Hello, this is my first post but I have a question that I know that someone can answer. I'm interested in hooking up a rheostat to my vantec tornado 92mm fan... I need to know exactly what I need so I can run out to radioshack or somewhere to get it. I'm also interested in learning how to do all the calculations, so if someone would kindly explain and help me out I would be very happy. I've looked through some articles but I really don't understand a thing...so explain as if I were a 3 year old
Rule #1: DON'T USE RADIO SHACK These 3 shops are your friends: Jameco Mouser Digikey Now, for the actual calculations: There are a few figures you'll need. The first one is the lowest voltage the fan can run at. Typically, this is around 5-7v, although it usually sides with 7v. After that, you'll need to find the current draw of the fan. Typically, it's on the sticker on the fan itself, along with the voltage, although sometimes the wattage will be listed instead. Two very important equations to learn: V=IR and P=IV. The first is Ohm's law: Voltage is the product of Current and Resistance. The latter (which I don't think has a formal name) is Power Dissipated equals Current times Voltage. These two will be the basis of our calculations. The first product we need to find is the ohms needed to get from our supply voltage to our desired voltage, which is found by doing so: (Vs - Vd) / I = R Vs = Voltage supply Vd = Voltage desired I = Current R = Resistance We have the resistance, now we need to figure out how much power will be dissipated when the rheo is set at full resistance, which can be found by doing so: (Vs - Vd) * I = P Now with those two figures, you can now look for the appropriate rheo for the job. You won't find the EXACT values, but it's ok to be close enough. Just remember though that you CAN'T skimp on the wattage unless you skimp on the resistance. It's best to work backwards to see how the rheo will work. I * R = (Vs - Vd) When you do that, plug in (Vs - Vd) into the 2nd equation. If the P figure is less than 90% of the wattage rated for the rheo, you're good to go. The most important rule of all is this: Read my signature, know my signature.
Thanks for your great reply. So, with my fan with the following specs: Rated Voltage: 12VDC Rated Power: 12.5W Rated Current: 1.0Amp Using your formulas, I could use 12v as the supplied voltage and 7 volts as a starting voltage, and to get ohms I would use an equasion: (12 - 7) / 1.0 = 5 ohm And likewise for the power dissipation: (12-7) * 1 = 5 Watts Would this rheostat work liked I want it to? its the closest values I could find. Digikey pt. #: RES6R0-ND "RHEOSTAT 6.0 OHM 12.5W"
Yeah, that's the closest you're going to get. Unfortunately, it's over $20. All Electronics has a 15ohm 12.5w rheo (RHE-15), which will definitely turn the fan way below the point where it stops spinning. But on the plus side, it's less than $2.
alright, thanks for all your help man, I'll probably go for that option to save me some money. also, it is likely that I would have to turn it all the way up during power on, correct?
Ah but, current I at (Vs - Vd) ain't 1A anymore, it's only 1A at 12v. For a rough guide it's the rated I*Vd/Vs, so if the 12R fan is running at 7v it will draw about 7/12A. The calculation is a tad more complicated, but given the limited range rheostats come in, picking one with a resistance close to the fan resistance is near enough. 10R in series with 12R will split the 12v into 12*10/(10+12) across the rheo, 12*12/(10+12) across the fan (5.45v & 6.55v) A 15R will take the fan down to 12*12/(12+15) = 5.3v. With rheostats it's safer to look at the current rating rather than the wattage, a 15R 12.5W is only rated to 0.91A max (from W=I2R) but there's usually a 25% safety factor so it will be OK on your 1A max fan.
