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Does anyone have any information regarding pelter cells? Answered

I am working on a pelter mini fridge and i need some info on what wattage would be appropriate for a 14 inch by 8 inch by 8 inch space. also, is there anything special this needs when i am wiring it? would a wall wart work, or do i need to hack a computer supply? thanks, any help is greatly appreciated. 



http://www.jcelectronica.com/articles/peltier.htm  tells how to design a Peltier cooling system and how to calculate the wattage.

 For powering it, the article says, "Driving a Peltier cell is easy. It behaves very much like a resistor, with practically negligible capacitance and inductance. A low-voltage medium-current BJT or MOSFET can be used as a switch."

Frack.  Wrote a long response and it got deleted accidentally closing a tab.  The old text editor never did that...anyhoo.

You need to know:
Specific heat capacity of the thing you want to cool.  For water its 1 joule per 1ml per degree c. 
How much of that 'stuff' you want to cool
thats ~350 joules per degree C per can of pop
Temperature difference you want to get - say, 25 degrees C to 5 degrees C = 20 degrees C.
Thats 20x350 = 7000 joules (7kj) to cool 1 can of pop 20 degrees.

Figure out how long you want it to take - say 1 hour.  Divide your number of joules (convertible to watt-hours)
convert (I use google) 7 kilojoules into watt-hours and you get 1.94 Wh.

At 100% efficiency it would take a 1.94 watt peltier 1 hour to cool a can of pop.
a 10 watt peltier would 1.94wh/10w = .194hours = 12 minutes.

Peltiers are much less efficient than that, but I like to work in that physics world of ideal conditions, then figure out the error after.


How to wire it - as others say, its dead easy.  Use whatever's available.

If an ideal wall-wart (3-12 volts, find one that matches your peltier, say 5 volts) can put out 2 amps of current, its 5v*2A = 10 watts.  You can have a maximum of 10 watts of peltiers on that wall-wart.

As cyberpageman says, you can use a mosfet to turn it on and off to a thermostat - depending on how deep you want to dive in.

Thanks. i was just wondering, do i need to worry about current draw? i would rather not destroy a wall wort. 

 Just so you know, its peltier, not pelter

Wall power supplies can supply 'as much as' they are rated for - best not to go too far over that limit, or heat, then fire, will be an issue :D

Make box. Put cup of water, with free ice in it (0 deg C). Measure time to rise to ambient. Power required for losses can be estimated that way.  

Or, design in a big cell, put on a variable power supply, measure temperature, adjust current to keep temperature stable in sample.

Its hard to predict, unless you know the thermal performance of the case, and how cool you want to be, in whatever  ambient it needs to work in. Lots of insulation is a good thing. P cells are not very efficient.

Thanks. I just wanted some ballpark info. Probably just one inch foam f/ insulation, and room temperature ambient. Id like it to be approximately as cold as a normal fridge. of course, there is no real way anyone could calculate  exactly how many watts i would need.

'peltier'- easy to spell if you pronounce then imagine it french..ly.  pel-tee-ay.

As steve says - its all about the r-value of the container.  If it insulates better, it will need less 'upkeep' energy to keep it cool.

The more wattage the faster it will get from ambient to your desired temperature.  With sufficient insulation, a very small peltier can cool a very large area - very slowly.  Conversely a very large peltier will get a small area cool quickly.  Also depends on the heat capacity of the area.  Liquids will take a lot more energy to cool than gasses. (by density).