I took a spare wall-wart type AC/DC adapter (12v DC @ 1000mA) and an old PC fan, and installed them into the back of my entertainment center to cool things down a bit.
It works great, dropping CPU temps from 150 to ~120, but it’s a little too loud.
Now I want to wire in a double pole switch so I can turn the fan off, give it 5vdc, or 12vdc.
What type of resister should I use to drop the voltage to 5 volts, ie. how many ohms, and is 1/4 watt enough?
I assume I want to put the resister on the positive side, right?
This is tricky to answer without knowing either the dc resistance of the fan or how much current it draws. The voltage drops across series loads is proportionate to the series resistance values. So if the fan has, for example, 10 ohms of resistance, the series resistor should be 14 ohms @ 5W (rounded up from 3.5W).
Do you have an ohm meter you can use to measure the fan’s dc resistance? Does the fan have a current rating on it?
Somewhere on the fan there should be a nameplate giving the current required by the motor.
A 1/4 watt resistor will almost certainly not be big enoough. You intend to run with 7 volts across the resistor. In order to limit the power in the resistor to less than 1/4 watt the resistor would have to have a resistance of 288 Ohms or more. But a 288 Ohm resistor would only allow 48 mA current max. and that might not be enough to start the motor. And besides, a 1/4 watt resistor dissipating 1/4 watt needs forced air cooling to avoid overheating.
A resistor is a not a good idea. You know that your motor requires less than one Amp. Suppose it is 100 mA. A 70 Ohm resistor would give a 7 V drop in voltage which is required. In that case you are dissipating .7 W in the resistor (7[sup]2[/sup]/70). You should use at least a 2 W resistor and 5 W would be better.
I’ve not though this out too well, but you might try using one of those light dimmers as a power source for your AC/DC adapter. If the waveform out of the dimmer doesn’t bother the adapter this ought to give you good voltage control for the motor. The dimmer will probably handle at least a 100 W bulb which ought to be plenty for your purpose.
And if you’re still set on dropping the voltage to 5 VDC, David Simmons’ idea of using a light dimmer might work O.K. But there are a few caveats:
If the wall wart has a regulated output, it might not work. (If the voltage on the output of the rectification stage decreases below the regulator’s drop out voltage, it won’t work.)
The transformer in the wall wart might overheat due to the high frequency components generated by the phased-fired switching in the light dimmer.
Another alternative to using a series resistor is to switch in a 5 V regulator. But I’m not sure if the 7805 can source enough current. Even if it can, you’ll probably need a big heat sink. (I’m just guessing; I haven’t worked out the math.) I’ve also seen high power regulators in TO-3 packages. I’ll see if I can dig them up.
A potentiometer would also work, and would be simpler and cheaper to boot. Finding ones that can dissipiate that much power won’t be a problem, and it’ll be about as simple a circuit as one can get…
Let’s assume you use a 7805 5V regulator w/ TO-220 package. A TO-220 package typically has a R[sub]JC[/sub] of 5 °C/W and a R[sub]CA[/sub] of 65 °C/W. If we assume the 12V/1000 mA wallwart has an output voltage of 12V, and if we assume the current draw is 100 mA, then the regulator would dissipate 0.7 W and the junction temperature would be 84 °C when the ambient temperature is 35 °C. This is below the 150 °C limit typically specified for 78XX regulators. Therefore, a 7805 regulator w/ TO-220 package might work O.K. without a heat sink attached to it.
But it would still be better to use a quiet fan (or multiple ones) operated at the correct voltage.
a soft iron bar, wrapped with 120 input turns and 50 output turns? Might not be great near a CRT or a hard drive, but a lot better and more energy-efficient than just throwing a series resistor in there.
You mean a step-down transformer before the wall wart? This has three problems:
The switch and transformer must be on the AC side, which kinda sucks from a safety and wiring standpoint.
If the wall wart has a regulated output, it might not work. (If the voltage on the output of the rectification stage decreases below the regulator’s drop out voltage, it won’t work.)
The cost of the transformer will probably be more than the price of a quiet fan operated at its rated voltage.
But at least you don’t have the problem with high frequency components generated by a phased-fired light dimmer.
Because stepping down a DC voltage using a series resistor, series rheostat, or potentiometer voltage divider sucks. This is because the voltage going to the fan will be a function of the fan impedance. If for example, the fan has low impedance at startup, then the voltage going to the fan at startup will be low.
Using a regulator chip is the way to go. In the case of a constant voltage, a regulator does not increase the part count; the series resistor is simply replaced by the regulator. In the case of a variable voltage, it only increases the part count by one. That’s a small price to pay for big improvement in performance.
I thought of using a potentiometer at first but didn’t think I had any in the Box’o’Crap. I did find a 10k pot, but it’s kinda wimpy looking at roughly 1/2" diameter… actucally scratch that, it doesn’t click off.
My #1 goal in this little project was to get something effective from scrounged parts laying around the house, so I really don’t want to buy another transformer and fan, but a resistor or pot at RS is no problem. Figure a $2 budget
Gotta go, thanks for the replies so far, I’ll post the fan specs tomorrow.
Oh, one more thing. Crafter Man you keep saying “correct voltage” for the fan, but don’t some motherboards step the voltage from 12 to 5 for high and low? I didn’t pick 5v at random, I thought that’s what computers did.
And if it’s an issue of having enough power to start the fan at 5v, I can just wire the switch off-high-low like most room fans.
You’re probably correct. I’m so used to dealing with AC motors that I forget DC motors tend to not be as picky about supply voltage.
Anyway, if you want to step the voltage down, I would still use a 7805 regulator in lieu of a series resistor. It’s only $1.59 at Radio Shack, Catalog no. 276-1770. The 7805 will not increase the parts count; it is used in place of the series resistor.
And for $1.59, what do you get? A regulated voltage and thermal protection. That’s a big improvement in performance for a small amount of change.
