Thermistor luxeon question

I realize I don't have any experience in luxeons but I was thinking how hard would it be to find a thermistor to wire into a luxeon driver to automatically limit current when it started getting too hot. In theory it sounds simple but finding the right thermistor would probably pose the most challenge. If you could get a small enough thermistor you could put it in any luxeon light and even put luxeons in plastic lights that will scream when cold and slowly dim as they heat up too much saving a burnout. This could also come in handy on underwater lights as the water may help cool more thus the throttling down of the thermistor would be considerably reduced.

The question is.. could it be done right and would it be worth the trouble?

Actually, what you want here is a PTC (positive temperature coefficient) resistor. I use these as self-resettable fuses sometimes. When current flow is low they have low resistance and don't get very hot. When a larger then normal current flows they heat up very abruptly, their resistance greatly increases, and the current flow is limited. When the fault is removed they cool down again and the circuit functions normally. By placing one in series with and in close proximity to the Luxeon it will be cold and low resistance until the Luxeon heats it up. Once it does, the PTC resistor will limit current flow. The hard part is picking the right one.

My opinion though is that it's probably not worth the trouble, and may not work at all well with step-up regulators. As the PTC resistance goes up the regulator will increase voltage to compensate, resulting in yet more heating and so forth. The end result might be a destroyed regulator. This scheme might work well with direct drive but direct drive is a bad idea on so many levels it isn't even funny.

Yes, I was reading about PTC, I guess you would have to design a circuit to use one of these, perhaps some simple boost circuits would work if you could find appropriate thermistor resistances throughout the temperature range of luxeons.

Hi all,

On the other hand, many of the ICs we are using to power our lux's have auto-shutdown thermal protection built into them. The problem there is that most of the temp thresholds are at ~140C while the lux max Tj is 125C.

It would be interesting to be able to hand pick some IC's with a lower temp thermal shutdown, then thermally tie that into the lux heatshink. If we got the setup right and the IC's throttle temp was low enough, we could have the circuit shut itself down if the lux got too hot.

The beauty of that solution is that it wouldn't require any more circuitry or components to implement. It would only take a different circuit layout and some thought added to the thermal path.

pb

I like the idea of an external thermistor so you can place the circuit elsewhere away from the luxeon and just run 3 wires to it, one of which is from the thermistor, one for the power to the lux and the third a common ground. For some reason I expect this to happen about the time luxeons get so cheap it may not be worth the extra expense replacement costwise but worth it for the repair labor costs to replace it.

Many converter chips have a shutdown pin that can be used to control the converter. A thermosistor and another resistor with a ref. voltage from the converter, should be able to act as a thermal cutoff.

Shouldn't be too hard. The trick is to get the thermosistor as close to the die/slug as possable.

It would be better if you could just use the thermistor to slowly reduce the output of the converter instead of turning it off. A light that gets dimmer as it heats up is more useful than one that keeps shutting off completely.

Hi LA,

Frankly, Lumileds states that a die temp of 125mA is tantamount to blasphemy, so I don't know why they don't just offer an emitter with a built-in PTC. I'll bet that a lot of people would love that.

There's also another thread going on right now here about a new Sipex CC boost chip. The datasheet has a schematic for driving a lux on pg.12 which shows an optional 10k pot to be used for dimming. I'm sure you could find a thermistor of the correct value for that one.

pb

[ QUOTE ]
CNC Dan said:
Many converter chips have a shutdown pin that can be used to control the converter. A thermosistor and another resistor with a ref. voltage from the converter, should be able to act as a thermal cutoff.

Shouldn't be too hard. The trick is to get the thermosistor as close to the die/slug as possable.

[/ QUOTE ]

Good idea! But I've noticed that most converters available have a several hundreds of milivolts on hysteresis like ZLT's having 600mv?(enable at 700mv or above,disable at 100mv or below) This very wide voltage will not create a sharp knee of response on temp.sensing. How about using the LBI/LBO of the converter? The LBI have a typical hysteeris of 10 to 15mv. Tying a voltage devider across it like the upper resistor is a thermistor while the lower side is ordinary resistor, then the output LBO to be connected to the shutdown pin will do the trick.