Confused about the basics - Page 1
Forum section: electronics / beginners

author: Ohmzilla
author_rank: Contributor
timestamp: October 24, 2016, 03:13:08 am
content: Ok guys, please excuse my noob with this question. So I have a standard 12v 3 pin computer fan. When I hook it up to the bench supply at 12v it lights and turns as expected. Now when I hook my meter up and inline a 1k resistor I get nothing at 12v.So my question is, why am I not getting fan spin at the same voltage with the resistor in line at the same voltage?Thanks

author: IanB
author_rank: Super Contributor
timestamp: October 24, 2016, 03:52:36 am
content: Are you putting the resistor in series with the fan? If so then it will limit the current and prevent the fan getting enough power to turn. For every 0.001 amps passing through the resistor you will lose 1 volt. So if 0.012 amps passes through the resistor you will lose all 12 volts and the fan will see no voltage at all.Can you explain what you mean by hooking up your meter and putting a 1 K resistor in line? What is it you are trying to do?

author: timb
author_rank: Super Contributor
timestamp: October 24, 2016, 03:58:21 am
content: Ohm's Law!Let's say the fan pulls 150mA at 12V; if you put a 1k resistor inline, the maximum current it can pass at 12V is 12mA. A resistor essentially limits current.What exactly are you trying to do? Control the fan's speed? You can't do this to a computer fan by simply limiting the current to it. Why not? Well, computer fan's aren't just simple DC motors. In fact, they're *brushless* DC (BLDC) motors.What does this mean? Well, instead of using a brush to commutate the electromagnets inside the motor, a BLDC motor performs the commutation electronically. Essentially, there will be some Hall-Effect sensors inside that monitor the position of the rotor; as it turns the two (or more) coils are switched on and off, out of phase. That's what makes it turn.Now, the BLDC motors used in 2, 3 and 4-wire computer fans have the drive electronics integrated into them. You can't control the speed by limiting the current, because the current only varies a small amount over the entire useable range of the fan. Instead, you have three options:1) Linear Voltage Control: Basically, you vary the voltage to the fan using a linear regulator or linear fan speed controller IC (like the LTC1695, or the MAX6300). For small fans, the linear regulator tends to not dissipate much excess power since the current is low, so this is a good solution.2) PWM Voltage Control: In this method, you basically hook a MOSFET between your voltage source and the fan's + wire, or between the fan's negative wire and GND. You feed a low frequency (50-500Hz) square wave to the FET, which turns the fan on and off at said frequency. Because this is happening hundreds of times per second and the fan has inertia, the speed can be controlled by varying the duty cycle of the PWM signal (that is, how long it's on vs off for each cycle).This method works well, especially for larger fans with a lot of current draw, mainly because you're not dropping any voltage across the FET, so it doesn't generate any heat. The downside is the fan can produce an audible "droning" noise at the PWM frequency. If you control multiple fans this way, from separate PWM sources, it can be much worse since you end up with a beat frequency. For *some* fans it can also be hard on the motor, since you're constantly interrupting the BLDC motor's own internal PWM.4-Wire PWM Control: This method uses 4-wire fans (that is +, -, Tach and PWM). Basically, instead of using a MOSFET to turn the fan on and off externally, you feed your PWM signal directly to the fan. These types of fans expect a frequency in the 10-50kHz range. Because the frequency is so high, the fan makes no audible noise. It's also better on the fans themselves, because the BLDC drive electronics inside the fan uses this external PWM as the drive signal for the motor.So, those are your choices for speed control!

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 24, 2016, 03:59:16 am
content: assuming fan is 0.1A 12V spec. you can imagine the fan as simple resistor of 12 / 0.1 = 120ohm (V / I = R), and then you put 1Kohm in series. using voltage divider or kirchoff rule you will get [120/(120+1K)] * 12 = 1.28V across the fan. so its like connecting a 1.28V to the fan without the 1Kohm resistor. I = V / R = 1.28 / 120 = 0.01A, one tenth of the specification to turn on the fan smoothly.

author: rs20
author_rank: Super Contributor
timestamp: October 24, 2016, 04:07:17 am
content: Quote from: timb on October 24, 2016, 03:58:21 amMy Noctua fans came with a "low noise" adapter that, IIRC, was simply a resistor inline with the power rail. Are you sure that a simple resistor in line with the supply isn't a viable (if slightly hacky) option? I mean, obviously the OP used too large a value of resistor, but that doesn't disprove the general principle.Quote from: timb on October 24, 2016, 03:58:21 amIf the current is nearly constant (which I'm not sure is true?), then surely a resistor of the right value will produce a nearly constant voltage drop... Ohm's law, right? Making an inline resistor look an awful lot like linear voltage control. Only catch is the startup current burst, which places a limit on how large the resistor can be.

author: stj
author_rank: Super Contributor
timestamp: October 24, 2016, 06:21:16 am
content: a resistor will probably get hot - not a good idea.just buy a fan with the CFM you want in the first place.

author: timb
author_rank: Super Contributor
timestamp: October 24, 2016, 08:40:28 am
content: Quote from: rs20 on October 24, 2016, 04:07:17 amA small resistor would be alright if you just want drop the speed a fixed ~25% and if the fan doesn't pull a ton of current. It would basically operate the same as a linear regulator, yes.I didn't mean to imply the current was constant. Basically, over the operational voltage range of a BLDC fan (which may be 5-15V for a 12V fan or 2.5 to 5.5V for a 5V fan), the current doesn't scale linearly.Here's an example:As you can see, once that regulator hits just under mid-scale (DAC code 25 or ~2V) the fan starts up and the current rapidly rises until it plateaus out around 4V, at which point it scales linearly with the voltage until 5V.Finally, most computer fans don't have much "burst" current at startup. The internal BLDC controller inside the fan will generally soft start the motor.As an aside, better fans will also monitor the Hall-Effect signal and stop powering the coils if the blades stop spinning; it'll keep retrying in short bursts every couple of seconds.I highly recommend this Auto-Restart feature as it's the only other way (besides monitoring a 3-Wire fan's tach output) to make sure the fan doesn't burn up if something gets stuck in the blade. (You can't monitor a fan's current draw for this either, since a stopped fan draws the same current as one running normally; though you *could* monitor the commutation current spikes, but it's fiddly and needs to be customized for each particular fan.)

author: Ohmzilla
author_rank: Contributor
timestamp: October 25, 2016, 04:10:32 am
content: First of all, thank you all for responding.I was not trying to "build" anything just a newbie playing with a breadboard and some old PC fans.I have done some reading along with what was noted here and have a idea now how the part functions.Again, Thanks, guys!