lo all, I'm building an external watercooling unit. The unit houses pump/rad/res. The rad is cooled by two 120mm papst fans that I'm hoping to throttle via a single rheostat control mounted on the front panel of the unit. As you can't buy rheobuses with just the one channel, I'm going to have to build the thing. Shouldn't be too hard... I've read through all the FAQS and have come up with the following. The fans I am using are 12V dc, 1.25W fans. This gives them a current rating of 0.104A (W=VI, 1.25=12I, I=0.104)and a resistance of 115ohms (V=IR, 12=0.104R, R=115). The beginners electronics FAQ says that to get a 6V - 12V range out of my circuit I want to try and match the resistance of the rheostat to the resistance of the fan. Now, I want a 7v - 12v range (the fans are only rated from 7v to 12v and I don't want them stalling on me) so I'm planning on using a 100ohm rheostat. Given the fans are 1.25W each I'm planning on using a 3W rheostat or above. To summarize; Fans = 2 x 1.25W, 12Vdc Rheo = 1 x 3W (or >), 100ohm So far so good...now come the questions... I'm planning on linking the fans to the rheo in parrallel. i.e. PSU -> Rheo ---> Fan ---> Fan Now according to the basic faq this would make the fans total resistance = 57.5ohms (1/R=1/R1+1/R2, 1/R=1/115+1/115, 1/R=2/115, 1=2R/115, 2R=115, R=57.5). Does this mean I should be using a lower resistance rheostat? I'm sure I must have got a basic principle wrong here...just doesn't seem to make sense. Where on earth (well, where in the UK) can I buy a 3W, 100ohm rheostat from? I've checked Maplin and RS and both don't have many Rheostats at all and those they do have start at around 25W, 1kohm! They have potentiometers but they start way down in the 0.2W range and never make it much above 1W. Help! Thanks, Det
If you're running fans in parallel you work out the resistance of the pairing as you've done and get a rheo to match - so a 50R will be better than the 100R for 2 fans equivalent to 57R. A 3W 50R is ample for two (you'd need a 3.5W 100R for the same fans, 3W 100R won't carry the >200mA current near max speed). Try Technology Supplies, they're about the only UK source at sensible prices. Or build a 2003 controller
Thanks Excellent. Glad to see I got most of it right. So basically, I want a 3W 50R rheo. I checked out that link of yours but they seem to be in the middle of a reorg at the moment. A pity as I someone else suggested I try to pick up some 25mm Acrylic Sheet from there as well. Ho-hum! Not sure I understand this bit. Do you mean if they were run in serial or if I wanted one rheo per fan? Also, what's the maths behind that? I want to get my head round this. Great site btw. I'm really interested in low noise computing. I've got my PC (pics and details here) down to 33dbA compound with all the components being sub 26dbA (they range from 19 - 26). The watercooling unit is my attempt to take things to the next level. I hope, by throttling the two papst 120mm (~26dbA) down I can make them practically silent. This probably won't make the PC much quieter as they'll only be replacing two fans inside the case but it'll enable me to run performance kit that doesn't yet have low-noise air-cooled solutions...and besides...it's fun. Anyway, I digress. I found a link to Farnell on your site which in turn produced the following potentiometer (Order Code: 351313). Does that look like it'll do the trick? It's 3W 50R and seems to be OK. I'm a bit worried by the 500V dc rating as I'm not sure what that means? I'm assuming it means that the potentiometer will work with voltages up to 500V dc...but I'm only guessing [bl**dy n00bies, eh? ]. Thanks for your help, m8.
Good old Ohm and the Power laws. A 115R fan takes 12/115 mA = 104mA. Two in parallel will take 208mA. So to the Power Law, Watts = Current^2 x R, Rheostats are really rated to the current the wire will carry, so a 3W 50R will carry sq.rt(W/R) = 245mA, but a 3W 100R only 173mA. I gather it's safe to add a 25% safety margin when cold unused turns take some of the heat away, so the 3W 100R can be pushed to 216mA (my mental arithmetic was off yesterday) When they're turned down there's no problem, the fan current drops to levels the rheostat is quite happy with. The danger point is if they're left at almost but not quite full on. The Farnell units would be fine, BUT (a) they're 40mm diameter. height of a drive bay = 42mm so a tight fit. And (b) the price - I think Tech Supplies were about £2 each. So for one 115R fan - 100R rheo. For two 115R fans in parallel (57.5R) - 50R rheo. In both cases they'll control down to 6.4v = 12*115/(115+100) =12*57.5/(57.5+50)
I getcha. Thanks for taking the time to explain that... I'm not installing the rheostat inside a drive-bay so size isn't that much of a problem. I'll wait until the techsupplies site is back up though...cost is always a consideration. Thanks again for your help m8. Very much appreciated!
Yeah? Trouble is I have to run both fans from the same rheostat. Basically, I managed to get hold of a broken one of these; The battery had given up the ghost so I stripped it down, disposed of the battery properly and am installing watercooling kit in it. I'll then pass a couple of pipes out of the back and to a manifold inside my PC which will distribute the water to the various cooling blocks inside the PC. The rad I am using is a huge job with 2 x 120mm fans on it. It's these that I'm hoping to throttle with a single rheo mounted on the front of the unit. The rheo knob will conveniently cover a raised area on the front of the unit that will spoil the look once it's painted (IMHO). Still, I'm in no great rush. I've been working on this for a good few months now...another couple of weeks won't hurt.
