Powering a peltier

[gamefreak]

For a school project I am working on a mini-fridge using peltier units and I am currently wondering how to power the beasts. I have five 136.8 watt peltiers whose stats are:
VMax 15.2
IMax 9
Ohm 1.3 - 1.5
deltaTMax(celsius) 67

Using the bench top power supplies at my disposal i've only been able to get them up to 8 volts(on a good day) with 3 amps(where they quit), at that the peltiers can quickly get cold enough to freeze water and water covered fingers to them within a few seconds.

I intend to power this by plugging it in since it would require an ungodly amount of batteries. In my limited searching I havent found any wall adapter that gets anywhere near those values and it is starting to look like I may have to fashion one myself.
I have zero experience with anything above 12 volts and zero experience with AC and would rather not play with the kind of power that could kill me.

Another problem is controlling the power to the peltier. It would be nice if once the fridge is cooled that I could lower the power to the peltier. From my early days in robotics I learned that trying to make pins of a microcontroller try to source more than a few milliamps is a good way to either break the entire chip or get lucky and only that pin dies. I imagine that trying to pull 9 amps from a microcontroller would be a fun way to make it explode...
I have found some things on this subject such as building a digital to analog converter out of resistors, anyone have any thoughts on this?

[Razor Concepts]

12v car batteries may be the easiest way

[billhowl]

For power supply you can use this Benchtop power supply from ATX computer power supply
http://letsmakerobots.com/node/1355
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

For control you can this Schematic for driving DC solenoids, using a transistor connected to an Arduino microcontroller pin and replace the solenoids with your peltier.
http://www.arduino.cc/playground/uploads/Learning/solenoid_driver.pdf
or you can use relays to control the peltier.
http://www.arduino.cc/playground/uploads/Main/relays.pdf

I have found some things on this subject such as building a digital to analog converter out of resistors, anyone have any thoughts on this?

Those are for DAC using R/2R resistor network, most for generate audio signal. Not for driving high current circuit.

[gamefreak]

I don't really have any power supplies lying about and buying one just to hack it would make me and it sad. Is it possible to wire transformers in parallel and get the amperage I want? RadioShack sells 12.6V 3A transformers, so could I simply wire their outputs in parallel and have a decent amperage? Along those same lines will the transformer just crap out at 3A or will bad stuff start to happen when I approach it?
In any case, after I transform it down to 12.6 volts I need to rectify, and filter it, correct?

As for the worry of playing with high voltage electricity, we were playing with an exposed Jacob's Ladder in class the other day. So long as I dont poke my fingers where they dont belong i'll be fine.

As for turning the power on/off wouldn't it be better to use a Mosfet instead of a simple transistor?

@Razor:
The original plan involved batteries, but they cost a lot, weigh a ton, and are annoying large.

[kd5kfl]

battery charger, or motorhome power supply

[SmAsH]

You could use the transformers in parallel if you wanted to...
Bad stuff shouldn't happen when you approach the max current, only if you go over it.

[Soeren]

Hi,

For a school project I am working on a mini-fridge using peltier units and I am currently wondering how to power the beasts. I have five 136.8 watt peltiers whose stats are:
VMax 15.2
IMax 9
Ohm 1.3 - 1.5
deltaTMax(celsius) 67

Using the bench top power supplies at my disposal i've only been able to get them up to 8 volts(on a good day) with 3 amps(where they quit), at that the peltiers can quickly get cold enough to freeze water and water covered fingers to them within a few seconds.

Freezing a tiny amount of water is nothing, even a small Peltier will do that easily.

A Peltier doesn't cool stuff, it moves heat. For a full use of your 5 elements, you'll have to get rid of the transfered heat PLUS the heat from the 684W worth of Peltiers, so to get any real use of them, you need serious cooling of the hot side(s), which means large heatsinks and fans.
Depending on the size of the fridge you're building and how fast you want it to cool down how large objects, a single peltier or two may be enough - I think each of your Peltier elements are about the size used in automotive cooled thermo boxes or close to (mine is about 12A @ 12.6V IIRC).

I intend to power this by plugging it in since it would require an ungodly amount of batteries. In my limited searching I havent found any wall adapter that gets anywhere near those values and it is starting to look like I may have to fashion one myself.
I have zero experience with anything above 12 volts and zero experience with AC and would rather not play with the kind of power that could kill me.

You'd need a 15V/45A PSU to fully power up all five Peltiers. That's not something you're gonna just whip out, but perhaps you can find a (semi-)local power supply producer that will help you out, since it's a school project.
On the other hand, you'd need a lot of forced cooling for their hot sides, so perhaps you should stick to one or two for that reason alone.

Another problem is controlling the power to the peltier. It would be nice if once the fridge is cooled that I could lower the power to the peltier.

A simple on/off will be quite sufficient, as the entire setup will have inertia so a thermostat could be made with an automotive relay, an NTC resistor and a small circuit in between.

[Soeren]

Hi,

You could use the transformers in parallel if you wanted to...
Bad stuff shouldn't happen when you approach the max current, only if you go over it.

Any difference will equate to powerloss (and hence heat), so that should only be done with 100% identical transformers from the same batch from a reliable source - And then they should be phased correctly to avoid very bad things (like melt down of the windings).
If the tools to assure correct phasing aren't available, I wouldn't recommend going there.

[gamefreak]

Im aware of how peltiers function, thats was step one in my quest. And for the sake of simplicity and cost I only intend to use one, the extras are there in case I break them, and its fun to freeze water with electricity.
I still need 12 volts 9 amps to fully power the little monster,

@Soeren:
So a few of these: http://www.radioshack.com/product/index.jsp?productId=2102702&tab=summary
Would not work if I wired them in parallel? I am pretty sure I have no tools for playing with transformers and so dont know how I would ensure correct phasing.

[SmAsH]

You could try an old pc supply, ive got one sitting on my seat that can supply 12v 12A that i got for free, great little things.

What soeren is trying to say is that if the transformers are not identical bad things might happen.
Those in parallel would work but only if they are exactly the same.

[gamefreak]

In a last ditch effort of saving money by digging through piles of stuff, I found a wall wart that has an output of 12 VAC 7 VA. Apparently Lego is trying to kill its customers with high voltage  . I figure that I can cut the chord, throw 4 diodes on the end for rectification, and put a large cap on to achieve a ~8.5 volt DC supply that is stable and has 7 amps to feed the peltier.

any thoughts?

[Soeren]

Hi,

@Soeren:
So a few of these: http://www.radioshack.com/product/index.jsp?productId=2102702&tab=summary
Would not work if I wired them in parallel? I am pretty sure I have no tools for playing with transformers and so dont know how I would ensure correct phasing.

You could phase them like this.
http://That.Homepage.dk/PDF/Phase_Check.pdf

[waltr]

I found a wall wart that has an output of 12 VAC 7 VA. Apparently Lego is trying to kill its customers with high voltage  . ............ d has 7 amps to feed the peltier.

any thoughts?

That is 7 volt*amp or 12V/7VA = 1.7 Amps. So this will never power your Peltier.
Also no high voltage! Its output is 12 V rms at 60Hz (if you are in the USA, 50Hz most other places).

[gamefreak]

Well that saddens me.
So the current list of things is:
Batteries: Simple, and effective. Weigh a ton, expensive, huge, got none just lying about
Computer Power supply: enough juice to power it. got none just lying about, requires modification
Transformers: Readily available, not to expensive. May end badly, involves high voltage
Battery Charger:I've got a few small ones lying about. The only one that has more than a few amps is my only good one


A quick online search of microcenter shows some cheap power supplies, I may just have to go out, buy it, and hack it to the way i need.
Thanks for all the help so far.

[SmAsH]

If you opt for the power supply method there isnt really much to modding it.
If it is a non-atx psu all you have to do is group the wires you want, eg. 5v, 12v and gnd.
There is nothing more to it! And you can pick up a used 300-500w for $30!

[Soeren]

Hi,

And you can pick up a used 300-500w for $30!

For free in the nearest dumpster - it's amazing how many is thrown out, most of them still in a PC.

[SmAsH]

Yeah, true... I have fallen in love with my local tafes dumpsters
Also, my mum brings me pcs home whenever her work throws them out...

[gamefreak]

I have yet to go dumpster diving, and wouldnt know the first place to start. My friends have pulled LCD computer monitors from peoples trash before but I have not ever seen anything above garbage in the trash around me. I blame those stupid greedy electronics recycling events that my school has.