Teach the stupid biologist basic electronics...

[Mokele]

Well, time for some electronics questions that are so basic you'll think I ride the short-bus to school...

 

Basically, there's a plant I want to grow, and it requires cool water around it's roots. Because I don't have the time to put icecubes on the damn soil all day, I decided that I wanted to find a way too cool it down automaticly. And I found the perfect things: Peltier junctions (apparently also called Thermo-electric coolers), which transform electrical energy into a heat difference across the device, with one side cooling and the other warming up. I can find lots of them, with lots of statistics on them, but here's the problem:

 

Although I took a bit of electronics (from the theoretical end) at a college level, I have almost zero practical experience and remember almost nothing. I remember stuff like V=IR, and that's actually confusing me.

 

I basically need a peltier, some associated stuff like thermal tape and a heatsink that I can handle, and a DC power source that doesn't need to be plugged in (no ac to dc converters) simple because there are no outlets on my porch (and I'll be damned if I'll let the mosquitoes feast on me for the sake of the plant).

 

What's confusing me is how everything is described. They'll say that the peltier can take up to ___ volts and/or (I dunno which) ____ amps. The power sources I've been looking at (rechargable batteries and solar panels) all say the output __ volts and ___ amps.

 

This confuses me, because I thought the power sources just set up a voltage difference, and how many amps was a consequence of the resistances in the circuit (via V=IR). So, seeing power sources also give amps is confusing me. Similarly, why are both max amps and max volts given for the peltier junctions? Shouldn't the volts over the resistance of the junction give the max current? Or is giving me those a way of telling me the resistance?

 

I found a little "kit" of solar panels online for less than $20 that apparently assumes you know nothing, and allows you to customize voltage and amperage, so I'm probably gonna go with that (especially since I only need the cooling when the sun is out), but I'm still confused about a power source giving amperage. Don't they just give volts, and the volts generate the amps in the circuit?

 

More specificly, how do I know what to hook up to what? I know the heat differential across a Peltier is proportional to current (thanks to wikipedia), but how do I modify that? By modifying the current of the power source or the voltage?

 

I guess I'm basically just confused because I'd always been taught that voltage produces amps when there's a circuit, so telling me that batteries and panels generate amps is throwing me off.

 

Anyone care to help the poor biologist?

 

Mokele

[mmalluck]

The current the power source supplies is dependant on the load resistance, but the power supply can only provide so much current. The current values you are given are the maxium current the supply can comforable source.

 

Using a resistive load that requires more current than the supply is rated will A) cause the outputted voltage of the supply to drop due to it's own internal resistance and/or B) cause your power supply to burn out.

 

You shouldn't assume the source will only try to supply so much current. Most of them are dumb and will eat themselves if given the chance.

 

Now I don't know a whole lot Peliter junctions. They may require a you to add a load resistance in series to keep them from drawing too much current (much like a LED), some of the data sheets I've just pursed have suggested this.

 

Like this one for instance: http://www.alltronics.com/download/25U012.PDF

 

Imax is 3 amps

Vmax is 16.4 volts

Module resistance is 4.9 Ohms at this power level.

 

Like a diode, the 'resistance' the device displays is dependant on the voltage across it and the current through it. They're odd little devices.

Lets play with these numbers.

 

We'll apply 16.4 volts across this module.

 

16.4V = I * 4.9 Ohms

 

Solving for I we find it wants to draw 3.35 amps. This would exceed our maximum current (3 amps), so we need to add a resistor to our setup if we're going to run it at this voltage level.

 

16.4V = 3 amps * (4.9 Ohms + x Ohms)

 

Solving for X we find we need an addtional .57 Ohms. Now resistors that small are had to come by. We can place a larger resistor in there and everything should still be fine. We go to our box of resistors and pick out a 1 Ohm resistor.

 

Going back to our equation:

 

16.4V = x Amps * (4.9 Ohms +1 Ohm)

Solving for x we find our setup will only draw 2.77 Amps. Everything is happy.

 

The only trick now is deciding how much power our resistor has to dissapate. Most resistors are only rated 1/8 a watt or a 1/16 a watt. They're not made to handle amps of current flowing through them.

 

P = (I^2 * R)

 

so

 

P = (2.77 Amps ^ 2 * 1 Ohm) or the resistor will dissapate 7.67 watts. That's a pretty beefy resistor you'll need.

[Mokele]

The current the power source supplies is dependant on the load resistance, but the power supply can only provide so much current. The current values you are given are the maxium current the supply can comforable source.

 

Ahh, that makes sense.

 

Like this one for instance: http://www.alltronics.com/download/25U012.PDF

 

I don't suppose you could take a guess what the different lines across the graph represent? Currents?

 

------------

 

Ok, so, alternatively, since I only need a very modest degree of cooling, and cooling is proportional to current:

 

3 volts, and a guesstimate resistance of 2 ohms, which gives an I of 1.5 Amps, so I should be good, right?

 

It seems like, unless the resistance decreases *really* fast, I can get away without a resistor. Especially since the one I'm looking at, on ebay, has a voltage range from 0 to 15, and amps from 0-10, but nothing online about the resistance and how it varies.

 

Also, the power source I'm thinking of is a kit which can apparently be customized from 0.5 V (amps not given) units by arranging them in series and parralel and such, so I'd be able to monkey with it.

 

Also, a bit of googling showed me that the resistance isn't dependent on voltage, but rather on temperature.

 

I'm tempted to just buy one and start messing around.

 

Mokele

[reverse]

yep...they are pretty "cool".

 

I played with some of these a few years back.

 

they get hot on the back though...so you need a fan to shift that away from the area in question...also heat transfer paste...it's a bit of a sidetrack form your core experiment...

 

If it was me I would get a real cheap 12 volt refrigerated picnic hamper and go all cannibalistic on that.

 

maybe even rout clear tubing through it and pump that around the plant.

[spamonkey8]

I found this a while ago, it seems perfect:

http://www.overclockers.com/tips1054/

[Mokele]

Ok, some more questions:

 

I purchases a peltier junction with a basic resistance of 1.45 ohms. However, the 1V, 800 mA power supply I constructed (4 small solar panels) is insufficient, and I'm thinking of just using batteries..

 

So, is there a limit to the amps batteries can dish out? I know that more amps=shorter life, but is there any reason that, if I hook it up to a few AA batteries in series that they'd do anything terribly bad, like explode?

 

Mokele

[Douglas]

Ok' date=' some more questions:

 

So, is there a limit to the amps batteries can dish out? I know that more amps=shorter life, but is there any reason that, if I hook it up to a few AA batteries in series that they'd do anything terribly bad, like explode?

 

Mokele[/quote']Yes, the rechargeable AA batteries are rated in mA hours. I've seen ratings as high as 2000 mA hours, meaning 2 amps for 1 hour. (check ebay)

 

You can hook up as many batteries as you want in series. When batteries are hooked in series the output voltage increases, when hooked in parallel, the output current increases