Slowing down a Color Classic fan with a resistor

[beachycove]

My Color Classic, converted to a CCII, has had since purchase a rather loud 80mm case fan. The noise is caused by the sheer quantity of air in circulation, and the sheer number of obstacles presented to that airflow in the Color Classic. It was not a case of a squeaky fan needing oiled.

I was on the lookout for a slower fan, or for a rheostat or the like to adjust the fan speed, but could not find anything locally and did not want to go to the trouble of buying on eBay and waiting for it to come. So, on an experimental basis, and during our recent little 68kmla-less interlude, I installed a 1v, 100 Ohm resistor on the positive line of the fan, in order to slow it down.

This worked fine, and I find that the noise is now greatly diminished. The air venting out the back is less than what was venting before, and while enough to be audible still in moving past the vent grating, is certainly no longer irritating.

The vented air, however, is slightly warm once the machine has warmed up (after 30 mins. or so of use). The heat is evidently being generated by the CRT, as when the screen enters sleep mode, the vented air will be cool. The computer's casing remains cool always, as does the physical screen itself.

Advice sought on whether or not to keep it this way, to replace the resistor with another — either with a higher voltage rating (cooler?) or with a lower resistance so as to allow the fan to spin faster — or else to return the machine to its former noisy self and play it safe.

 

[phreakout]

You've pretty much nailed it. But I would keep an eye on the resistor. If the fan stops working, you can't hear or feel air moving out by the cooling fan, then chances are the resistor has burned out. This means you ought to try a higher tolerance rating in Watts power. For example, if you installed a 1/4-watt or 1/2-watt resistor and it blew, try using a 1 watt or higher. You can also experiment with a 50-ohm resistor if need be.

73s de Phreakout.

 

[equill]

... Advice sought on whether or not to keep it this way, to replace the resistor with another — either with a higher voltage rating (cooler?) or with a lower resistance so as to allow the fan to spin faster — or else to return the machine to its former noisy self and play it safe.

You have taken the first step along the path used in many contemporary (with us) fan-cooling systems, namely, reducing fan noise and cooling effect by dropping the supply-voltage to the fan. The voltage-drop may be arbitrary, as in your case, or mediated by a circuit responding to a thermistor incorporated into the fan. Your way, if you take account of the necessary heat dissipation from the dropping resistor—dropping 2V across a fixed 33Ω resistor to supply 10V to the fan needs that the resistor safely dissipate 0.15W, which must be added to the (reduced) heat generated by the fan—you may need a 1-W or larger (wattage) resistor to cope, but it is certainly not an impossible ask. (I have assumed a 12-V 80-mA 40-mm square fan for the example.) Use of a larger (say, 75Ω resistance) resistor to drop the voltage to the fan further to 8V will increase the wattage to be dissipated in the resistor with the square of the current passed by the resistor and with the first power of the resistor's ohmic value to 0.64W. You can cope with that by juggling the resistor's wattage rating, but powering a fan built for 12-V supply with 8V may cause it to fail to shift much air. In fewer words, it is a trade-off.

de

 

[Mac128]

The heat is evidently being generated by the CRT, as when the screen enters sleep mode, the vented air will be cool.

Not sure about this reasoning. When the computer is in sleep mode, the hard drive also stops spinning. I replaced my 3.5" HD with a 2.5" PowerBook hard drive and found the interior of mine much cooler. Also, I took the thermostat approach and find my fan comes on mainly when the hard drive and CPU have been powered up for some time. I don't recall hearing it when just the screensaver has been on.

Can't remember if you can independently spin down the hard drive, but if you can, see what happens when you put the HD to sleep and run something only in RAM like a screen saver without putting the system to sleep. I think some later versions of System 7 allow this. If not, you should be able to unmount the HD, or temporarily unplug it an load your system from an external drive or floppy.

If you gain better heat performance, I would definitely consider going with a PowerBook drive or a flash drive instead.

 

[beachycove]

Not a screensaver in sight.

The CC goes into energy saving mode by darkening the screen, and it is after that that the air runs cooler. I am not sure whether or not the system spins down the drive (the drive is very quiet in mine), and as the usual energy saving control panels aren't installed (stock System 7.5 install), I can't seem to control the timing of putting the drive to sleep in any way just at present. I do recall that I took care to install a cool-running drive, though as it is now 3 years since, I can't remember which model specifically. It's certainly a 1GB 3.5" scsi.

In terms of heat and the CPU, you'll know as well as I that a 68030 generates no heat to speak of. As posted elsewhere, I recently installed an LC550 logic board (33MHz 68030, same as a Color Classic II), and am very happy with the result. Until recently, I had an LC575 logic board in there, with full 68040, and that did get rather toasty. In fact, I ran it as a "Mystic" with no rear panel in order to encourage ventilation.