building electric cooler

i want to build an electric cooler, the cooler is 9"x6"x9" . my question is how powerful of a tec is needed, i found a 33w and a 51w at allelectronics.com and an 80w at directron.com . this cooler will stay plugged in my truck for 3-8 hours while i cut grass at different houses across town. the two heatsinks will be 3"x3"x1" and the hot side will have a 80mm fan.

It would probably be best to get this moved into extreme cooling so that you can grab the people who work with that stuff all the time. Or you could just leave this post here and I am sure Thelemac will move it for you as soon as he drops by.

your real limiting factor here is the TEC's current draw. Any more than about 8a for 8 hours will discharge your car battery, and that's if it's a good battery. 8a x 12v = 96w, and that's 96w draw, not heat transfer. I'd say the most you can go with is like 50w, but I did all the calculations in my head.

I'd agree with Greenman, since you only want to keep cold beverages cold, but I'd use the 33 watter (or smaller yet).
The larger Coleman ones use one around 50 to 80 watts, and because they don't have a constant heat input like with a processor, they don't need to be large.

If you still end up freezing your drinks, you could use a PWM circuit to cut back on the voltage applied to the fan, which will increase the temp of the cold side as well (somewhat). I don't think a PWM could handle the TEC itself. Maybe a small transformer from radio shack to change the voltage from 12 to 9.

Sounds like something I want to try too, since I work out on the road all day driving from city to city. I also have one of those little six pack coolers.....and a couple of TECs....and a couple of heatsinks....darn you!

Diggrr said:

Maybe a small transformer from radio shack to change the voltage from 12 to 9.

Click to expand...

Good idea, but you'd have to change (invert) the DC to AC, run it through the transformer, and then rectify it to DC again. Losses and expense would not be worth it. A pulse width modulation circuit (PWM) would be best.

Or, if it's freezing your drinks, move it into the sun. If it's too warm, move it into the shade.

the longest that i ever have the engine off is around 30mins or less. time is money so i hurry to cut the grass and then im off to the next house about a 15-30mins drive. so if i slow down the fan, the tec will not cool as fast then? my fan blows around 20cfm, it came from an old 200w powersupply.
thanks for your help

Okay, that helps. You need two fans and two heatsinks. The hotside fan needs to be running at max all the time, but you could vary the coldside's fan to change temperature. 50-80w should do it.

You should not need to change the current from ac to dc. If you use a PWM to control a transistor you can run all the current you need. Also, I would reccommend using an oscillator that allows you to vary the duty cycle so that you can dial in the temp that you like.

Word of warning: make sure that you use a fairly high frequency oscillator for this (like around 20khz) as TECs do not live very long when you do this at 20hz...

he suggested transformer, I was stating that wouldn't work.