# Warming a laser diode using a thermistor

• warfreak131
In summary, a green laser pointer can work fine when it's warm, but it becomes almost useless when it's cold. The solution is to use a thermistor to create a positive temperature coefficient, and to keep the pointer in the same pocket as the pocket warmer.
warfreak131
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?

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.

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.

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.
http://www.fairchildsemi.com/ds/PN/PN2222A.pdf

But that's more complex than your solution , which is elegantly simple.

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.

the second it touches the cold, it fizzles out and gives me almost zero output.
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.

## What is a laser diode?

A laser diode is a semiconductor device that emits light through the process of stimulated emission. It is commonly used in various applications such as telecommunications, laser pointers, and optical storage devices.

## Why is it important to warm a laser diode using a thermistor?

Warming a laser diode using a thermistor is important because it helps to regulate the temperature of the diode. Laser diodes are sensitive to temperature changes and excessive heat can damage or degrade their performance. By using a thermistor, the temperature can be carefully monitored and controlled to ensure optimal functioning of the laser diode.

## How does a thermistor work in warming a laser diode?

A thermistor is a type of resistor that changes its electrical resistance in response to temperature changes. In the case of warming a laser diode, the thermistor is connected to a heating element that warms the diode. As the temperature increases, the resistance of the thermistor decreases, allowing more current to flow through the heating element and raising the temperature of the diode.

## What are the benefits of using a thermistor to warm a laser diode?

Using a thermistor to warm a laser diode offers several benefits. It helps to prevent the diode from overheating, which can lead to damage or reduced performance. It also allows for precise temperature control, which is important for ensuring the diode operates at its optimal temperature for maximum efficiency and output.

## Are there any risks associated with warming a laser diode using a thermistor?

While using a thermistor to warm a laser diode is generally safe, there are some risks to be aware of. If the thermistor or heating element malfunctions, it can lead to overheating of the diode, potentially causing damage. It is important to carefully monitor the temperature and ensure the thermistor and heating element are functioning properly to avoid any potential risks.

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