Dew heater ?

GrahamL · #1 04-08-2006, 04:29 PM

Does anyone have an idea would a small rechargeable batt pack run
8 or more resosters for long ?

Tia .. ns

RAJAH235 · #2 04-08-2006, 05:44 PM

NS..It all depends on exactly how much 'heat' errr.. wattage/power, you require out of the batteries. Need to know what you're going to 'keep warm'??? Better off with a 'larger capacity' battery for any kind of heating purposes. Heaters are a constant drain/load. More info req'd please..

L.

acropolite (Phil) · #3 04-08-2006, 06:37 PM

A standard 7ah Gel battery will run my dew heater for a lengthy session, but I only use it on my corrector and use a home made switch mode controller. Using resistors you will need to know what current you are going to draw, then it's simple arithmetic, e.g. a 7ah (amp hour) battery will allow 1 amp draw for (almost) 7 hours 1/2 amp for 14 hours etc. When the current drawn rises to above 1/10 of the AH rating the battery capacity de-rated. The number and value of resistors will depend on what you're trying to heat so as Laurie said there are lots of variables.

drmorbius (Randall) · #4 04-08-2006, 07:48 PM

If you know the value of each resistor, you can calculate it... Now electronics was a long time ago for me, but I'm sure I'll be corrected if I'm wrong

First you need to know the total resistance, which you can get by adding up the resistance of all the resistors. Let's say each resistor was 15 ohms, the total would be 8 x 15 = 120 ohms. If you don't know the values, see if you can borrow an ohm meter and measure the resistance through the dew heater.

Divide the total resistance into 12 (volts) to get the total current... 12 / 120 = .100, or 100 milliamps.

Divide that into your Amp-Hours rating to get the number of hours your battery can sustain that current. Let's assume you have a 7AH (Amp Hour) battery... 7 / .100 = 70 hours.

You can also calculate the power your resistors put out by multiplying your current by 12 (volts), in this case .1 * 12 = 1.2 Watts.

Hope that helps rather than confuses...

RAJAH235 · #5 04-08-2006, 08:09 PM

FWIW. You can also use what I did, & get an old electric blanket & strip the heater wire out...Approx 420 mm length will give about 1 1/2 to 2 watts of heat on 12 volts...more than enough for most apps...

L.
ps. The end terminations can be a bit of a hassle if you're not careful...

GrahamL · #6 04-08-2006, 10:30 PM

think i get it .. was looking at the article in projects and looking at all these 2500 mah rechargables i have managed to accumulate

useing your example dr m
and bumping up the draw 10 times
( i think this is closer to what they would be actually useing )???
a 12 v supply of these batteries would barely run for 2 hours ???

thanks all .. thought it didn't seem possible somehow
but didn't hurt to ask

RAJAH235 · #7 05-08-2006, 12:36 AM

NS. My heaters consume about 150 mA, or 1.8 Watts (a 450 mm length equates to approx resistance of ~ 80 ohms), so your 2500's should give you around 14/16 hrs??, given they're in top condition & peaked the night before. They can be run in parallel if req'd. Good thing bout electric blanket htr wire is, it's nice n flexible.

L.

sheeny (Al) · #8 05-08-2006, 07:31 AM

Quote:

Originally Posted by drmorbius

First you need to know the total resistance, which you can get by adding up the resistance of all the resistors. Let's say each resistor was 15 ohms, the total would be 8 x 15 = 120 ohms. If you don't know the values, see if you can borrow an ohm meter and measure the resistance through the dew heater.

NS & Dr M,

The above calc is true for resistors in series. For resistors in parallel, such as in the dew heater project its a bit different. For parallel resistors:
1 N
_ = _
R Rr , where N = number of resistors, Rr = resistance of 1 resistor, R = total resistance.

Or... you can calculate the current through 1 resistor, I = 12/Rr and than just add up the currents (or multiply by the number of resistors).

Or, you could use the design spreadsheet in the project. It do it for you.

Hope this helps!

Al.

drmorbius (Randall) · #9 05-08-2006, 08:01 AM

Good point Al... Boy, does this bring back memories...

I think I'll go build something...

Dujon · #10 05-08-2006, 01:27 PM

This post has been removed due to apparent duplication.

acropolite (Phil) · #11 05-08-2006, 02:01 PM

Al forgot to tell you that he has written an article on this very subject. Also this site has some good info and a design for a simple controller.

GrahamL · #12 05-08-2006, 02:05 PM

other than spelling resoster wrong.. repeatedly.. j/k .. looks good

this is all a help to get my head around the basics

edit>>>Yeah its a great read and contribution acropolite thanks al.. I was a little sketchy
on the principles behind it .

thanks everyone

Dujon · #13 05-08-2006, 02:21 PM

Ah, sorry, I didn't realise that posting existed.

*crawls back into shell trailing bottle of settler-downerer*

GrahamL · #14 05-08-2006, 04:21 PM

hope you can post that up again john .. its all helpfull imo
even if what you posted is similar in content to als article .. it does add alittle more
to the bigger picture .. and for someone like me the more info i can jam down my
earhole the better.

take care ns

Photon · #15 06-08-2006, 01:52 PM

In relation to the link provided by Acropolite, a similar low cost dew heater controller that can be knocked up quickly is kit K3070 DC Motor Controller available from D Smith. The kit is around $16 in Tasmania. I build one and it's perfect. Output current is 1 Amp which will control heaters up to 12 Watts. The unit is so small I intend to knock up three of them in one case so I can control my Dew Heaters individually. With one controller supplying 3 dew heaters (EP and Finder scope) I find when I get one optimised the others can be too warm or too cool, this fixes that problem nicely and ultimately saves unnecessary battery drain.
In relation to interference to other electronic scope devices there are methods of addressing this so one battery or 12v power source can be used. When I get some work off my plate later this week I'll post an article to this thread.
Kind Regards Ian Gillespie

Merlin66 (Ken) · #16 07-08-2006, 08:41 AM

Jaycar have 4m lengths of 13 Ohm/ metre NiChrome wire for $2.80 and double length would give approx 20Watt around a 10-12" corrector; much easier than soldering all those resistors!!!

GrahamL · #17 02-09-2006, 03:50 PM

yeah merlin that wire works well
dew and a telrad sure go together.. so thats all I'm going to heat for the moment.. I'm not really happy with how this turned out.. I should of done some things a little differant but couldn't without starting over.

9v @ 300 m/a warms up nicely probably around 70/80 deg/cel
can't get it to fog up at all so far .. see what happens tonight i guess