#21  
Old 09-11-2008, 01:25 PM
TrevorW
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Ok got the temperature controller working with the help of my techo BIL now the Peltier only draws power when cold plate temp rises above a certain level just fine tuning now
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  #22  
Old 11-11-2008, 11:40 PM
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Tandum (Robin)
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Looks like a very affordable and worthwhile project Trevor. Having delt with with these things before, I'd be a little concerned about condensation/ice forming inside the camera myself. They can get pretty cold especially with a heat exchanger off the Titanic as you have
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  #23  
Old 12-11-2008, 08:37 AM
TrevorW
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Quote:
Originally Posted by Tandum View Post
Looks like a very affordable and worthwhile project Trevor. Having delt with with these things before, I'd be a little concerned about condensation/ice forming inside the camera myself. They can get pretty cold especially with a heat exchanger off the Titanic as you have
I'm not sure about condensation as the intent would be to use the Peltier only on warmer summer nights but will have too test and see. Some stats are attached

material cost so far $60

Cheers
Attached Files
File Type: doc Condensation.doc (76.5 KB, 28 views)

Last edited by TrevorW; 13-11-2008 at 02:15 PM.
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  #24  
Old 13-11-2008, 08:23 PM
TrevorW
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Test results

Taken inside ambient temperature 24 degrees C

Images all 105 seconds ISO800 camera turned off between image 1 and 2 left on between 2 and 3

After 15 minutes the cold plate temp had dropped to about 1 degree then jumped to steady 2 degrees although the Peltier draws 4amp at 12v DC only 10.2v was being fed into it. My battery supply was running low, so obviously the Peltier was not drawing the max current and running at full effeciency the heat sink hardly warmed up.

1st image is start up, 2nd after 30 minutes and 3rd after 45 minutes, no change in cold plate temp between 30 and 45 minutes.

A good reduction in noise is apparent, further testing will be done when sorted power supply issues ie: buying a 12v 5amp regulated power supply adaptor. At full current draw Peltier should acheive sub zero temperatures.

Attached Thumbnails
Click for full-size image (dark shot 1 start up.jpg)
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Click for full-size image (dark shot 2 30 minute.jpg)
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Click for full-size image (dark shot 3 45 minute.jpg)
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  #25  
Old 21-11-2008, 08:55 PM
TrevorW
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Its apparent that to get the cold plate down to sub zero temps you'll have to look at a multi-stage Peltier.

I've changed a resistor in the controller so the circuit draws 11.60V but the cold plate only gets down to around 1 degree after 15 minutes.

Another option would be to use copper for the cold plate.
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  #26  
Old 21-11-2008, 11:08 PM
Craig_L
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Definitely try the copper. Better than 99% pure aluminium I think.
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  #27  
Old 22-11-2008, 12:43 PM
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Looks like you are making headways there Trev! I don`t think you shall get it much lower even with copper as I think thats about the differential limit of the module. You will have to stack them to get it lower I say..
Now if you can chill the innards to 1-2 degrees things shall be very nice!
Camera modding time!
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  #28  
Old 23-11-2008, 10:04 AM
TrevorW
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Second set of test shots showed that although it appears there was no reduction in noise level you may notice a gradual drop in the file size after the Peltier was turned on ?? Each frame was reduced to 800X532 in PS7 saved as JPEG quality level 5

A further test run will be turning the Peltier on and leaving it on for a least a half an hour before turning the camera on and taking frames

Attached is the second run of test images ambient temp was 24C, camera was turned on over 10 minutes before 1st frame then Peltier was turned on, each frame is 15 minutes apart camera was left on during each frame,

Camera setting; Auto shutdown OFF, frames 90 seconds ISO 800 INCR OFF.

Final cold plate temp was around 2c
Attached Thumbnails
Click for full-size image (peltier test 1.jpg)
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Click for full-size image (peltier test 2.jpg)
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Click for full-size image (peltier test 3.jpg)
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Click for full-size image (peltier test 4.jpg)
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Click for full-size image (peltier test 5.jpg)
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  #29  
Old 23-11-2008, 03:19 PM
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Hi Trev, you should be able to get it cooler than that.
Here's a pic of my experiment a few years ago, tho not for an astro related use(homebrew ).

It's a single 33W 40x40mm peltier and the cold side is at, from memory, about -10c.
Thats with a large heatsink on the cold side that can absorb a fair bit of ambient temp... tho ofcourse the ice would be insulating it. With a little finger it could go much lower.
Room temp was probably around 20ish(room rarely varies, hence used for homebrew) and the heatsink temp up around the 40 mark somewhere.

The trick is to keep the hot-side as cool as possible, and it's massively important here to reduce all thermal resistance, I don't mean the heatsink, it's only a small part of the issue and the easiest to deal with. The heatsink might feel cool but the hot-side of the TEC could be much hotter.
The max temp differential(hot to cold side) for a Peltier is approx 68c.
If your cold side is at 1c, and the heatsink is at say 30c, the differential is only 29c, where is the other 39degrees? In all the materials and joints.

Imagine every interface, and every material used, as an extra resistor in an electrical circuit... voltage loss here, voltage loss there, it all adds up. Same with TEC's, temperature loss here, temp loss there.
Even a tiny space of air(the best insulator) between two mating surfaces will cause problems, thats why it's recommended to use lithium heatsink transfer compound.

In the photo I have used an aluminium CPU heatsink, about 3"x3"x2" size that has a copper core running through it, a rod about 1" dia by 2" long, the 1" diameter was too small for direct contact to the TEC (40x40mm) so I had no choice but to use a plate between the two. This adds another material and another joint, both with their thermal resistance, each could easily add a few more degrees to the hotside temperature, which means a few more degrees to the cold side too.
Each interface had atomised-silver heat transfer compound to increase contact area(reduce air) and reduce thermal resistance.

Will be building a temp controled brew fridge soon, so will be doing this again with all copper and silver construction and fluid radiator systems instead of traditional heatsinks.
Want to get the fridge as efficient as possible and be able to get the brew to lagering temps (1 or 2 deg) and possibly go all solar/wind powered.
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Last edited by MrB; 23-11-2008 at 03:30 PM.
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  #30  
Old 23-11-2008, 08:27 PM
TrevorW
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Mr B thanks for that

the further test as mentioned above showed an increase in the noise level the longer the camera was left on, the first image taken after the Peltier had been left on for half an hour and the camera off shows minimal noise but as the CCD heats up while the camera takes exposures 3 minutes apart the noise level rises between each image.

The best thing would be to turn camera off for a few minutes between each image which unfortunately is not practical except if long exposures are being taken.

Each of these images is the same ISO etc as previously mentioned although the time between frames is reduced.

From this it seems that the Peltier cold plate does make a difference depending on how it is applied.

Although another issue may be that the Pentax ccd has a higher noise to signal ratio than other camera's.

Attached Thumbnails
Click for full-size image (peltier 1a.jpg)
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Click for full-size image (peltier 2a.jpg)
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Click for full-size image (peltier 3a.jpg)
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Click for full-size image (peltier 4a.jpg)
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  #31  
Old 24-11-2008, 08:08 PM
TrevorW
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By removing the bottom plate I get the surface of the Peltier cold plate down to -3C after about 15 minutes with ice forming on the plate. Although I'm using a 12v power supply to run the camera will removing the batteries make any difference as batteries can cause heat???
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