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  #1  
Old 03-06-2013, 09:20 PM
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philiphart (Phil Hart)
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A Lightweight, Cheap and Simple Dew Heater for Night Sky Photography

The commercial controllers for dew heaters have three issues as far as night sky photography is concerned:

Expensive: $100+ just for the dew heater controller. This might be ok when you need multiple outputs to protect a telescope and guidescope, and possibly finderscope/eyepiece/camera lens as well, but that's an expensive solution to keeping just one camera lens free of dew. These controllers also make sense when you need fine control over the amount of heat applied to avoid telescope tube heat currents and the thermal distortion of delicate telescope lenses and mirrors. However, with wide angle camera lenses these issues are not generally relevant.

Complicated: It's another black box and an extra length of cables and connectors to carry and hook up.

Require heavy batteries: These controllers are designed to run off lead-acid batteries providing greater than 12 volts (and are often powered by DC power supplies connected to mains power). All the newer controllers will cut-out if the voltage drops below 12 volts, to protect the lead-acid battery from being over-discharged. This is great for protecting your battery, but a lead-acid battery that can provide several amp-hours of heating in cold conditions and still stay above the cut-off voltage ends up being quite heavy and inconvenient to carry up a mountain for example.

Lithium-ion batteries offer a great lightweight alternative to lead-acid batteries, but because their supply voltage quickly drops below 12 volts, they cannot be used effectively with the existing commercial dew controllers. In fact with the high resistance of the small dew heaters for wide-angle lenses, they could be run directly from 12 volt supply without any controller, but there is no easy way to plug the standard RCA connector on a dew heater to a DC power supply (RCA was a dumb connector to choose as the standard for dew heaters).

The Solution

My solution is to simply hack the connectors and plug the dew heater straight into a lightweight 12v lithium-ion battery pack.

Rather than cut off the RCA end and stick on a 2.5mm DC connector, I've made up my own adapter cables so that I can still use the dew heaters with the controller when I'm using the scope and mains/big battery power etc (see attached pic). You can make this adapter cable very easily from components on ebay.

Power Use and Specifications

The Lithium-ion batteries run at around 11.5 volts (anything from 12.6 fully charged down to less than 10 at full discharge). The Dew-Not 2" heater has 38 ohms resistance. This draws current of 0.3A and power output of 3.4W which is plenty for just a lens. On paper you should get 22 hours run-time from the 6800mah battery but I would not assume anything like that. I do comfortably get 8 hours run-time out of them (perhaps less in extreme arctic cold conditions though). If they are flat, the batteries can take a *long* time to recharge so put them on to recharge early in the day if you want to reuse them again the next night.

The 2" Dew-Not heater (~20cm) plus elastic and velcro is long enough to go round all my lenses, although there can be a bit of a gap in the heater material with just the elastic reaching round the last bit of the lens, but in practice this is fine.

The 2" Kendrick heater (~15cm) is a bit shorter leaving a larger gap and in some cases even the velcro does not reach round to attach at all around very big lenses. But the higher resistance of 49 ohms draws less power (0.23A, 2.7W) meaning longer battery life. In many situations this may actually be preferable. The 3" Kendrick heater (~25cm) has a lower resistance of 29 ohms and probably draws more than ideal (0.4A, 4.5W) from the Lithium-ion battery.

The dew heater and battery can now all fit in one pocket - much easier to carry round and hook up!

I've copied this content from a post on my site (http://philhart.com/content/dew-heat...ky-photography) for anyone who wants to do the same thing.

cheers
Phil
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  #2  
Old 03-06-2013, 09:27 PM
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Astroman (Andrew Wall)
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Nice write up Phil, very handy to know, may look into this for some wide field Meteor observing.
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Old 03-06-2013, 10:19 PM
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h0ughy (David)
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i had been thinking about something like this and bought two lithium battery packs from Aldi just the other weekend, but afterwards i realised that they are only 5V output. but i still can use it to power the polarie. like what you are achieving Phil
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Old 03-06-2013, 10:29 PM
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philiphart (Phil Hart)
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Quote:
Originally Posted by h0ughy View Post
i had been thinking about something like this and bought two lithium battery packs from Aldi just the other weekend, but afterwards i realised that they are only 5V output. but i still can use it to power the polarie. like what you are achieving Phil
Can you post a picture of those packs houghy? What capacity and price were they.. sounds interesting!

5V would still be enough to get a useful heat output.. probably good enough for many conditions?
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Old 03-06-2013, 10:44 PM
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h0ughy (David)
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Originally Posted by philiphart View Post
Can you post a picture of those packs houghy? What capacity and price were they.. sounds interesting!

5V would still be enough to get a useful heat output.. probably good enough for many conditions?
Power Bank
ea $ 29.99 *

Charge your smartphone, tablet, digital cameras or any other device with a USB charging function
7400mAh high performance
Li-Polymer battery
2 USB connections
Has an AC Adaptor for fast charging of Battery
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  #6  
Old 03-06-2013, 10:49 PM
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philiphart (Phil Hart)
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Quote:
Originally Posted by h0ughy View Post
Power Bank
ea $ 29.99 *

Charge your smartphone, tablet, digital cameras or any other device with a USB charging function
7400mAh high performance
Li-Polymer battery
2 USB connections
Has an AC Adaptor for fast charging of Battery
Interesting (and affordable). But it only has USB sockets? Anybody know how to adapt a 2.5mm x 5.5mm DC plug to that?
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  #7  
Old 03-06-2013, 11:01 PM
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Another couple of alternatives.

http://tinyurl.com/kvxxsc5

5VDC - 15000mAh
8VDC - 8500mAh
12VDC - 65000mAh
$36.90 free shipping
3 different voltage outputs from the one battery.

http://tinyurl.com/kwyd5af

12VDC - 9800mAh Li ion
$43.69
This one I have and is quite good. I will now make a couple of adapters to suit.

These from dealextreme.
Great company to deal with.

Thanks for the idea Phil
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Old 03-06-2013, 11:08 PM
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If you do need a cheap controller - these single channel controllers are $4.38 each - shipped!

http://www.ebay.com.au/itm/180924175...84.m1497.l2649

It's also worth keeping in mind that Li-ion batteries are notorious for exploding / catching fire... might be a good idea to keep them stored/transported separately to anything expensive.
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Old 03-06-2013, 11:16 PM
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And here's another one just come up on Zazz.

http://tinyurl.com/kp3l79b

16000mAh but at 5V.
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Old 03-06-2013, 11:21 PM
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Quote:
Originally Posted by naskies View Post
If you do need a cheap controller ...
Dave
I assume that the circuitry is PWM'd as they state they are for dimming LED's so shouldn't cause any problems with heaters?
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Old 03-06-2013, 11:42 PM
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Excellent Phil, thank you! I've been needing to get a dew heater organised for some time so will look into doing this. And thanks to you other guys too for the info on batteries and controllers.
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Old 09-06-2013, 03:53 PM
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Those batteries work well I got mine off ebay for $20 6800 Mah the scope lasts all night long and the mx2 with the Orion Teletrack share one and last all night as well so Thumbs up for the batteries I have had them for we'll over a year and work and charge well I never have run them flat and left the scope on and the dolly setup out the back and still and plenty of juice left over in the morning.
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Old 09-06-2013, 04:55 PM
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Quote:
Originally Posted by naskies View Post

It's also worth keeping in mind that Li-ion batteries are notorious for exploding / catching fire... might be a good idea to keep them stored/transported separately to anything expensive.
I know about this problem with Lithium Polymer batteries and that they shouldn't be left charging unattended but is this also true of the standard Lithium Ion??
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Old 09-06-2013, 06:16 PM
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Quote:
Originally Posted by naskies View Post
If you do need a cheap controller - these single channel controllers are $4.38 each - shipped!

http://www.ebay.com.au/itm/180924175...84.m1497.l2649

It's also worth keeping in mind that Li-ion batteries are notorious for exploding / catching fire... might be a good idea to keep them stored/transported separately to anything expensive.
I've ordered a controller today like the one shown and will find out how well it works!

Phil
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Old 09-06-2013, 07:51 PM
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Quote:
Originally Posted by Occulta View Post
Dave
I assume that the circuitry is PWM'd as they state they are for dimming LED's so shouldn't cause any problems with heaters?
Yeah, I think so... I first found out about these controllers reading through the DIY archives where others had used them successfully to control heaters.

Quote:
Originally Posted by Regulus View Post
I know about this problem with Lithium Polymer batteries and that they shouldn't be left charging unattended but is this also true of the standard Lithium Ion??
Most of the high profile cases I've heard of in the past 10 years have been with Li-ion batteries (LiPo being a more recent invention). It's not only just a recharging issue; they can spontaneously short circuit too.

Omaroo recently had a close call:

http://www.iceinspace.com.au/forum/s...d.php?t=107376

Quote:
Originally Posted by philiphart View Post
I've ordered a controller today like the one shown and will find out how well it works!
I'm still waiting on mine to arrive... surface shipping from China takes a while! I notice that another seller now has them listed for AU $3.68 each, shipped

http://www.ebay.com.au/itm/Brightnes...p/400476463622
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Old 09-06-2013, 07:58 PM
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Nice write up Phil!

Davewaldo put me on to those chinese dimmers a couple of years ago too - I've got a couple I've used to dim my lightbox. Heck of a lot easier and cheaper than any other duty cycle dimmer circuit kit I've ever been able to find.
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Old 09-06-2013, 08:00 PM
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Hi Dave,

I ordered a couple of those dimmers too. At that price who wouldn't? Also a big Li-Ion battery from DX. Next time I have ACP driving the big scope I'll grab some star trails without having to worry about dew.

Thanks for the pointer.

Cheers,
Rick.
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Old 09-06-2013, 08:02 PM
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Davewaldo put me on to those chinese dimmers a couple of years ago too - I've got a couple I've used to dim my lightbox. Heck of a lot easier and cheaper than any other duty cycle dimmer circuit kit I've ever been able to find.
That's a good idea, Rob. Wonder if they will work with my EL panel?
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Old 09-06-2013, 08:36 PM
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That's a good idea, Rob. Wonder if they will work with my EL panel?
They're reasonably voltage fussy I thought Rick? Hard to see how it could hurt to try - I'd be very interested to hear how you go. Getting flats >3 secs is still the bane of my life with the QHY9.
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Old 09-06-2013, 09:08 PM
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Careful buying those battery packs. There was a notice on overclockers last week about fakes coming from china.

http://www.overclockers.com.au/news.php?id=1087397
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