LED help for a beginner

[sonko2010]

Hi everyone,

I am working on a project where I am trying to illuminate a small flat stone sculpture that will be recessed into wood. The wood will have a small arch over the sculpture. My idea is to install 3 single LED's recessed into the arch to light the stone from above, and then mount a switch and battery pack to the back of the wood. I would prefer a neutral-white LED versus the cold white that I have in some of my flashlights. So lots of questions:

1. Where can I purchase several neutral-white LEDs? Just the LED's themselves if possible.

2. Will they remain cool enough not to burn the wood that they will recessed into? Don't want to cause a fire.

3. What kind of battery supply will I need. Will 2 AA's do the job? And for how long?

4. Can I also install a dimmer switch to adjust the intensity?

Thanks in advance for the help. I appreciate it.

Chris

 

[AnAppleSnail]

0: What kind of light are you overcoming? That will settle light balancing (don't want a nuclear sun under a sculpture in a dim gallery, or piddling glimmers outside)
1: I've had good luck at Cutter.com.au. You'll need to have enough voltage to supply whatever driver you have along with the LEDs.
2: That depends entirely on the drive level. Think of each LED as a heater at the drive power, and wood is an insulator. It'd be hard to retrofit this, but you might be able to drill into the wood and insert metal heatsinks. The heatsinks need a way to lose heat or else you just delay the inevitable. Speaking of hiding infrastructure, how will you hide these? Soldering wires to a bare LED is tricky (doable, but tricky)
3: Depends on drive level and driver efficiency. You can get cheap (spotty quality) constant-current drivers or (my suggestion) use a resistor to limit current (V = I*R)
4: There's a few options for dimming. Buy a multimode driver, or use that LM317.

Choosing the drive current is an interesting choice. You can get 90 lumens out of a cheap neutral-white Cree LED by driving it at 350 mA (1 watt LED power). But then you're heating a wood-mounted LED at 1 watt, and LEDs get dimmer and eventually cook when hot.
Using a resistor to limit current is easy, although current is then dependent on battery voltage; with alkalines it changes a lot over battery life. NiMH batteries have less voltage change (1.45v to .95v when dead-dead-dead). Remember you'll want to exceed the Vf of the LED at whatever current you're driving it at.

I'll run through one setup in my next post.

 

[AnAppleSnail]

Second post, for fear of losing it all in one go.
The Cree XRE isn't bad. The newer XP-G is more expensive but brighter at a given drive level (or more efficient for given output).

Scroll down to the list of radio buttons. The LED descriptions are as follows. I refer to charts in this PDF

[Flux bin] Flux [color space] Tint ([low]K to [high]K) (+cost in cents)
P4 Flux 7A Tint(3200k to 3500K) (+0.49)

This is a P4 flux bin LED. On that pdf, the first page identifies the flux bins - the output at 350 mA drive current (1 watt power). This LED will emit about 80.6 lumens. The flux bin is -entirely separate- from the kelvin temperature, although warmer LEDs tend to be dimmer - there's more phosphor globbed on. You'll want a nice Neutral White in a high flux bin, because these will allow you to get the light you want at lower power levels. Q4 isn't on the chart, but expect about 100 lumens at 350 mA.

Briefly, the datasheet:
Page 4 of the Cree datasheet has some facts about the Cree LED, including the maximum temperature the LED can withstand - an infrared thermometer can approximate this when you point it directly at the diode. Page 5 gives the spectral distribution, showing the Green Hole and the Red Tail, both of which contribute to the corpseriffic look given to people under (cheap, cool white) LED lights. Page 6 shows the "Luminous flux at given junction temperature." You lose a percentage of light with an overheating LED. Page 7 shows the "Drive current at a given voltage." This is how LEDs are usually killed; a high-capacity source at a slightly-too-high voltage will pour amps and amps through. Page 8 shows the increase in output for a given current; LEDs are more efficient at lower drive levels. Page 11 shows the LED itself. The page numbers may be different for other Cree LEDs. Luxeon releases similar information for its LEDs, I haven't worked with them as much.

Planning:
Let's say that this statue will be in a fairly bright area (but not outside), and that you want about 180 lumens of light coming from below the statue. With a bare LED there are no losses for optics or diffusers, so we just want 180 lumens split among 3 LEDs (60 per emitter). That requires about (according to page 8) 200 mA per LED. Using page 7, we see that the forward voltage is about 3.1v. The heat from each LED will be approximately Current times Volts = .2 Amps times 3.1 volts = .62 watts. How hot will each LED get? Not too hot, but we'll want to account for some loss of LED output. Page 6 tells us that a 100 centigrade LED die will lose about 20% output - that's the boiling point of water, hopefully nothing we'll lose. In testing you may find that setting the LEDs at exactly 200 mA isn't enough, and you'll adjust it.

LED Wiring:
LEDs can be wired in parallel or series. With 3, you have to pick all parallel or all series. You'll need (at least) 3 times the LED voltage in battery, or have triple the current, or have a boost driver (electronics that step battery voltage up by sucking extra current; ex. turn 1 amp at 1.5 volts into .4 amps at 3.7 volts). If you do it with resistors and fewer batteries, you'll want voltage. AAs will hurt here; a 9v battery or CR123 may be a better option. You could really use any voltage source that can deliver the current and voltage you want - either:

200 mA and 3*3.1v = (at least) 9.3v
or
600 mA and (at least) 3.1v

The resistor to limit current will depend on exactly one trick. The voltage across the resistor is (Vbattery - Vled). The forward voltage of the LED matters here. Let's say you use 3 AA batteries in series (the ever-popular 2000 mAh Eneloop). On a fresh charge that gives 1.45 volts, and when it's nearly dead it will have dropped to about 1 volt. This voltage, 4.35v, is only enough to drive the LEDs in parallel.

Vresistor = I * R
Vresistor = (Vbattery - Vled) = (3*1.45 - 3.1v) = 1.25v
I = 3 * 200 mA = .6A
R = ?
1.25v = .6 A * R
1.25v / .6A = R
2.0833 = R

So a 2 ohm resistor that can handle a power of (.6A times 1.25v) .75 watts will limit the current. As the batteries die though, the LED will be dimmer. A slightly lower resistance will make the LEDs start slightly brighter, end slightly brighter, and kill the batteries slightly faster. A resistor of exactly 2 ohms will give you about 4% more power than we planned for, which is ok. The eneloops in series have 2000 mAh at 4.35 volts, and a 2 ohm resistor with these LEDs would drain them at about 625 mA per hour. These batteries alone would last for about 3.2 hours - 2000 mAh divided by 625 mA gives hours. If you'd used a high-voltage battery (9.3 volts or more) then you could run it at .2 amps instead, but the formula is identical.

Any questions so far? Tl;dr?

 

[sonko2010]

The light to be overcome is just ambient indoor lighting, and not very good at that. But in the vast majority of situations I would only turn on the LEDs at night. So I guess the LEDs do not have to be too intense...certainly less then a modern LED flashlight (and hopefully less hot). The stone sculpture is small 5 inches x 3 inches.

To hopeuflly address heat and infrastructure, I would plan on drilling clear through the arch and the top of the wood mount giving a little air circulation as heat from the LED rose and to also run the wires out and over the wood to the back side where I can hopefully recess a battery pack and a small switch.

All sounds good in my head, but I need the advise of experts to see if this is going to be possible or just a bag of hurt.

 

[AnAppleSnail]

Last reply as a sort of summary in case that double-wall of text is hateful. Hopefully breaking up posts is ok?

This setup will give about the planned 180 lumens for 3.2 hours; which may not be enough for an art show. Using more batteries in parallel (6 Eneloop AAs gives 6.4 hours) or using lithium-ion batteries can give longer runtime. If you'll be in a dim place, the LEDs can be dimmer and run for much longer.

Summary:

3 LEDs giving about 180 lumens.

A neutral-white Q4 flux with no board would cost $5.35 USD (plus shipping)

.62 watts per LED to dissipate or you fry them. They probably won't get over 100 centigrade, but I can't remember what temperature wood scorches at.

Given the datasheet information, we'll want 60 lumens per LED, reached with about 200 mA per LED. This gives a Vf of about 3.1v.

3 AA Eneloops, 1.45v each and 2000 mAh
Current is given by the voltage across the resistor (Vbatt minus Vled) times the resistance. Instead, V/I = R; or (Vbatt - Vled) / current = R; in the case I gave it was (4.35v-3.1v) / .6 mA = R, giving R=2.0833.

Using a 2 ohm resistor will give current draw of 625 mA, close enough for approximation. Fiddly adjustments can be made with 2 resistors in series or parallel. This resistor would dissipate .75 watts of heat on a fresh battery - more than one LED.

2000 mAh drained at 625 mA gives (2000/625) = 3.2 hours. Change batteries to taste, although matching the Vled to the battery wastes less power.

 

[AnAppleSnail]

But in the vast majority of situations I would only turn on the LEDs at night. So I guess the LEDs do not have to be too intense...certainly less then a modern LED flashlight (and hopefully less hot). The stone sculpture is small 5 inches x 3 inches.

It's quite doable. With one rat-shack part you can make the multiple settings, although the size and neatness depends on your soldering abilities. A "Single pole double throw" switch has one switchy bit with two output pins. You get two settings here, so you can pick two resistors for different levels.

The same LEDs run at a much lower level would noticeably brighten up a room at 50 lumens total - that's 16 lumens per LED, a drive current down near 25 mA each (hard to eyeball on the datasheet's chart). This output can be easily done with cheaper 5mm LEDs, but many of these have very crummy quality. I don't know where to source good-quality nicely colored white 5mm LEDs.

The math is similar; (4.35v - 3.1v) / (3 * 25 mA) = 16.667 ohms. This one would run for 24 hours on a 17 ohm resistor, given 3 AA eneloops. Expect similar life from decent alkalines.

 

[HKJ]

The light to be overcome is just ambient indoor lighting, and not very good at that. But in the vast majority of situations I would only turn on the LEDs at night. So I guess the LEDs do not have to be too intense...certainly less then a modern LED flashlight (and hopefully less hot). The stone sculpture is small 5 inches x 3 inches.

5 mm led might be enough for this (The plastic is 5 mm in diameter):

Run them at 10mA and you do not need to worry about cooling.
You need 3x1.5 volt batteries (or one LiIon) and a series resistor to drive them.


To get some, buy a very cheap flashlight and extract them from it, this is good enough for test. Then you can later get a better quality and correct color temperature, if you are satisfied with the light level.