- Joined
- Sep 7, 2002
- Location
- Tucson, AZ
I think I found a cheap and simple alternative to PWM, but first let me explain the problems I've had with PWM. 7V'ing fans was out of the question, as they were still too loud, any lower and the suckers wouldn't always start.
I've done a number of PWM fan controllers, and they all exhibit one common problem. Certain fans (like Delta) create a humming sound depending on frequency. Even at frequencies that should be inaudiable (proven on my scope) harmonics come in to play. On most fans, it's not a problem.
The second issue is that they all have ICs, meaning they are overly complex for the average person. Granted, they are simple ICs (555's and the like) but a lot of people freak on anything over 3 pins.
The problem with simple voltage reduction is that a fans start-up voltage is a bit higher than the minimum it will run at, i.e. setting a fan to 6 volts it will still run, but it may not start.
The advantage of PWM is obvious: An retrigerable timer connected to the fans feedback allows you to pump VCC to the fan if it isn't running, meaning it can be set to a lower speed safely.
Last night, I figured out a REALLY simple solution. Power On Reset, just like we put into digital ciruits. No chips or anything.
Basicly, the whole circuit is just a TIP-120 (could use FET), a pot, and a large cap (I used a 220uf). A thermister could be used in addition.
The cap is attatched between the base and VCC (12V). Center of the pot is attatched to the base. The pot is also connected to VCC and ground. The collector is attatched to VCC, and the emitter goes to the positive lead on the fan.
The net result is the fan gets VCC for a couple of seconds, and then drops down to the set voltage.
Why is this so neat? Say you have a thermistor. When you first fire up your computer, it's relatively cold, and thus the minimum speed would be the fan start-up voltage vs. the slowest speed the fan can run at. By adding the RC circuit to the base (power on reset), the low could be the minimum running voltage, making the system much quieter.
In case your wondering, yes, the pot is doing double duty. Not only is it setting the voltage of the fan, but it's also acting as the "R" in the RC circuit.
Despite chips like 555's and 741's being dirt cheap, they aren't free. The entire circuit I described can be built for a few bucks.
$0.65 TIP-120
$0.85 10K miniature potentiometer
$0.30 220uf electrolitic (radial)
These are all qty 1 prices from a local supplier here in Arizona. They can be had cheaper elsewhere, especially in higher quantities.
On a side note, I have a couple delta 120mm fans running off it with no heat sink on the TIP-120. For more than that, a heat sink might be a good idea, but they can be had for pennies.
It's kind of neat when these Delta 120mm's kick in for a couple seoncds, and then drop down to barely moving. They are still pushing a moderate amount of air, but you have to put your ear right next to them to hear them (watch your hair if you don't have a fan guard!!!). Even at 7V, these fans could be heard quite easily, so this is a very noticable improvement.
Anyway, if this is all just common knowledge, I appologize. It's the first time I personally have ever used (or seen) a Power On Reset in an anolog circuit, though.
I've done a number of PWM fan controllers, and they all exhibit one common problem. Certain fans (like Delta) create a humming sound depending on frequency. Even at frequencies that should be inaudiable (proven on my scope) harmonics come in to play. On most fans, it's not a problem.
The second issue is that they all have ICs, meaning they are overly complex for the average person. Granted, they are simple ICs (555's and the like) but a lot of people freak on anything over 3 pins.
The problem with simple voltage reduction is that a fans start-up voltage is a bit higher than the minimum it will run at, i.e. setting a fan to 6 volts it will still run, but it may not start.
The advantage of PWM is obvious: An retrigerable timer connected to the fans feedback allows you to pump VCC to the fan if it isn't running, meaning it can be set to a lower speed safely.
Last night, I figured out a REALLY simple solution. Power On Reset, just like we put into digital ciruits. No chips or anything.
Basicly, the whole circuit is just a TIP-120 (could use FET), a pot, and a large cap (I used a 220uf). A thermister could be used in addition.
The cap is attatched between the base and VCC (12V). Center of the pot is attatched to the base. The pot is also connected to VCC and ground. The collector is attatched to VCC, and the emitter goes to the positive lead on the fan.
The net result is the fan gets VCC for a couple of seconds, and then drops down to the set voltage.
Why is this so neat? Say you have a thermistor. When you first fire up your computer, it's relatively cold, and thus the minimum speed would be the fan start-up voltage vs. the slowest speed the fan can run at. By adding the RC circuit to the base (power on reset), the low could be the minimum running voltage, making the system much quieter.
In case your wondering, yes, the pot is doing double duty. Not only is it setting the voltage of the fan, but it's also acting as the "R" in the RC circuit.
Despite chips like 555's and 741's being dirt cheap, they aren't free. The entire circuit I described can be built for a few bucks.
$0.65 TIP-120
$0.85 10K miniature potentiometer
$0.30 220uf electrolitic (radial)
These are all qty 1 prices from a local supplier here in Arizona. They can be had cheaper elsewhere, especially in higher quantities.
On a side note, I have a couple delta 120mm fans running off it with no heat sink on the TIP-120. For more than that, a heat sink might be a good idea, but they can be had for pennies.
It's kind of neat when these Delta 120mm's kick in for a couple seoncds, and then drop down to barely moving. They are still pushing a moderate amount of air, but you have to put your ear right next to them to hear them (watch your hair if you don't have a fan guard!!!). Even at 7V, these fans could be heard quite easily, so this is a very noticable improvement.
Anyway, if this is all just common knowledge, I appologize. It's the first time I personally have ever used (or seen) a Power On Reset in an anolog circuit, though.