Low voltage heater

[petersull]

Hello,

I have a little project to build, but have virtually no knowledge of electronics, although I can handle a soldering iron, and many years ago, managed to follow instructions and successfully build a working transistor radio from a circuit diagram and individual components. I decided to join this forum in the hope that someone can help me with advice and a suitable circuit diagram I can work to.

Since my childhood over 40 years ago, I have been a keen amateur astronomer. I have recently discovered the convenience of using a green laser pointer attached and aligned with my telescope to help find objects easily in the night sky. I am aware of the effects on aircraft and possible eye damage, but mitigate these by only using the beam for several seconds at a time, which is all it takes to find an object, after which it remains off.

The laser works great in warm weather, since the operating temperature is specified to be above +10 degrees centigrade. The bulk of my astronomical observing is done in the colder months when night time temperatures are usually well below 10C and often below zero. At these temperatures the laser will work fine for several minutes whilst dimming until invisible. Using Lithium batteries (2 x AAA) seems to slightly prolong the life of the beam but not by much. Once the beam has failed, I can reactivate it by removing it and warming it in my hands for a few minutes, but almost as soon as I have fitted it back into its bracket and accurately aligned it again with the screws, the beam has dimmed and is virtually useless.

My laser pen is approximately 150mm long by 14mm diameter, more than half the length holds the 2 AAA batteries. I propose to carefully wrap a coil of Nichrome wire around the business end of the laser pen, on top of a thin layer of electrical insulation and then to insulate the outside of the coil. I need a simple controllable circuit to run off a small battery pack, possibly two or four AA batteries or whatever is recommended so I can vary the power in case of overheating, but keep the laser warm enough to function as and when required for a few hours. So I really need to be able to raise and maintain its temperature by up to a maximum of say 20 degrees C.

For your information, the Nichrome wire I propose to purchase from Mindsets Online is a 10 meter coil costing only a couple of pounds, and specified as 0.1mm thick, with a resistance of 1 ohm per centimetre.

Is this a viable idea or are there better ways to do this?

Any help, comments and advice on this would be very welcome, since I really don't have a clue what I am doing.

Cheers,
Pete.

[piratepaul]

It is a viable idea... you could control temp with a moveable contact... perhaps wound on glass or clay, surround it in insulation.
The heater batt is a bit on the small side... perhaps a 6v motor bike batt would be a better option.

I am in the car right now and cannot look look it up in my books (will be back home in a few days) ... the temperature constants are in electricity made simple (ebay £2 or so) it tells you how long the wire must be and how hot it will get.

[piratepaul]

A variable resistor in series with the heater wire would simplify temp control... The required current is dependant on ambient temp and thermal conductance/resistance of the lazer housing... (torch body).

[piratepaul]

May be stabilize it with a thermistor to give constant temp.

It is poss to break a bottle and grind the neck to a smooth surface on a flat stone... thus making a heat proof insulator on which to wrap the heater wire... might take a few attempts though.

[piratepaul]

The resistance of the wire is dependent on its temp...

At what temp is it 1 ohm per cm?

It has a temp coeficient of resistance, increase in temp = resistance x (current squared) x coeficient of nichrome...
Or resistance x (current squared)/inverse coeficient of nichrome.

[petersull]

Thanks for the info so far Paul.

This is the web page for the Nichrome wire...
http://www.mindsetsonline.co.uk/product ... cts_id=549
Maybe you know of another supplier that publishes more specs.

More battery power is no problem, but it would be good to be able to calculate the numbers required. I originally envisaged two/four AA's as a lightweight battery solution that I could stick or velcro to the telescope tube, but could run a longer cable to anything up to a rather heavy 65 Amp hour battery which would obviously need to go on the ground. I probably already have an old copy of Electricity Made Simple or maybe Teach Yourself Electricity so will look that out.

The variable resistor sounds like a much slicker method than the moveable contact, but I'm sure these come in various specifications that I am unfamiliar with.

Not sure about grinding a bottle neck, surely just coiling the wire carefully around the aluminium laser housing on top of electrical insulation would do it.

The coil, if wound just on the business end of the laser (excluding the unscrewable battery compartment) would be 60mm long and 14mm diameter.

If you have time to study this a little more when you get chance, a circuit diagram would be very helpful.

Here is a photo of the scope showing the laser in its adjustable holder...
https://dl.dropboxusercontent.com/u/104047272/798.jpg
The laser is towards the top middle of the photo, zoom in for a more detailed view.
You will see that in the laser mounting bracket, there is only room for a small extra thickness around the laser pen.

Thank you very much for your interest.

Best regards,
Pete.

[piratepaul]

http://en.wikipedia.org/wiki/Nichrome I will do some sums and get back to you... I will be home tomorrow calculator needed...

[piratepaul]

Hi the formula I gave earlier is poss the formula for a boiling kettle and should be x time hmmm ... I should check my facts befor I post...
This formula http://answers.yahoo.com/question/index ... 331AArXk4k is more like it...
There are lots of wire options on fleabay.

[piratepaul]

http://www.ebay.co.uk/itm/colvern-preci ... 2871wt_958 May or may not be a bit big but quality stuff.

[piratepaul]

I have sent off for a new XD card reader which should arrive soon, I will post a diagram if and when I get the camera to work.
It is a variable resostor in series with a switch and the heater wire, I think a meter of thicker wire (will get the size in a bit) would work well... this gives about 19 turns and disapates a couple of watts or so max which can be turned down to next to nothing or switched off.