PSU-info

Hi, after much considering I have changed my mind again and am going for peltiercooling after all...

Now, I'd need some info on PSU requirements!
I'm a little confused when it comes to powering the pelts...

What I need to know is:
1. how much power I need for the pelts (if the pelts is @ say 226W, do I need exactly 226W power from the PSU)?
2. Can I use negative volts to create a higher voltage (like +12V and -12V to get 24V)?
3. What does the Imax do.. Is that the amount of current the pelts really has to get in order to give (if it is a 226W) 226W cooling? And what if the PSU is rated higher on one rail? (If Imax is 28 and the +12V rail is rated higher, will it kill the pelt?)

So it's pretty basic but very complete information I need...
If there is a thread that answeres my questions but I have missed it just give me a link...

Thanks in advance!

The deal with pelts is that the input voltage decides what current will be used. If your Vmax is 15Volts and Imax is 24Amps, then at 12V you would need 19.2Amps.
The relationship is V = IxR and seeing as R stays pretty much the same, as V drops, so does I.
Using the -12V on the PSU won't work. It isn't meant to handle a lot of current and also because of the way PC PSU's are designed it won't give the right voltage.
The Wattage rating of the pelt is how much heat it can transfer from one side to the other, the power it comsumes in doing so is gotten by this equation : Power = Voltage x Current, so for my example above, the pelt would use: 15x24 = 360W of power to be able to transfer 226W of heat form one side to the other.
One good thing about pelts is that as the voltage you give them goes down, their relative efficiency goes up, e.g. I am gonna use a 220W at 5V, which only consumes 40W to be able to transfer 73W from one side to the other.
But get a pelt rated for say 15V, and use a 12V PSU (a normal 500w computer one or a special 12V only one) to pwer it, as the difference in it running at 12V and 15V will only result in about a 2 to 3 degree difference in temps when runnning an XP/P4 at load.

Finken said:

1. how much power I need for the pelts (if the pelts is @ say 226W, do I need exactly 226W power from the PSU)?

Click to expand...

Like strokeside said, Qmax is max cooling capacity. The power necessary to achieve this is Vmax * Imax.

Finken said:

2. Can I use negative volts to create a higher voltage (like +12V and -12V to get 24V)?

Click to expand...

Not with a computer PSU. The -12V rail does not have the current capacity to power a pelt.

Finken said:

3. What does the Imax do.. Is that the amount of current the pelts really has to get in order to give (if it is a 226W) 226W cooling? And what if the PSU is rated higher on one rail? (If Imax is 28 and the +12V rail is rated higher, will it kill the pelt?)

Click to expand...

The Imax is the current the pelt draws at its maximum cooling point. You need a power supply rated to supply at least as much current as the pelt draws at whatever voltage the supply operates at. It's a good idea to use a supply rated for at least 10% - 20% more than the pelt will draw at the output voltage of the supply. A pelt behaves fairly similar to a resistor. In other words you can calculate a resistance using R=Vmax/Imax and by knowing that resistance you can calculate the current draw at another operating voltage.

Voperating/R=Ioperating. (It's not really as simple to calculate as that, but it gives good enough ballpark numbers to determine an adequate power supply.)

Unless you have an amazingly efficient watercooling system you do not want to power a pelt at Vmax. You will most likely get less CPU cooling at Vmax than at 75% of Vmax, because of the extra heat that your watercooling system has to deal with.

For additional information search for posts by Colin. He frequently lists a bunch of links to pelt cooling data.

Thanks, that helped me a lot... but what do I do if the pelt requires 24V? (I know of a 172W-pelt the does)

If your Vmax is 15Volts and Imax is 24Amps, then at 12V you would need 19.2Amps.
The relationship is V = IxR and seeing as R stays pretty much the same, as V drops, so does I.

Click to expand...

That just can't be right! Then by lowering the volts enough, you should be able to run it from a battery. 1V at 1.5A (very roughly..) So I think the correct way is that it needs more amps running at lower voltage. That should equal to the same amount of power..

You could run it at 1V from a battery, but I don't think a battery would give enough amps, plus you would only get about 14.5W transfer form hot to cold side, which would only do for RAM.

Ok, how about this...

The peltier:
Imax = 24 A
Vmax = 15.2 V
Qmax = 226.1 W

The PSU:
+3.3V = 28A
+5V = 37A
+12V = 16A
-5V = 0.5A
-12V = 0.8A

Max: 370W

How should I set up the connectors?


And by the way... if I give the pelt more V or A then I- and Vmax, will I destroy it?

Veland said:

That just can't be right! Then by lowering the volts enough, you should be able to run it from a battery. 1V at 1.5A (very roughly..) So I think the correct way is that it needs more amps running at lower voltage. That should equal to the same amount of power..

Click to expand...

Sure it's right. The current draw goes down as the voltage goes down. The pelt will "run" at 1V but it won't do much cooling. You could keep a 12 watt heat load at ambient with decent air cooling and a 226 watt pelt powered with 1V.

Finken said:

Ok, how about this...

The peltier:
Imax = 24 A
Vmax = 15.2 V
Qmax = 226.1 W

The PSU:
+3.3V = 28A
+5V = 37A
+12V = 16A
-5V = 0.5A
-12V = 0.8A

Max: 370W

How should I set up the connectors?


And by the way... if I give the pelt more V or A then I- and Vmax, will I destroy it?

Click to expand...

That power supply really won't do the job. You need one with a 12V output rated at 20 Amps at least. 25 Amps would be better. You really don't want to power a pelt with more than 80% of Vmax so 12V is about right.

The best retail deal I've seen on a supply is here. Although you can frequently do better on Ebay.

You can destroy a pelt by exceeding Vmax and Imax, but there is no benefit to exceeding Vmax and Imax anyway. Vmax and Imax are by definition the highest cooling operating point of the pelt. Unless you have an EXTREMELY good watercooling system connected to the hot side of the pelt, you're going to hit the maximum cooling point for your system somewhere below 13V.

About the 24V/172W pelts. I wouldn't recommend them for most people because of the difficulty in finding a supply. However, take a look at the PFC250-1024 on this page. If you are willing to work with something like this, I can give you the email address of a salesman there who has sold me one for $32 plus shipping. You would need to do more to use this supply, and you would need to provide air flow to it, but it is a good brand name for this type of supply.

Wasn't paying attention. Sweden huh? I guess my supply sources aren't going to help so much.

Ok, now it's getting clearer all the time
To bad I couldn't take advantade of your supply sources

Well, thanks a lot!

Sure it's right. The current draw goes down as the voltage goes down. The pelt will "run" at 1V but it won't do much cooling. You could keep a 12 watt heat load at ambient with decent air cooling and a 226 watt pelt powered with 1V.

Click to expand...

Ahh.. Sorry, I misread the original post, I thought is said that the pelt moved the same amount of heat. So in that context, lowering the volt would implie that the amps must increase. I understand now..

But another thing:

You can destroy a pelt by exceeding Vmax and Imax

Click to expand...

Shouldn't this be only Vmax? As the PSU should have more amps then Imax to make sure it doesn't overload.. My rule of thumb about powering stuff:
Don't exceed Vmax and make sure the PSU has more then Imax available.

Veland said:

Shouldn't this be only Vmax? As the PSU should have more amps then Imax to make sure it doesn't overload.. My rule of thumb about powering stuff:
Don't exceed Vmax and make sure the PSU has more then Imax available.

Click to expand...

The current draw is dependent on the voltage across the pelt so if you are keeping the hotside at 25 degrees and exceed Vmax across the pelt, you will exceed Imax.

Think of it in terms of water flow through a waterblock (Resistor). If you apply no pressure (Voltage) with a pump, you get no flow through the waterblock (Current). If you apply 1 PSI of pressure (Voltage) at the pump outlet you get some amount flow (Current) through the block (Resistor). If you apply 2PSI of pressure at the pump outlet you get a higher amount of flow through the block.

It's by no means a perfect analogy but it somewhat illustrates the relationship between supply voltage, load resistance, and current flow.

Your rule of thumb is generally correct.