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12V DIY Dew Heater Resistance

DIY Accessories Astrophotography Equipment
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16 replies to this topic

#1 Ss00scott

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Posted 02 October 2023 - 02:34 PM

Hey guys and gals

I was hoping to get some help for a DIY dew heater. I make my own but I need to know the resistance for a 12V heater. If anyone knows the value off the top of their heads or can test their’s that would be appreciated. I’m going to make a heater for the ASIAIR

Edited by Ss00scott, 02 October 2023 - 02:35 PM.


#2 gcardona

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Posted 02 October 2023 - 02:52 PM

Here's a page showing the resistor arrangements of various dew heater configurations and how to make them. The resistance is going to depend on how big the heater is, the bigger it is the lower the resistance because it has to draw more power to heat a larger area.


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#3 kathyastro

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Posted 02 October 2023 - 03:20 PM

Decide how many watts you need.  The resistance you need is V^2 / W, or 144 divided by the watts you need.


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#4 BucketDave

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Posted 02 October 2023 - 04:40 PM

Since the ASIair consumes power, I'm sure that power would be sufficient to keep it above dewpoint if you enclosed it in an insulated box. Something like 1/2" of polystyrene.

If you just wrap a dew heater around it, the heat will just convect away into space. You need something to keep it in - hence the external box.
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#5 NearVision

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Posted 02 October 2023 - 08:24 PM

You shouldn't need a heater for the ASIAIR unless ambient temp is way below zero. Heat is an enemy of computers and to be avoided.


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#6 Ss00scott

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Posted 03 November 2023 - 12:04 PM

Sorry if my initial post was not clear enough. It’s a few heater for my optics that is powered by the air. Thanks for everyone’s advice

#7 Woodbridge_Dave

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Posted 06 November 2023 - 11:08 AM

I would not use individual resistors to make a dew heater (that is so 90's).  Use simple nichrome wire.  The wire can be purchased with various resistances.  If we assume you can afford 1 amp of power per dew heater, then at 12 volts your total resistance should be 12 ohms.



#8 don clement

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Posted 06 November 2023 - 12:05 PM

I would not use individual resistors to make a dew heater (that is so 90's).  Use simple nichrome wire.  The wire can be purchased with various resistances.  If we assume you can afford 1 amp of power per dew heater, then at 12 volts your total resistance should be 12 ohms.

Nichrome wire can't be soldered and must be crimped or similar. Nichrome wire would only be needed if the current density and heat would be too great for resistors like if one were making a toaster or room heater. If that were the case for a dew heater then the heat generated in a telescope is going to affect seeing thru the scope. Why not just use a hair drier which most likely uses nichrome wire for  the heater element?


Edited by don clement, 06 November 2023 - 01:08 PM.

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#9 kathyastro

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Posted 06 November 2023 - 04:28 PM

I used individual resistors for the dew heater for my all-sky camera.  It works just fine.  The resistors were easier to work with than nichrome, and I get 3 watts of heat, which works for most conditions.


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#10 Woodbridge_Dave

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Posted 06 November 2023 - 06:53 PM

Nichrome wire can't be soldered and must be crimped or similar. Nichrome wire would only be needed if the current density and heat would be too great for resistors like if one were making a toaster or room heater. If that were the case for a dew heater then the heat generated in a telescope is going to affect seeing thru the scope. Why not just use a hair drier which most likely uses nichrome wire for  the heater element?

it appears you’ve never used nichrome wire.  I’ve been using it for years for dew heaters.  It can be crimped and soldered if done right.  As well, the idea that it would generate so much heat as to affect seeing is utterly untrue.  What do you think is used in commercial dew heaters ?  It’s nichrome tape (a thin, flexible version of nichrome wire).


Edited by Woodbridge_Dave, 06 November 2023 - 06:55 PM.


#11 star acres

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Posted 07 November 2023 - 08:35 AM

I'd hit a coffee maker on the head and make off with the brew warmer. When you take a 50 watt string cord and run it on 12 volts, you only draw a few watts. Perfect.

#12 don clement

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Posted 07 November 2023 - 10:35 AM

it appears you’ve never used nichrome wire.  I’ve been using it for years for dew heaters.  It can be crimped and soldered if done right.  As well, the idea that it would generate so much heat as to affect seeing is utterly untrue.  What do you think is used in commercial dew heaters ?  It’s nichrome tape (a thin, flexible version of nichrome wire).

I've used nichrome wire for a dew heaters. I haven't been able to solder it. Please post on how you were able to solder nichrome. 

 

I said:" Nichrome wire would only be needed if the current density and heat would be too great for resistors..."  I didn't  say that nichrome would generate so much heat as to affect seeing.Get your facts straight! Using too much heat will definitely affect seeing though. 

 

I did design a simple closed loop circuit  back in the 90's that measures the ambient temperature and the temperature of the optics and keeps the optics temperature a very small amount above ambient temperature so as to minimize the dew heaters affect on seeing but that's "so 90's" right. 


Edited by don clement, 07 November 2023 - 10:37 AM.


#13 triplemon

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Posted 08 November 2023 - 04:19 AM

Your garden variety electronic solder with rosin core isn't agressive enough to solder NiChrome. You would need acid based flux as sheetmetal workers use, you can get it at Napa or HomeDepot.

BUT
 

That stuff happily will destroy almost anything electronics related. Like copper wire, PVC wire insulation, most plastics. So its at most good enough for a few hours lifetime frown.gif
 

Another thing is heat. An even just moderately hot heater wire will over time degrade your solder contact. So you will at least want to use a higher temp silver solder. And now you think - I'm not using it to get THAT not. Well, the wire has a very small surface area and even at quite low energy densities it does get quite warm ... in particular at the ends where its not having a good heat sinking for it.

 

So in reality - you crimp/compression contact NiChrome wire, only. Use a copper or brass ferrule or some wire clamps.

 

I do use NiChrome wire for the corrector plate of my 6" Mak. Its directly wound onto the glass, with thermal paste and all held in place with Capton adhesive tape, which is very heat resistant. Some 1-2 watts is enough to keep the somewhat big thing just at ambient temperature or a tad above. You really just want to replace the radiative lost heat.
 

And that is why you want MUCH finer control than just a series/parallel connection of two resistive loads. Most dew heater controllers use some PWM circuit to adjust the average power. I'll nowadays would just get some step-down buck converter module to supply it with a lower  and variable voltage.


Edited by triplemon, 08 November 2023 - 04:36 AM.


#14 don clement

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Posted 08 November 2023 - 09:57 AM

If I am going to use a resistance wire for a dew heater my choice is constantan. Unlike nichrome wire, constantan wire can be easily soldered with typical  60-40 pb/sn solder. BTW constantan has a low thermal conductivity so I used constantan wire to wire up a CCD chip on a cold finger. Constantan wire is available from Omega and even Amazon.


Edited by don clement, 08 November 2023 - 10:02 AM.


#15 briansalomon1

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Posted 08 November 2023 - 10:41 AM

I used individual resistors for the dew heater for my all-sky camera.  It works just fine.  The resistors were easier to work with than nichrome, and I get 3 watts of heat, which works for most conditions.

Using resistors will make it easy to change the heater down the line. The OTA/surrounding Aluminum will help distribute the heat.

 

https://twcontrols.c...s-and-pie-chart

 

2 watts/12 = 0.1666 amps.

12V/0.1666 amps = 72 ohms

If you used 6 resistors for the heater that's 72/6 = 12 ohms each.

Each resistor is only dropping 2V and making only 0.333 watts, so resistors rated to operate at 0.5 watts would be fine.

https://www.amazon.c...r/dp/B00VVOROV2

 

 

 

3 watts/12 = 0.25 amps

12V/0.25 amps = 48 ohms

If you used 6 resistors for the heater that's 48/6 = 8 ohms each

Each resistor is dropping 2 volts and making 0.5 watts so you would need resistors rated to at least 0.5 watts

https://www.ebay.com...KMjxNY9XBWOfwac


Edited by briansalomon1, 08 November 2023 - 10:42 AM.


#16 don clement

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Posted 08 November 2023 - 11:56 AM

I think the best material for a dew heater would be a resistive paint that could be painted on optics. The connections to the resistive paint could be conductive silver paint. One could roll their own resistive paint by adding graphite powder to a paint. 



#17 triplemon

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Posted 10 November 2023 - 02:16 PM

There are all kinds of flexible heating pads commercially available, either embedded in silicone (which can take a ton of heat) or on capton adhesive tape.

https://www.amazon.c...mc_bcc_2_i?th=1
https://www.amazon.c...d/dp/B0BHWLRDM3




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