so the cheapo fan in ATX supply is about shot ...looking at 120mm fans on Ebay trying to find a bargain. found some nice ballbearing fans rated at 48vdc. had a note about
24vdc starting volts and i was wondring if they would turn slow and quiet at 12vdc. the seller said he never tried so i dug out some 12v 40mm ballbearing fans i have..good ones Sanyo denki or something like that. i put 3.5 on them and they semd to like it , turning slow and quiet. can i expect the same from the larger 120s anyone know? i tried the same with a 120mm i have but did not work at 3v tho the fan was rated for 12vdc.
my question is does anyone have a 48vdc fan they run at 12vdc? or just wont work?
Most of the fans I've seen and used have been electronically commutated and will run at voltages much lower and higher than the voltage they are rated at. The problem is that they don't move as much air at lower speeds and the drag may not allow them to start up at all if you reduce the voltage too much. I would not want to vary the voltage by more than, say +/- 25% ie 36v for a 48v fan. You can pick up 12v ex equipment fans for £1 or so, that are all but new.
divxdude wrote in news:edb4a4fa-f298-4617-b4e1- snipped-for-privacy@o4g2000vbo.googlegroups.com:
I've run 12V fans at 7 volts, from the 5V and 12V lines on an ATX supply). That works ok. The speed drops off sharply as voltage falls from intended values though, so you won't get a lot out of a 48V fan on a 12V supply, you'd be lucky it it got enough torque to start, or even maintain running if you nudged it first. And a stalled fan will overheat fatally even if you're feeding it from a lower voltage. (Has more to do with the electrics than the lack of airflow).
snipped-for-privacy@zekfrivolous.com (GregS) wrote in news:hu5nt6$rj2$ snipped-for-privacy@usenet01.srv.cis.pitt.edu:
Using one when you haven't got one? :) Lot of people haven't. If any of them are reading now, Google LM317, then try to find one, the datasheet tells you a few useful circuits. Very few parts. A car battery charger or similar 12V supply makes a useful raw supply for it. Even standard fixed regulators can be made to regulate arbitrary voltages at useful currents.
Re testing fans for low speeds, I just stick resistors in the supply wires and read volts and amps for the fan when I find a resistor that gets it turning how I want it. That's the easiest way I know. IF you have a meter... :)
Lostgallifreyan wrote in news:Xns9D8BB43B6B0A4zoodlewurdle@216.196.109.145:
And if you haven't, just leave the resistor there and tape it up after twisting wire connections firmly together and removing stray strands. This is something I could do when I was too young to have my own money never mind be trusted alone with a soldering iron. Just use your wits and try to think of what can go wrong, like dust, grime, blocked airways, slowing the fan till it stalls. Don't leave too weak a margin of low speed or it won't work for long.
snipped-for-privacy@zekfrivolous.com (GregS) wrote in news:hu6ca0$30b$ snipped-for-privacy@usenet01.srv.cis.pitt.edu:
Interesting. I've wired thermistors to an entirely passive network as part of a laser diode constant current drive, to match temperature coefficient inversely for constant(ish) output. I think the variation might be too small for fan speed control though, unless some fairly large and expensive thermistors are used. Cheaper to use a small cheap one and an active gain stage to control the fan.
Older PC PSUs often had a small sub panel clipped to a convenient heatsink with a few transistors between the thermistor and fan, they were easier to salvage and use for other things.
Later PSUs have the fan control circuit integrated onto the main PCB, but the circuit is pretty simple - usually no more than 3 transistors and a few resistors, so its easy to trace the circuit by hand and copy it on a bit of stripboard.
Some variable speed fans have a thermistor built in. Its usually in the air flow path so as the air gets hotter the fan spins faster. You can often find them in computer PSU's.
"ian field" wrote in news:E0ANn.58284$bb1.41145@hurricane:
Worth doing for sure, few parts to deal with. I like passive though, and I suspect that some systems overcompensate wildly, starting strong after an initial almost idle state, then backing off sharply. These weren't on/off, they had various states they'd settle at, it was just a bit extreme, not very proportional. Judging by how much change in speed a smallish change in series resistance makes, a passive network might do it, though calculating it might be more trouble than using a simple active control.
Mine wasn't entirely passive, it was an NTC thermistance wired into the sense resistance of an LM317 in constant current config, so the whole thing is a 2-terminal system that can be added like a series resistor. It varied laser diode current to over 25% at 50°C above what it would be at
0°C. As far as I remember, wiring for more extreme swings was possible. It did depend on the three parallel SMT thermistors being somehow mounted on a heatsink though, dissipation was great enough that waving them in the air stream of a fan wouldn't be good enough. They could be mounted on the first heatsink the air hit though.
I wrote it all up neatly one time thinking of posting it somewhere but didn't, for long. At the risk of overdoing it, here it is. Word wrap might make it look ugly but I'm trying to post with no wrap to prevent that...
Opnext/Hitachi Laser Diode HL6526FM; current for 80 mW output at specified temperature.
Sample 0°C=107 mA Sample 0°C=108 mA Sample 0°C=109 mA Sample 0°C=109 mA 1: 25°C=116 mA 2: 25°C=118 mA 3: 25°C=118 mA 4: 25°C=118 mA 50°C=133 mA 50°C=136 mA 50°C=136 mA 50°C=136 mA
RS order code 247-7244, 150 ohm, SMT NTC thermistor, b constant 2750, 5% tolerance and low cost.
Power rating at 25°C 125mW Optimum working dissipation 1mW Resistance tolerance 5% b constant 2750 to 4100 Thermal dissipation constant 1.5mW/°C Thermal time constant 4 seconds Operating temperature range -55°C to +125°C Dimensions L=2, W=1.25, TO5
Equations for deriving values from the technical data, 1K scale, 2.326K at 0°C and 0.49K at 50°C:
T = K(R/1000) T is the value of the compound thermistor at given temperature, K is the known value of a 1K thermistor at given temperature, and R is the known value of the compound thermistor at 25°C.
1/F + 1/T I is the resulting current in amps, supplied by the regulator. I = --------- F is the value of the known fixed resistance, 0.8 and T is carried over from before.
P = T(1.25/T)^2 P is the resulting power dissipation at given temperature.
If T is 75 ohms, made from two 150 ohm NTC thermistors in parallel, and F is a network of metal film resistors equal to 12.4 ohms at 1% tolerance, the regulator tracks the current/temperature curve of the laser diode extremely well, and the total regulator parts cost is less than £2.00.
|¯¯¯¯¯¯¯¯¯| ¡--/\/\/\--¡ I for 0°C-50°C | | ¦ F ¦ -----------| LM317 T |---¦ ¦---¡----------- 0°C=107.97 mA | | ¦ T /¯ ¦ ¦ 25°C=117.47 mA |_________| !--/\/\/\--! ¦ 50°C=134.82 mA ¦ / ¦ ¦ ¦ !-----------------------!
Dissipation.
0°C=8.96 mW They will be dissipating a lot more power than the optimum 1 mW, 25°C=20.8 mW but well within specs for the operating temperature range. They 50°C=21.3 mW will be thermally clamped, bonded to the laser diode heatsink.
Note: While scaling is excellent, the offset isn't, given only 5% tolerance in the thermistors, so a 1% metal film resistor network must be built with a variable resistance to finely tune it. The statement (1/75+1/(1/(1/(4*3.3)+1/(100+470))))/0.8 allows a 470 ohm multiturn preset to set the current between about 4 mA and 6 mA off-range, just enough to accommodate a
5% error. While it looks safe, the calculation predicts a very non-linear response, so this one must be tested. The main resistance will be four 3.3 ohm resistors in series, parallel with a network of series values, 100 ohms and the 470 ohm preset. Which is wired so the current falls, not rises, if it goes open circuit.
Baron wrote in news:hu6kjt$q3t$ snipped-for-privacy@news.eternal-september.org:
Nasty things. :) Well, there's the poor thermal coupling between sink and air, and a horrible lag in response, plus a tendency to wildly overshoot in a crude compromise for that lag. I like the ones with a probe on wires that you can place onto a heatsink though. They can usefully protect something very specific then.
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