Cooling - what is Peltier???

[davidpretorius]

Ok, i have read about birds peltier cooled mirror and about peltier cooled toucams.

Now which bit is the peltier??? or is peltier a process or heat action?

Do i simply buy a heatsink and a fan? and whammo -> i have a peltier cooler???

[Starkler]

A peltier is a semiconductor device that works something like a reverse cycle aircon in that it transfers heat from one surface to another. 12V fridges have a big peltier cooler in them.

[Striker]

Yep what Geoff said.....the peltier is a small flat surface that exchanges the heat from one side to the other...making one side very cold the other very hot...normaly this hot side has a fan to cool the heated side as much as possible...the cooler you can keep the hot side the colder the cold side will be.

Here is a picture of a Peltier....they normaly will run on 12 volt but come in various sizes.

http://www.oatleyelectronics.com/images/gp1.jpg

[davidpretorius]

thanks guys, i am now off to mr bird to get some ideas on using em with my scope!

[Starkler]

Fans not doing the job for you Dave?
Hows things with the sucking fans now?

[Photon]

Hi Dave,
I've had a little experience with peltier effect modules and put one in a small Esky just to have a play a year ago. Oakley Electronics is where I purchased a module. Try this link.
http://oatleyelectronics.com/peltier.html
You could mount one on the back of a mirror for quick temp stabilisation if that's what you where thinking, considering you have the temp sensors to monitor what is going on it might just be a practical option

Regards Photon.

[davidpretorius]

not bad, but a cooling rate of 2 degrees an hour is not enough with the largish range i get during summer. from say 27 down to 5 or 6??? that is 11 hours of cooling!!

i need to go to the next step. birds is very pratical and sensible solution of blowing cold air around the mirror. i have ordered some peltier from the new jaycar store in town, so i will start building after xmas. the plan is to keep it real simple and hopefully in a kit form!

[davidpretorius]

thanks photon, jaycar have just opened near the cinemas. i talk to the guy today and he has quoted me some 50+ pricing for 4 peltiers, so i have ordered them in.

He will get in what i need to make this. I will also visit some local aluminium fabricators to help knock this together.

[acropolite]

BTW Dave peltier effect devices draw substantial current when operating.

[davidpretorius]

i have ordered the 4 amp version. my 7amp hour battery is in a bit of strife, but if i can get the cold plate happening quickly and blow that air around the mirror and get it colling quickly, then i might get away with a practical sized battery

[acropolite]

Dave, at least at home, there's always a long extension cord...

[janoskiss]

4 amps, or about 50 watts, sounds way too much! Did you check with Bird? I'd forget about running it off the 7Ah battery. I don't think they can handle that sort of load without dying quickly (I mean dying for good not just going flat).

[davidpretorius]

bird has 2 or 3 x 8 amp versions.

I have grabbed 4 of the smallest i can get and will start with 1 and then test and then add a second etc.

I want to try and get a handle on how much cooling one peltier does compared to two compared to three, compared to the 6 amp compared to the 8 amp.

Some guys may be happy with 240 volt or a 80 kg of batteries to drive the cooling system, others may only want to use existing car batteries.

i am very happy with my 7amp hour battery from jaycar. It ran the fans from 10pm to 4am and then tracking and panning the scope for 2 hours.

[RAJAH235]

Davo, the fan only draws 150/170 mA, while your t'scope may draw 2 or 3 amps when slewing. Not sure when tracking,(~200/300mA???).
The peltier requires a constant minimum 4 AMPS to work 'efficiently'. (if they do at all)!
IMHO, You would be better off buying the biggest capacity battery you can to start with. You will not get a true test result using the 7 AH. T'will 'die' within 15/30? minutes. Put a DMM across the battery terminals & watch the voltage. If it falls below ~10.5 volts, stop. They don't like going below that.
HTH. :D L.

[davidpretorius]

thanks rajah,

was wondering when it is best to recharge the battery. I have a car battery as well to play with a 12n-24-4

i am going to gets some sensors to monitor ie amps and volts i think and integrate into the system

[acropolite]

Dave asked... The best time to recharge any lead acid or sealed lead acid (gel) battery is immediately after use. The ultimate life of any lead acid battery is dependant on the depth of discharge and the number of charge discharge cycles. i.e. for a shallower discharge you will get a greater number of charge discharge cycles. Even "deep cycle" batteries, which have special additives to acheive better deep discharge performance, are best charged in this way. The process of discharge causes pure lead to change to lead sulphate, which starts to harden rapidly to the point where after as little as a couple of weeks it will not reconvert to lead when thge battery is recharged.

[davidpretorius]

thanks phil

[Starkler]

Dave, the sucking fan is more about removing the boundary layer than fast cooldown of the mirror. Do you really need to go to this extent ?

[davidpretorius]

two things:

i do notice the odd vibration at 250x through the eyepice, but am yet to test it.

I need to take imageswith the 5x powermate of say saturn with fan on and fan off when there is a temp difference and then wait for no temp difference and try again.

It is hard to isolate bad seeing at the moment due to temp difference, jetstream and a warmer mirror and not to mention "idiot: factors like orgetting to check collimation

Plus i like to tinker!. If i get the cooler happening, then i can do the experiment all in the space of say 10 minutes where i am hoping the atmosphere stays realtively consistant so i can isolate fan vibrations, boundary layers etc

[bird]

Hi guys, I run my peltiers off a $50 computer power supply, the +12v rails are rated at about 20 or 25 amps, more than enough to do the job. Put it into an instrument case bought from Jaycar (or **** Smith, can't remember) with sockets and switches etc on the outside for all the bits and pieces I run.

Course this means I'm permanently tethered to 240v...

Bird

[RAJAH235]

Davo, fwiw. Might retrieve something from here.......
http://www.rsaustralia.com/cgi-bin/bv/initSession.jsp?goto=/browse/Browse.jsp&catpath=XA/All%20Products/Batteries%20and%20Power%20Conversio n&logText=au0098
Merry Christmas. :D L. :xmas:

[davidpretorius]

thanks rajah, have a great chrissie