How do you make a Dewheater from Nichrome wire

[[1ponders] (Paul)]

Ok all you techi gurus out there.

After looking at the Orion Dew Zapper I bought the other week, I thought "It can't be too hard to make one of these and save myself some money". So I went and bought myself 4 meters of 28B&S Nichrome Resistance Wire from faithful old DS. And that's as far as I've gotten. I know about as much about playing with electronics as a chicken does about scuba diving. Well maybe a little bit more but not much I'd love some ideas about the various ways I can go about this.

I've heard that if you take about 2 ft of the stuff and connect it to a 12v battery it'll get to the right temp to keep the dew off a 8" lens but not do any damage. The prob I have is that I already have one for the 8" and I want to make one for each of my finderscopes, my guide/widefield refractor and my camera lenses. I think I know enough to know that if I cut a piece short enought ot go once around my refractor the resistance will be too low and the wire will get too hot (is this correct?)

Any ideas? Also does anyone know if DS or Tandys or Jaycar have something that would do a similar job as the Thousand Oaks or Kendrick controllers.

[elusiver]

LAURIE!!!

it's the resistance in the wire that causes the heat.. so I'd figure to maybe add a resistor before the nichrome to drop it so that you can use shorter lengths and keep the temp down. Maybe even a small Pot would work.. and you'd be able to control the temp that way from zero to hottish worm. I wouldn't know where you'd put the wires though on a refractor, probably wrapping it around the outside of the tube where the lens is.

el

[[1ponders] (Paul)]

Finding a place to put them el won't be a problem. Once I sheath it I can wrap it around the back of the lense cells.

[RAJAH235]

Hi Paul,Tandy & D.Smiths have nothing relevant. A simple way to get the correct heat out of your nichrome wire, is to > connect one end to +ve, > then using a pair of pliers or a crocodile clip, starting at the other end, slowly slide the -ve wire down the nichrome until the desired temp. is reached... Taking into account the temp.diff. between inside your house/room & outside???? Bit iffy really!!! Then you have to insulate it somehow!! See original > http://www3.sympatico.ca/mark.kaye/dewheat.htm
This might be worth a try >http://www.dewbuster.com/heaters/heaters.html
I used an old electric blanket heater wire. If you cut a piece about 300 mm long, it will just be warm enough to keep your finderscope dry. I wrapped it round twice, (8 x 50 finder), & it works fine.
You will have 1 problem, ie; because it's resistance wire, you cannot solder it, but if you twist them together well & then solder, it works O.K. Sounds complicated but it's just fiddly. I used a small BP,(screw type), connector as security. You can also use a piece of 2 way terminal strip.
Mine runs all night no probs. Along with the E/Pc box heater & red LEDS. All up they total around 350 mA.
Hope the links are a help.
Regards, L.
ps. You need around 1.5 to 2 watts of heat to keep the dew off.
Ohms Law > I = E/R. E = IR. R = E/I. P = EI. E = P/I. I = P/E. I = current. E = voltage. R = resistance. P = Watts.

pps. ELLEX, what is a resistor? What is 'nichrome' wire?
Same thing, only packaged differently. See 'Dewbusters' link. L.

[elusiver]

paul.. stick with lauries answer.. he knows what he's talking about.. sometimes


el




ok.... all the time.

[RAJAH235]

Just the facts m'am! L.

[[1ponders] (Paul)]

thanks for that Laurie. Looks simple enought for even me to have a go.

[[1ponders] (Paul)]

So Laurie, going by your formula above I could connect the leads of a dewheater to some kind of pot/dial to adjust the voltage which would also adjust the heat generated.

[RAJAH235]

Paul, Because the heater only runs @ around 2 watts, you could use a simple pot, ie; potentiometer, to adjust the temp. BUT you must buy a 'WIRE WOUND' pot, rated @ a minimum 3 Watts. The normal carbon track ones are not suitable. They are only rated in mW., ie; the carbon track will cook.
A suitable value would be a 100 Ohm 3 Watt Wire wound pot.(Readily avail.). This will give a very cool to warm range. Max current with a length of 300 mm of electric blanket heater is 170 mA with 12 volts applied. This equates to around 2 Watts.(Resistance = about 70 Ohms). Should be more than enough to heat your finderscope objective lens. (same as mine).
I used a simpler idea, in that I 'tapped' into the heater wire about 60 mm from one end of the 300 mm length, & ran another wire to it. This gave me, LOW HEAT = FULL LENGTH, HIGH HEAT = @ SHORTENED LENGTH. I hardly ever use the HIGH heat tho! The heater, being in contact,ie; 2 turns around the finder, maintains a fairly constant temp. all night. ( warm mass). A single pole changeover switch is all that's required to go from H to L.
I posted a pic somewhere. Will try to find if req'd.
HTH. L.
ps. http://www.iceinspace.com.au/forum/a...achmentid=1484
Details are slightly diff. but close enough..... L.

[[1ponders] (Paul)]

Thanks Laurie. I think I'll pop down to the Slavos/Endevour/lifeline and see if they have any electric blankets that didn't pass electrical safety muster. Did you encase your finderscope heater with tape or anything to keep it bound together. If so did you include some sort of insulation to the outside to reduce heat leaking away into the air?

[Starkler (Geoff)]

For 2 watts at 12v you need 0.166 amps of current, therefore 72 ohms of resistance.
Do you know anyone with a multimeter to measure this for you?
I like Rajahs idea of a tapped point. That way all of the battery current is going toward heating your finder and not wasting it heating the pot instead.

Perhaps use a longer than 300mm length for low power and tap at 300mm for high power? If 300mm gives you 2 watts, then 424mm will give you 1 watt ( sqrt 2 x 300)

[h0ughy (David)]

Hey Paul, why noit email Renato, he made one at duckadang in a few minutes, I will pM you his email and phone number.

[[1ponders] (Paul)]

Thanks guys, you've given me plenty of options. I do like the idea of the Tapped point as well. High, low and off. Looks like I'm off to DS or tandy tomorrow for a few fittings. Maybe my wife will sacrifice her electric blanket in the name of science. Not I think

[RAJAH235]

Paul, No I didn't find it nec to 'encase the 2 turns' but you could, as this would keep the heat in a little better. Poss. use 'Glass' tape with a layer of 'ripstop' gaffer tape over it.
Geoff, Thanks for comments. Actually you only need around 300 mm, max of 350. This was found by practical use, outdoors. Anything longer was pretty useless, heat wise. I couldn't feel any heat in my hand/fingers at all. You just need to measure 300 mm + 10 mm for terminating each end. No DMM req'd.
HTH. L.
see link; http://www.iceinspace.com.au/forum/s...ead.php?t=1640

[acropolite (Phil)]

Signor Ponders.....here's everything you need to know. The best way control the dew heater is with a switch mode controller of which there are heaps of designs floating around on the net. Here's one, together with a very nice solution for the LX90 (same ota as yours) the cct is as simple as you will find and he's even documented the PCB layout.
I think the gauge of wire that DSE sells isn't quite ideal but all that's required is to vary the length. http://www.backyard-astro.com/equipm...dewheater.html .....

[[1ponders] (Paul)]

Thanks for that Phil. I'll have a closer look at it but me like Ballaratdragon wit comptr. 'lektrik spark, sharp hert [1ponders] . I put a screw driver in a wall socket when was about 8. Didn't want to have anything to do with electricity after that.

[acropolite (Phil)]

Paul apparently 3-5W of power is sufficient to keep the dew off an SCT corrector plate and on the site that I have quoted the author used a resistance of 14 ohms to give about 10 watts (at 12V) maximum heating. BTW, you can't solder nichrome wire. As I said the design is as simple as it gets (cheap too, bits would be under $10) and because it's switchmode it is also efficient power wise...
Go on have a go, short of sticking your tongue on the battery there's no way you'll hurt yourself....

[[1ponders] (Paul)]

done that too Phil I already have a dew heater for my SCT, its the guidescope and finderscopes I need to work with.

[RAJAH235]

KISS. Is the way to go. Paul, I can build you a PWM,(pulse width modulation), controller if you want, but it's way OVERKILL. Mine's been going for about 5 yrs now, with no repairs nec. L.

[Orion]

Laurie will the electric blanket heater wire work for an secondary heater and a eye piece heater?