation for LED Lighting

In article <Xns9C11487C46FE1geoffschultz@xxxxxxxxxxxxxx>, Geoff Schultz <geoff"at"@geoffschultz.org> wrote:
A couple of years ago I installed DoctorLED
(http://www.doctorled.com/p24.htm) Mars Dome interior lights throughout
BlueJacket. This was the first shipment of these to the US and I literally
had to wait for them to get off of the container and into my hands before I
could depart for the NW Caribbean. They may have changed the design since
then, but this is what I've got.

Anyhow, about a year later I realized that they were getting dimmer and
dimmer and recently took a failed unit over to a friend who's an excellent
electronics engineer to determine what had failed. He found 2 problems with
the units:

1) There's a layer of thermal paste which is supposed to conduct the heat
between the circuit board that the LEDs are mounted on and the aluminum
backing plate which also functions as a heat sink. For all intents and
purposes, there was too little paste to provide contact between the two.
However, after running a working unit for about 10 minutes, there was no
appreciable heat buildup, so we don't suspect that my problem was due to
thermal overheating.

2) The basic design of the circuit is a transistor which limits the voltage
to 9V to 2 parallel stacks of 3 LEDs in series. This provides 3 V to each
LED, which is just fine. However, the transistor is too slow to handle fast
transient voltage spikes, and there's no voltage dump circuitry on the
boards to handle this. A spike probably occurred somewhere along the line
and fried some of the LEDs.

DoctorLED will sell me replacement circuit boards at about 1/2 of the cost
of the whole unit, which is still expensive. I plan to do that, but I want
to protect against this happening in the future. All of the lighting is on
2 separate circuits. What can I do to protect against this happening again?
I was thinking about a 12 V DC to DC power supply. Any suggestions?


-- Geoff
www.GeoffSchultz.org

I would try to add some capacitors across the 9 volt. Get what fits,
maybe a 100 uF electrolytic 25 volt. This does not rule
out a defect in the design circuit, which a dc/dc converter
may not fix.

I have been fooling with various high power leds. I had no problems
with the Lumiled devices, but some Cree devices wen bad
after I attached to the power supply. There is some kind of spike
that takes out these leds which are supposed to handle 1 amp peak.
There are nice leds with a nice warm glow. The warmest I have seen.
http://ledsupply.com/creexre-ww.php

greg
.

Relevant Pages

  • Re: High brightness white LEDs damaged by custom switcher
    ... you only drive one string of leds. ... In a chip, ... The LEDs are in series, so all see the same current, and the voltage ... I could also add a similar circuit to detect output ...
    (sci.electronics.design)
  • Re: Bicycle dynamo circuit
    ... There are four basic parts to the circuit: ... Posn 2 - detector ... Designed to drive a 12 Ohm bulb so voltage is quoted a 6v but will in fact rise to meet the Vf of the diode string providing you're going fast enough! ... The LEDs are Cree XP-Gs which have a Vf of about 3.25v at 700mA so maximum voltage reached should be about 13v ...
    (sci.electronics.design)
  • Re: Induction Heater Progress Posted
    ... >> You can eliminate that entire relay time delay circuit and RL3 ... >> are better ways to limit the turn-off leakage inductance spike. ... voltage is also slightly less than twice the supply voltage, ... resonance will be used little. ...
    (sci.electronics.design)
  • Re: Bicycle dynamo circuit
    ... There are four basic parts to the circuit: ... Momentary latch circuit to bypass three LEDs for when going slowly ... Place an LED power supply module where the 1K trickle resistor is, ... ditch the whole low voltage bypass circuit. ...
    (sci.electronics.design)
  • Re: 7805 failure mode
    ... and with about 1uF of load capacitance. ... output voltage again spikes high. ... used in this circuit, a hand-wired prototype, ... But it makes a voltage spike each time it starts up. ...
    (sci.electronics.design)