Dew strip on the cheap?

[Ags]

I'm thinking of getting some resisting wire, a cheap toy torch as a battery holder, and then running the resisting wire through some velcro and into the torch instead of a bulb... The idea being to keep dew off my camera lenses. Would this work, do I run the risk of melting my lenses, and can I vary the temperature by varying the length of the wire?

[Gina]

I've just finished struggling with nichrome resistance wire and insulating tape to add dew heaters to my scopes and 200mm camera lens. Nichrome wire has a mind of it's own, very springy and very sharp ends where cut as I've found with the end of my finger! Amazing how much blood comes out of a pricked finger!! There's plenty of nichrome wire on ebay in a good selection of gauges. I'm running mine off my main 12v supply with LED strip dimmers to control the power. TBH I'm not sure if you'd get much life out of a small torch battery. In fact I don't really know how much power is needed to stop lenses dewing up - I've probably gone OTT with mine but won't know until I start using them. That might be tonight - it's clear ATM and I'm just about to set up. Won't be doing any serious imaging though due to the very bright full moon.

[Ags]

A little torch produces a lot of light so it should produce a fair bit of heat :-) I used to be good at electrical circuits at school, don't remember any of it now. Intuitively I would expect the heat produced to decline with the length of the wire, reaching zero when the resistance wire is long enough to qualify as insulation. But I guess if I only had one mm of resistance wire I would also produce only a little heat, so it must be kind of bell curve.

Remembered to google...

http://webhome.idire...w/resister.html

Power = v^2/R so assuming 1 ohm resistance, a 12V power supply gives 144W, while my 2 AA batteries give ~6W (2.4 squared). Sounds like I need 4 AA batteries for 25W. They each pack 1.7AH or 6.8AM total. V = IR, so I = V/R which gives a current of 5A. Just under an hour and a half of dew control - assuming 25W will do anything, that is.

Useless trivia: my little dog's nickname is Ohms.

EDIT: remembered that these calculations need to take into account the internal resistance of the battery, which changes everything somehow.

Ohms law must break down for superconductors (or even plain copper wires)... Power = V^2/R = V^2/Zero = infinite power!

EDIT again: From the link above "On an average Midwestern night, I find 4 watts in combination with the dew cap gives me plenty of protection and enough battery life to last all night. Nights of higher humidity, and thus higher dew point, may require more." No info on the surface area being warmed though... Anyway, it sounds my 2AA battery solution would be plenty for a couple of hours imaging (and my tracking isn't good enough for more).

[Ags]

After doing some more maths I think I can get 2W for 3 hours from two rechargeable AAs, using a resistance of 3 ohms.

I've also learned that a bettery rated at 1700mAh can deliver more or less than that - for example the same battery also states it can give 170mA for 16h - obviously its peak efficiency. So perhaps I should design a circuit to draw 170mA and get a 50% boost to the capacity of the batteries! :-)

EDIT: A 170mA circuit only gives 0.5W of heating... scratch that idea.

[Ags]

After a little research, I have found this wire:

http://www.amazon.co.uk/Brimal-Nichrome-Nickel-Chrome-length/dp/B0071KZMP0/ref=sr_1_5?ie=UTF8&qid=1350248647&sr=8-5

...giving 1 ohm per 10cm, or 30cm wrap for 3 ohms. My lens hood has a diameter of 8cm, so a circumference of 25cm, which would yield 2.5 watts if I keep the wire straight, lasting for about 3 hours.

My naive idea it to wrap the wire round the hood, attach it in place with black insulating tape and solder a blob on the end of each wire. I will attach wires from the battery holder with little crocodile clips. Does that sound reasonable?

Assuming that works out well, my Mak's corrector plate would be the next target. Assuming roughly 5V from 4 rechargeables, and wanting 5W heating, I would need 5 Ohms resistance. That's 50cm of this wire, and it is 35 - 40cm circumference around my corrector plate. So I would do one wavy circuit with the wire.

[Gina]

I did the same thing but several turns round the lens hood and 24swg nichrome wire. I went for 12 ohms with a 13.8v supply which gave plenty of heating - I usually find half power adequate. I've done the same with my scopes (ED80 imaging and ST80 guide scope).

I did find though that nichrome wire doesn't solder well so I used chocolate block screw connectors. I also used twin 3A figure 8 cable to my dew control box where it connects with phono plug and socket.

[Ags]

Thanks Gina. It's good to have something for comparison - what was the resistance in your circuit?

[Gina]

Thanks Gina. It's good to have something for comparison - what was the resistance in your circuit?

12 ohms.

[beamish]

I used Nichrome wire for my "integrated dewstrip" and used this site as a good guide

[Ags]

Wow, a lot more sophisticated than a pack of rechargeables, crocodile clips, black tape and resistance wire! I think I will keep things simple for now with a dedicated unit for my camera lens - no need for complicated control circuits. I'll have enough wire to make something for my scope and RDF though.

[Chris Rowland]

I've made dew heaters from a series of low value resistors strung together in a series/parallel way then enclosed in heat shrink tubing. They work well. You need to know some O level physics (Ohms Law, series and parallel resistors) and be able to solder.

IIRC it ended up as 5 or 6 3.3 ohm resistors in series and enough sets of that in aprallel to reach round a C11.

Chris