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Help me find a 80watt rheostat please

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May 24, 2001
NY where the cows out number the people
Here's my setup. I'v got a 80w peltier on my PIII 733. Right now I'm getting idol temps around -10C and full load temps around 5C DD waterblock and DD cooling cube, 300gph pump.

Here's what I want to do. Put a rheostat on my peltier so I can set it to different temps. Then I can do some testing on my CPU @ different temps. Neat. Only one problem, I'm having a heck of a time finding a large enough rheostat. Here's the specs it would have to handle:
80 watts
12 volts
6 amps

There must be one out there on the net that I could buy. All I need is a push in the right direction. Or, if there's something I could build my self....let me know. I would need specific details to do it. ( The local Radio Shack is no help) Thanks
you arnt going to find one .... Start looking into building a voltage regulator. It will have to be based on the LM-338K chip ... I think Hoot built one for his blower...

Yep he did http://www.overclockers.com/tips461/index03.asp realy easy circuit ... it will handle 8 amps and it would be way cheaper than a huge *** rheo.
...but it will not deliver full voltage, only about 10V for a 12V input. That's the price you pay. You can put a switch across the regulator and bypass it to deliver full voltage, but you lose variability from 10 to 12V. In the case of using one for fan control, this is especially annoying because the majority of the speed is developed in the last 2V.

Ah shoot. You lost me allready. :) Sorry, I know about electronics, but not that much. I read Hoot's article and it described the LM-338K and had this pic. I'v got some questions you need to clear up, then I'll try this if I have a good enough understanding. Can't go to radio shack unless you can TELL those kids what your doing, and have enough info to back up your insanity. I hate it when they argue with you, but i'll save that for gripes and moans.

First question is...is the LM-338K something that is allready built at radio shack and you can just buy it, or do you have to make this thing form scratch ?

Second - You mentioned I would only be getting 10v what would I have to do to get the full 12V you lost me on the switch the dooblei thingie to the golbldonger.

Third - Hoot used a pointmeter as well, where whould I connect that ? What size do I get ?

Thanks for your help.
First off I found a rheo http://bgm.bgmicro.com/prodinfo.asp...914092&prodid=PWR1044&page=1&cri=rheo&stype=3 its a 150watt but will only handle 4.47 amps so I dont know if it will work for you or not .... it will have to have a larg heatsink and possibly a fan to handle that kind of load. It will get HOT.

The LM-338K is a chip so you can just buy it. A potentiometer is like a reho stat up with way less wattage they will have these at radio shack. You will probably have to order the chip... Ill see if radioshack has on today and get back to you.
LOL I'v got a full tower case, and I intend on filling it. LOL that will give me a good head start :)
I think your right, being under spec. for a 6A load would cause problems, it would get very hot when it's wide open and even hotter as I trun it down. My favorite words come to mind "Molten Lava in my computer" hehehe I thought they made them big, i'v been looking at some for cars, but havn't hit gold yet. I know there all big, the car ones are also like 3.5 inches accrost hehehe. Whopers they are.
Radio shack does not carry the LM-338K, but Mouser, Digi-Key, Allied, and other "serious" electronic suppliers do. You may be able to find a local Electronics supply shop that carries the ECG or NTE general replacement line. They both make a general replacement for the LM-338K, but you will pay dearly for it (200-300% more).

Since the Lions Share of the current goes through the 338, you don't need a rheostat to control it. A simple potentiometer will suffice, as it only sees a few milliamps of current.

The LM-338K, in theory, will deliver the input voltage minus the 1.2V internal referrence. Since it is wired in series with the electrical path, you can jumper across it and get the full input voltage at the output terminal. Kind of like shorting across the rheostat to eliminate the voltage drop it causes.

I don't know how to word this better.

Hoot, what you said makes sense. I get it now. I think :) I'm not sure where to find a LM-338K or something like it arround here. We have a radio shack, auto zone, parts america. That's it for electronics here. Might have to get one online ?
Whoo, found this.....

Mfr.'s Part #: MP25A1
Allied Stock #: 934-7300
Manufacturer: ANGSTROHM
Description: Rheostat, 100 Watt, 3 5/32", 1
Catalog Page #: 617*
Qty. In Stk.: 10
$44.920 Ea.

Mfr.'s Part #: MP25A10
Allied Stock #: 934-7308
Manufacturer: ANGSTROHM
Description: Rheostat, 100 Watt, 3 5/32", 10
Catalog Page #: NIC
Qty. In Stk.: 7
No Back Order Item

$36.150 Ea.
Mfr.'s Part #: MP25A100
Allied Stock #: 934-7316
Manufacturer: ANGSTROHM
Description: Rheostat, 100 Watt, 3 5/32", 100
Catalog Page #: 617*
Qty. In Stk.: 1

$36.140 Ea.

Don't know what all the numbers are though watts, someting, something any clues ?
Ok, the attachment tells what the specs are. So what one should I get. Here's an easier list to read form. The only difference is the resistance.

All 50 watts I think 25 Amps

1 5 25 50 75 100 200 300 500 1000 2500 5000 Ohms, what one would be best for my setup ?
What are you using to power the tec? I always recomend a seperate PSU for the tec. If you look inside most PSU's you will find a pot for adjusting the voltage. Just solder a couple of wires on the board and move the pot outside. Various pots and knobs are available at the shack and on mine I also added a digital voltage /amperage meter as well.
It all depends on how many amps you want to reach the pelt. If you use 50 ohms, then the minimum amperage going to it would be 12/50 (Volts/Ohms)Amps, which is 0.24A. If you use the 200 ohm resistance, you would have a minimum of 0.06A running to the circut.

If pelts are voltage dependant, and not current dependant, I could think up a variable voltage circut. It would take a while though. I don't remember exactly how to make one from my microelectronics course.

ken257, you have a good idea there. Im sure that could be done for a Whole lot less than what one of those monster rheostats go for. I'm using a Deer 300w ATX power supply to run my peltier and 4 case fans (of course on rheos) So, If I used the pot to turn down the voltage on my peltier I would also effect my case fans :(
(This brings up another post I was going to make later about my case fans being too slow but I'll post that problem later. There on the 12V line but I don't think there getting the full 12 volts. I'll put that in a new post though. )
My other problem with moding the power supply is that I don't think I can get it out of my case, It is in-between my main power supply and my DD cooling cube, In order to take it out would be a major strip down of my system. Don't want to get into that mess again as I just got it in there.
So, what I think I'm going to do is just bite the bullet and get a large rheostat, I saw a large lot on ebay with two 100w and two 75w rheos for a less money. I'll probably end up getting that and selling the others in the classifieds here or something along those lines. A rheostat should be easier for me to install and it would save me time, and save me the grief of ripping my whole setup apart to mod the power sup'
Thanks for all the ideas and help. I've learned a lot form this thread.
Computer PSU's are not that good for powering tecs. You don't always get a full 12v out of them unless there is a load on the 5v line as well. Sometimes these PSU's will fail under the heavy load of a tec and when that happens your cpu may go into meltdown as well.

Instead of going with a monster rheostat I strongly recomend getting a good PSU for that tec. Check here http://www.teamdelta.com/products/prod3.htm I have 1 of these myself and it is great. How can you beat a 12v 21amp supply for $45 and it has a voltage adjustment to boot. As a plus if u decide to upgrade the tec to something like a 120w unit in the future this supply will give you the headroom to do so, a computer psu will not.
Can you explain to me the 1amp load on the 5v line please. I got into an argument with one of the Radio Shack BOYS about this and he insisted that it wasn't right. that the suply always gave off 12v. I gave up because I didnt' know enough about the reasoning behind it. Help me out so I can go back and school this boy. also see my post "Dual ATX power suplies fans too slow"