Newb needs power & interface

ScottF

Hi:
I would like to turn a peltier cooler on and off with my STAMP.
The cooler can handle 1.40 watt, 3.0 amp max.
a pdf for the cooler can be seen at:
http://www.customthermoelectric.com/tecs/pdf/00711-5L31-03CA_spec_sht.pdf
·I would like to use batteries to power the cooler but not sure what the best interface to my STAMP is and how to power my cooler. I would like to use batteries for the cooler if possible. Any suggestions would be appreciated.
Thanks
Scott
·

phil kenny

Some more info on how and where you plan to use this device
would enable people here to provide assistance.

For instance:

1) How long is this going to be operated? (minutes or days)

2) What sort of temperature reduction do you expect?

3) Any details on the environment it will be used in?

phil

Post Edited (phil kenny) : 1/11/2008 6:04:39 PM GMT

ScottF

I don't wish to engender the wrath of the Anti-Vivisectionists. Our lab has to get a few degrees of cooling of grams of biomass for 2 to 3 days. We can change the batteries used on the peltier easily. I am trying to educate my self on how I can power the peltier with it's own power source and switch it on with the stamp. Are there private communications in the forum? I can send more details if you need them.
Thanks.
Scott

Franklin

Phil asked some good questions, if you could answer those we might be able to help. We are all here to learn and the best way to do that is in the public forum.

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- Stephen

phil kenny

A Thermoelectric cooler (TEC) prefers to operate from a current source as
opposed to connecting it directly to a voltage source (aka a battery).

If you do connect it directly to a battery, you'll have to include some form of
current limiting such as a resistor. This is a simple solution, but most of the
energy supplied by the battery will end up wasted as heat.

A switching regulator type of current source would be more efficient and
greatly extend the battery life. It could be as simple as a FET switching an
inductor that is in series with the load (the TEC). Sizing the components
involves supplying some numbers for the load current and battery voltage.

To give any more specific solutions you will have to provide further details
regarding the design goals.

A Stamp can certainly be programmed to switch on and off the power to the
TEC if it is monitoring the temperature of the cooling chamber..

phil

ScottF

Thanks!

The experimental animal is a Rat and has to wear the power supply without connection to the rest of the world. It also has to wear the stamp and it's power. It won't be moving (much)·while this is going on. The TEC only has to cool a small area a few degrees for 2 days. It will be sealed from moisture and placed under the skin near the area to be cooled.

I can't picture using anything but a battery despite it wasting heat. I am not aware of any small self contained switching regulater power source. Is there such a source? Can you help me figure out how to calculate the resistance I need for different batteries for the TEC described?

Thanks again!
Scott

phil kenny

Hello Scott,

That's puts your problem in a wholly different perspective.

Even though you only _want_ to cool a small volume, since the TEC
is implanted, my guess is that you're actually going to be cooling
the entire animal. This is because the TEC will be situated in a
mass that is a fairly good thermal conductor. Circulation of the
blood will be causing the cooled location to heat back up.

Have you attempted to create a thermal model of the TEC and
its surroundings after being implanted? You need to do that in order
to get a good estimate of the rate at which the mouse dumps heat
into the TEC. Only then can you begin to figure out the current
needed in order to achieve the desired cooling effect.

Let's say you wanted to cool the TEC by 10 deg. C. Then the
operating point would lie somewhere on the black dashed line
of the curve given in their spec. To get the 10 deg reduction, may
take as little as 300 ma or as much as 3 amps. It all depends on
how much thermal energy the animal's metabolism is generating.

This is where the vertical axis of the TEC performance curve
comes into play.

Once you have a good estimate of the current needed, then you
can start investigating power sources.

phil

Post Edited (phil kenny) : 1/12/2008 11:48:22 AM GMT

ScottF

Our estimate of the amount of heat we want to remove is 0.42 watts. Our first thought was to put the TEC in series with 1 Ohm resistor and 2 AAA NiCads.

phil kenny

Scott,

That info helps clarify things a lot.

I estimate the TEC current will be about 2.3 amps if you use a 1 ohm
current limiting resistor.

This was found by looking at the drop across the TEC (about 0.7 V)
and assuming 3.0 v for the two AAA batteries.

The 1 ohm resistor will need to be rated at 10 watts since it will
be dissipating 5.3 watts. I hope it isn't in contact with the subject.

AA batteries have about 2000 - 2500 maH capacity. AAA have even less.
Therefore, you'll be needing to change batteries every half hour at least.

Depending on internal resistance, you may not be able to draw 2.3
amps from a pair of AAA cells. It might be necessary to use additional
cells in parallel.

You may want to experiment on a dead rat first to see how much
cooling you get with 2.3 amps and how long the batteries last.

Unless you are monitoring the temperature somehow, the Stamp won't
know when to turn the TEC on and off.

phil

ScottF

Thanks, this is very reassuring. That's about the value we were contemplating for a current limiting resistor. We haven't gone on to temperature measurement yet but in the planning stage felt this would be an easier problem for us. I appreciate your time and insights.
Scott

sullydog

Hi guys. I'm one of the mad scientists working with Scott on this project. Thanks for helping us to this point. I wanted to put in some clarifying information. The peltier will actually be mounted outside the animal. The cold side will connect to the carotid through a short, thermally insulated copper conduit that tunnels under the skin and is bonded to the cold side with thermal adhesive. Based on mathematical modeling projections, a total of 0.42 W of heat will have to be pumped out of the carotids to achieve the desired brain temperatures. The dT across the Peltier will be about 30 degC. The Peltier we're currently fiddling with has a power curve indicating that at a dT of 30 deg C, the device can pump 0.4 W at about 0.5 V and 1.5 amps. We may ultimately be able to find a Peltier with more favorable specifications, but so far we haven't.

The microcontroller will receive input from a very fine gauge thermocouple implanted in the rat's cerebral cortex.

Crude schematics attached. Obviously, this system is still in early development phase.

The system has to function for no more than 24 hours and perhaps as little as 3 hours...that's one of the parameters we'd like to explore experimentally.

The AA batteries were a sort of default mode--the power source must be compact and not too massive. If anybody can suggest something different, that is portable, will deliver the necessary power, and is small and light, I'd love to know about it. Button batteries don't seem to have the juice.

Thanks for all your help. This is a neuroscience lab, and we don't have much expertise in electronics. We're going to incorporate some of your suggestions into some test circuits this am.