Just knocking up a quick dew heater for my coma corrector/DSLR

[SteveBz]

Hi People,

I thought I got my dew problems sorted out years ago, but this winter showed me that I really hadn't.  The very few clear nights available were wrecked by condensation, so I've knocked up about four 12v dew straps.  I'm now on the 4th and I thought I'd photograph it as it's really easy:

1) Measure the circumference of the DSLR (or whatever).  Mine is 25 cm.  Normally I would allow 1 resistor per cm (ie 1/3 W per cm) this should be 25 watts, but because of the peculiar design of this one I've used much less.  Also I don't want to overheat my camera. In this case I used only 8 (it turned out warm, so let's see it in action later).  I was using some standard 1mm lighting cable.  I just wrapped the cable round the focuser and marked it.

2) Buy some 470 ohm resistors (Yellow, Purple, Black, Black - 4 band), these are about 1/3 w at 12v.  Allow 1 per cm circumference.  So I should have used 25.  You want these big chunky ones from ebay rated at 1/2 w.  

3) Cut your wire at the mark and strip it.

4) If you are building a narrow heater, as I am, measure the gap.  Mine is about 15 mm:

5) Lay the the wires and the resistors out on your soldering pad and stick them down with tape.  Make sure they are 15mm apart (or whatever) .  Even if you are not width restricted, they need to be parallel.

6) Solder them in place:

7) Remove the tape and snip off the extra wire:

😎 Make sure it fits:

9) Add a cable to it:

10) And a 2.1 mm dc plug:

with insulation:

**** End of part 1 ****

 

Edited January 21, 2021 by SteveBz
spelling

[SteveBz]

*** Part 2 ***

Here you need some duct tape (mine is called Duck), and some leftover black material left over from flocking your Newtonian.  Or anything else.

11) Layout the tape and hold it down:

12) Lay your heater on the tape:

13) And wrap it:

14) Use your left over flocking and cut a similar size piece to the tape:

15) Remove the sticky backing

16) and wrap

17) Shape it:

Now it's ready for testing:

18) I knocked up a quick test rig with a 12v supply, which I'll use later as a hub:

19) It got warm.  How warm?  My IR Sky Thermometer says 42 C :

 

I'm about to mount it with a cable tie when I realise I've run out.  I order some off Amazon for £5 and I'll finish tomorrow.

I had a lot of fun doing this project.  Maybe it took 2 hours, apart from waiting for Amazon Prime.

Regards

Steve.

 

 

Edited January 21, 2021 by SteveBz

[DaveL59]

last one I made up for the skycam I wired them in series using ceramic cased resistors. That one had 5x 15R 3W, also a temp and LDR controller board in parallel to turn it on as needed. Seems to have largely worked at keeping the dome clear tho it can struggle on very cold sub-zero nights so I may need to drop one of the resistors (when its warmer and dry, it can wait lol). I used heatshrink to cover the chain but I happened to have some laying around which was handy.

At 25 resistors in parallel you're effectively at 18R for the string, that's 666mA and 8W, pretty warm I expect? Less resistors would give a higher resistance overall so perhaps better? 

I'd worry on a camera lens if its plastic bodied that the plastic would suffer in the long run.

Edited January 21, 2021 by DaveL59

[SteveBz]

14 minutes ago, DaveL59 said:

last one I made up for the skycam I wired them in series using ceramic cased resistors. That one had 5x 15R 3W, also a temp and LDR controller board in parallel to turn it on as needed. Seems to have largely worked at keeping the dome clear tho it can struggle on very cold sub-zero nights so I may need to drop one of the resistors (when its warmer and dry, it can wait lol).

At 25 resistors in parallel you're effectively at 18R for the string, that's 666mA and 8W, pretty warm I expect? Less resistors would give a higher resistance overall so perhaps better? 

I'd worry on a camera lens if its plastic bodied that the plastic would suffer in the long run.

So I used Nichrome wire for my earlier ones and they don't really cut it.  The parallel resistors are much better.  I do use those cheap PWM controllers from China to control the temp, but it works fine without too.

The nice thing about parallel resistors is that you can always trim to length and reuse the cut-off resistors for the next one.  

I have one for my secondary mirror, one for the primary opening one for the focuser and one for the guide-scope.

Edited January 21, 2021 by SteveBz

[Icosahedron]

Yellow, violet, black, = 47 ohm, not 470 ohm (yellow, violet, brown). This might lead to overheating or overloading of your PSU.

Edit: Sorry, I made a mistake, it is a four colour band resistor. Yellow, violet, black, black is indeed 470 ohm.

Edited January 21, 2021 by Icosahedron

[SteveBz]

1 hour ago, DaveL59 said:

last one I made up for the skycam I wired them in series using ceramic cased resistors. That one had 5x 15R 3W, also a temp and LDR controller board in parallel to turn it on as needed. Seems to have largely worked at keeping the dome clear tho it can struggle on very cold sub-zero nights so I may need to drop one of the resistors (when its warmer and dry, it can wait lol). I used heatshrink to cover the chain but I happened to have some laying around which was handy.

At 25 resistors in parallel you're effectively at 18R for the string, that's 666mA and 8W, pretty warm I expect? Less resistors would give a higher resistance overall so perhaps better? 

I'd worry on a camera lens if its plastic bodied that the plastic would suffer in the long run.

Dave,

I'm going to add one more point.  I did buy a professional dew heater and it's much hotter than any of the DIY dew heaters I made.  So I think I wouldn't worry.  This one here with 8 resistors is quite cool in comparison.  The other ones I made with 1 resistor / cm are about the same as the professional one. 

Steve,

[DaveL59]

Thanks Steve.

For me if I do use the DSLR it'll be with vintage lenses which are metal bodied so much less of a worry. I don't plan on dew heaters for my OEM Minolta ones which are more plastic encased but have yet to really do much with either at night, not sorted dew heaters for these yet either, one on the to-do list one day...

[Seelive]

1 hour ago, SteveBz said:

Buy some 470 ohm resistors (Red, Yellow, Black)

Red, Yellow, Black? In the 3 band resistors colour code that's 24 ohm,  at 12V they would certainly get hot! 

[SteveBz]

6 minutes ago, Seelive said:

Red, Yellow, Black? In the 3 band resistors colour code that's 24 ohm,  at 12V they would certainly get hot! 

See above

59 minutes ago, Icosahedron said:

Yellow, violet, black, = 47 ohm, not 470 ohm (yellow, violet, brown). This might lead to overheating or overloading of your PSU.

Edit: Sorry, I made a mistake, it is a four colour band resistor. Yellow, violet, black, black is indeed 470 ohm.

So I should have been clear its a 4 band.

Edited January 21, 2021 by SteveBz

[SteveBz]

1 hour ago, Icosahedron said:

Yellow, violet, black, = 47 ohm, not 470 ohm (yellow, violet, brown). This might lead to overheating or overloading of your PSU.

Edit: Sorry, I made a mistake, it is a four colour band resistor. Yellow, violet, black, black is indeed 470 ohm.

I've edited it.  Does that look right now?

[SteveBz]

17 minutes ago, Seelive said:

Red, Yellow, Black? In the 3 band resistors colour code that's 24 ohm,  at 12V they would certainly get hot! 

Sorry, checked again.  Typo.  Yellow, Purple, Black, Black.

[Seelive]

3 minutes ago, SteveBz said:

I've edited it.  Does that look right now?

It's now showing as ...Buy some 470 ohm resistors (Red, Yellow, Black, Black - 4 band)... 

... Sorry, checked again.  Typo.  Yellow, Purple, Black, Black ... Correct.

[faulksy]

i used 330ohm 1/4 watt for mine, small enough to bend. coupled to a cheap pwm

[faulksy]

this is the pwm controller i bought. even goes to open circuit (off position)

https://www.amazon.co.uk/dp/B076Q99K6D?psc=1&ref=ppx_pop_dt_b_product_details

[SteveBz]

12 hours ago, faulksy said:

this is the pwm controller i bought. even goes to open circuit (off position)

https://www.amazon.co.uk/dp/B076Q99K6D?psc=1&ref=ppx_pop_dt_b_product_details

Nice.  It's already in a box.  Mine isn't.  I used this one:

Mini DC Motor Speed Control Driver Board 3V-35V 5A PWM Controller / LED dimmer | eBay

Edited January 22, 2021 by SteveBz

[faulksy]

9 hours ago, SteveBz said:

Nice.  It's already in a box.  Mine isn't.  I used this one:

Mini DC Motor Speed Control Driver Board 3V-35V 5A PWM Controller / LED dimmer | eBay

i seen that one, then seen the one i got 🤣