15W Rheostat Question

[Lord Twilight]

At FrozenCPU they have 15W Blizzard Rheostat's. Would this rheostat require all the stuff that a LM317 Voltage Regulator kit would need (heatsinks, circuits, etc), or could I just wire up the 15W rheostat like they have here? I want the fan control, but I don't really feel like making all the associated stuff with the LM317 kits.

I would be running 2 Panaflow H1A's(at most 5.5W ..If I remember correctly) 120mm fans.

I've looked at the rheostat guides before, but I don't feel that I have a very good understanding of them. I just want something that is simple that I can wire up to control the fans.

[Puzzdre]

you dont need additional electronics...
just hook it up like here:

http://www.bit-tech.net/article/56/

good luck!

[Lord Twilight]

I don't understand... if the rheostat can be wired up easily like that, then why use the LM317 stuff?

Also, on the article, it says they have to use a 3W Rheo for fans up to 10W.. hmmm.. I don't know anything about all this Watt and Ohm stuff...

So would the 15W rheo be major overkill for 1 (or even 2) 5W Panaflows? Anyone have any links that could explain this better for me? :shrug:

[Puzzdre]

I think you're looking into this wrong way.

As far as I understand it, if a rheo is 15W, that means that it can handle max 15W load, so you can easily hook up 2 x 5W fans.
The thing is not to overload the rheo with fans (or other device), not the other way around

I would hook it up, but, You can wait for 2nd opinion

[Puzzdre]

and for the Ohm(s)...

look it this way, more resistance (Ohm) ---> lesser voltage ---> lower rpm on fan... until the point when fan stalls.

If you got 15 ohm rheo (not watts, ohms, max. resistance) you can introduce 0 to 15 ohm max resistance into the circuit by turning the knob.

[Puzzdre]

hmmm, just found this...

http://www.fanbus.com/thelab/Rheo_Ti...ostat_tips.htm

http://www.overclockercafe.com/Articles/Rheostat/

[Lord Twilight]

Well, I understand that overloading the rheo is bad (right?) I'm just saying, that with a 5W fan, I could use a much lower voltage rheostat that will work just as well. I just don't understand why they say that they are using a 3W Rheo for fans up to 10W.. Also on FrozenCPU's website, they say with the 15W Blizzard Rheo, they used fans up to 23W.

So basically, I would want a rheo that can handle more Watts than the fan will use, and the Ohms just says how much resistance the Rheo can do, which means how much the rheo will slow down the fan, right?

[Puzzdre]

Well, I understand that overloading the rheo is bad

right

I'm just saying, that with a 5W fan, I could use a much lower voltage rheostat that will work just as well

not lower voltage, lower wattage..

I just don't understand why they say that they are using a 3W Rheo for fans up to 10W.. Also on FrozenCPU's website, they say with the 15W Blizzard Rheo, they used fans up to 23W.

thats confusing, and a little out of my knowledge base

So basically, I would want a rheo that can handle more Watts than the fan will use, and the Ohms just says how much resistance the Rheo can do, which means how much the rheo will slow down the fan, right?

right again!

You're on the safe side as long as your devices (fans) wattage is lower or equal to the rheo wattage

[Lord Twilight]

Ok, I think I am starting to see the light after reading those links. Rather than looking at the Watts, I need to look at the amps too. So for a fan that is rated at 5.52 Watts (Panaflow H1A) it would use .46 Amps (calculated from Cliff's website), so when the Rheo says it can handle up to .70 Amps, 1 fan should be Ok per Rheo. More than .7 Amps = bad.

The only other thing I don't understand is that the Rheo's say "5 Watts" and those fans would be ~5.5 Watts.. so does that mean the Rheo would not be able to let the fans run at their full speed, or overload the Rheo, or am I still missing something?

Or would I need something like a 6W Rheo for these fans.. (or 1 15W rheo for both)

BTW, I'm looking at these Rheo's as an example.

[aabtek]

that is massively overpriced, you can pick rheostats up at any electronics store for $3-$4 ... just like the one on the picture, you just have to wire it up.

the more you slow down the fan, the hotter the rheostat gets, at full fan power the rheostat does not get hot at all.

I use a 5W/100ohm rheostat, actually called a potmeter or variable resistor arround here, with two 3.5W ys-tech fans. I had to put a 40mm 7v fan next to it as it got pretty hot at the slowest setting ...

the reason that people are using the lm317 version is that it is actually cheaper in parts, and is a more elegant solution.

[bigben2k]

Thank you aabtek!

It's very simple, really.

If you run a 12V fan at 12 volts, then it'll draw the maximum # of watts.

If you use a rheostat (a rose by any other name...), and you run the fan at full speed, then the fan still gets 12 volts, and there is no voltage drop at the rheostat.

If you run the fan at 7 volts (by adjusting the rheostat), then the rheostat is dropping 5 volts (12 - 7 = 5). The rheostat basically adds resistance to the circuit, in just about the same amount as the fan (a bit less since it's actually 5 and 7 volts), but overall, the total resistance almost doubled, so the total current is a bit more than half of what it was.

V=R*I

P=V*I

P=R*I*I

If the current is about half, and the resistance is almost doubled, then the total power is about half of the fan at full speed.

So the Rheostat needs to be rated for at least half (edit: no 1/4, see below) of what the fan is rated at.

A 15 Watt Rheo could power 30 Watts worth of fans. It's overkill.

A 3 Watt rheo won't cut it (edit: yes it will, see below) for a 10 Watt fan.

[Lord Twilight]

Originally posted by bigben2k
So the Rheostat needs to be rated for at least half of what the fan is rated at.

Ok, so if I have a 6W fan, then I should look for a Rheo that is at least 3W?

And when the Rheostat is turned all the way up (no resistance), then it's letting through as many Watts as the fan will pull, or as many as it's rated at?

If it's letting through as many as the fan will pull, and it's only rated at half the Watts, then what? it gets hot? nothing happens? What?

Well, anyways, if I need to look for a Rheo that is at least 3W, then I'll go to RadioShack tomorow and see what I can find.

[The Tick]

id avoid the shack if i were you. i made (about a year ago) my own baybus out of the jameco 5w/100ohm pots and a 12.5w/25ohm pot i got through Newark for my ex-cpu fan/jet engine. i went to the shack first to look for parts like those but all i could find were 1W/1kohm and the like. nothin over 1.5w unless you went for the rectangular ones, much less practical for a baybus.

but then again who knows, maybe your radio shack is more upscale than mine. at the very least, you could go to get some helping hands for soldering all those wires

[bigben2k]

The problem with rheos is that when they're used in the lower range of resistance (near short), they have to dissipate a lot of power. In this particular case, the rheo has to dissipate the most amount of power when its resistance is the same as the fans.

A correction to my previous post: if the total power is halved, and the resistance of the fan and rheo are about the same, then the power dissipated by the rheo is half the total power, and since we dropped the total power by half, the rheo has to dissipate 1/4 of the fans' rated power.

So a 3 Watt rheo would work for a 10 Watt fan, since it would need to be rated for at least 2.5 Watts.

If the rheo is not sufficiently powered, it will burn up. Radio-Shack probably doesn't carry anything like that, but it might be worth a look.

I ran the numbers in an Excel spreadsheet. If anyone wants a copy, let me know.

[Lord Twilight]

Ok, looks like RadioShack has a 3W, 25-Ohm (+-20% tolerance)Rheostat. Seems like this will work for a 5W fan, correct? If it does get hot, I may just get some thermal adhesive and slap a little RAMsink on the back.

[nuclear]

Well i think you have part of the equation right big ben. The fan will suck the most current when it is starting or when the motor is near it's limit of power. So when you underpower them, you are actually using more current. I think you should read the article of brian256 link for the article
The part that you will want to read if the last paragraph of the voltage regulator section
Oh and by the way, if the motor doesn't turn, more current pass trought it.

[jaydee]

The Blizzard 15watt 15ohm reho's come from here http://www.allelectronics.com/cgi-bi...-15&type=store

They are a whole $1.65 each. This is the only place on the planet I can find them for this price without buying 10million at a time.

[bigben2k]

Originally posted by nuclear
Well i think you have part of the equation right big ben. The fan will suck the most current when it is starting or when the motor is near it's limit of power. So when you underpower them, you are actually using more current. I think you should read the article of brian256 link for the article
The part that you will want to read if the last paragraph of the voltage regulator section
Oh and by the way, if the motor doesn't turn, more current pass trought it.

That's partly right, indeed!

The resistance of a motor is not continuous: it switches on and off all the time. For most purposes, averaging the resistance works. It's not like the pot is going to get real hot, then cool back down to ambient, 10'000 times per minute, now is it? (5 coils, 2'000 rpm)

In Brian's case, he's using a PWM, and there can be an issue there.