Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Warming a laser diode using a thermistor

  1. Jan 7, 2013 #1
    I use a green laser pointer at night to do astronomy related work. I got a cheap $5 green laser pointer off of ebay, and when it's a warm summer night, the device works fine. However, the second it touches the cold, it fizzles out and gives me almost zero output.

    My idea was to use a thermistor with a positive response curve, that is, as the temperature goes up, the resistance goes up as well. When it is cold, resistance is low, and current flows through the thermistor, generating heat, warming the batteries and diode. As it warms up, the resistance increases, diverting current do the now warm diode.

    I realize this design is not without its flaws. Does anyone have any suggestions?
  2. jcsd
  3. Jan 7, 2013 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Your idea is sound in principle.
    Look into how many joules you need to develop , the battery size required may surprise you. I doubt the pointer's battery will suffice.

    Old TV sets have a positive temperature coefficient thermistor in the degaussing coil surrounding the front of picture tube, if you scrounge one it may get you started experimenting on the cheap. But it's too big to go inside your pointer.

    I think i'd experiment also with those pocket warmers that outdoorsmen use - any sporting goods store has them this time of year. Keep the pointer in same pocket.
  4. Jan 7, 2013 #3
    Well what I feel it will really boil down to is how much and how quickly the heat is generated.

    Assume your typical alkaline AAA battery has 1200 mAh of charge (http://en.wikipedia.org/wiki/List_of_battery_sizes#Round_batteries). And assume you choose a thermistor such that it draws 100mA. At that rate, you should be able to run the pointer for 12 hours roughly.

    If the thermistor generates and radiates its heat quickly, maybe on the order of a few seconds to a minute, then the amount of charge that the thermistor has used will be negligible compared to the total capacity of the battery. Then at higher temperatures, depending on the design of the thermistor, it will draw almost zero current, making it nearly an open circuit. And this should work for intermediary temperatures as well. When the diode/thermistor are cooling down, the thermistor resistance lowers, pulling the system back up to the warm state.

    The good thing about the cheap ebay lasers is that it unscrews at the middle, exposing the negative bettery terminal. The positive side of the battery is connected to the chassis. So I could fit a thermistor (they're only a few mm wide) into the circuit area, and solder one lead to the negative terminal, and one lead to the case. Then the thermistor and the circuit/diode will be in parallel.
  5. Jan 7, 2013 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Indeed AAA's are somewhat more stout than i thought. Here's a fellow who actually tested a variety of them.


    Next thought is this -
    the thermistor need not dissipate all the heat, if you have room for say a 2N2222 in there. Thermistor between its base and collector would control current. Need just a bit of emitter resistance to limit current.

    But that's more complex than your solution , which is elegantly simple.
  6. Jan 8, 2013 #5
    I didn't expect a laser diode to bet less efficient at cold.
    Could it be the battery that weakens at cold? They are known for that.
  7. Jan 8, 2013 #6


    User Avatar

    Staff: Mentor

    I'd back the battery suggestion, but the short time constant quote tends to rule it out. So I'd go searching for a dodgy pressure contact, some place where metal contracts with the cold and a gap opens up. Maybe the terminal contacts in the battery holder? Some manufacturer's batteries are a tad shorter than other brand's, and this may be at the heart of your problem.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook