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Making my own fan controller - Page 2

post #11 of 28
doh.gif 3-wire fans can't use PWM control, only voltage control. the yellow wire is the tach output for the PC to detect the fan speed, and is NOT an input. PWM only works with 4 wire fans. They have the required PWM input, as well as power, ground, and tach.
post #12 of 28
Thread Starter 

Well ok, but I would think that this fan is like a DC motor, and if i just use the 12V input and the GND, I could use pulses to turn it on and off quickly to regulate the speed?

I feel bad now tongue.gif I'll have to buy some other fans then biggrin.gif
But I find it strange that it can't be done, because I found some documentation on people who had 3pin fans and they regulated the speed..

I'm gonna look for some nice RED Led fans with PWM then. Does any1 know some good fans (prefered silent)?

(btw sorry for my crappy english tongue.gif)
Edited by St0p - 11/6/13 at 10:46am
post #13 of 28
Thread Starter 
scrap my entire plan, is it possible to use this schematic to PWM control the brightness of LEDS? Or will I need to take some things into consideration?
post #14 of 28
It CAN be done to voltage control the fan - its just instead of the output going to the PWM pin, you'll have to add a power transistor into the circuit. See the first diagram I linked to, you basically need to do that, with the transistor connecting to the fan ground. Then the pulses will turn the power on and off to the fan in PWM fashion.

Same applies if you use the circuit to control LED brightness - though if the frequency isn't high enough, you may notice them pulsing (LEDs turn on and off very fast) Again, you'd need to use the transistor for switching the LEDs, as it'll handle a higher current. Also remember you'd need a current limiting resistor for the LEDs too. If you can give me the supply voltage, no of LEDs, how they're wired etc, I can give you a rough idea of what value you need. EVen better if you know the forward voltage and MAX forward current of the LEDs.
post #15 of 28
Thread Starter 
Hi, thanks for the reply.

Well, I connected everything the way the schematic said and try to control the fan speed using a transistor. But do I need a MOSFET or will a basic transistor be enough? Like I said before, the fan kept running at full speed tongue.gif (so I connected the 12V pin to 12V and the GND pin to the transistor who was controlled by the PWM signal to ground)

I think the leds wer 20mA. I'm not that sure, it's been a while since I bought them.
post #16 of 28
A basic transistor should do the job......but it will have to be able to handle the current of the fan, and be able to dissipate the heat generated when controlling the fan. Probably a transistor that can handle about 1A or so would probably be of the required size for controlling the motor. It may also need a wee heatsink, keep an eye on the transistor temp.

If the LED's are 20mA. it sounds like they may be the standard voltage of 2.2V forward voltage, only superbrights or white LEDs tend to have higher, usually 3V - 3.5V. To work out the resistor you need, we need to take the Supply voltage - each LED Voltage (in series) - the transistor voltage (generally 0.6V or 0.7V) / the forward current in amps.

so lets say you have 2 LEDs in series connected to a 12V supply and through the PWM transistor switch. That would give us -

- - - - -LED1--LED2--transistor
12V - 2.2v - 2.2V - 0.7V
- - - - -
0.020 (20mA in Amps)

= 6.9
- - - - -

= 345 ohms

So you would pick a resistor close to that value - probably a 330 in that case. You can just add more LEDs into that calculation as required, as long as they are the same forward current. Also remember to be careful of the LED polarity, make sure the anodes and cathodes are the right way.

also remember you can only string together LEDs with the forward voltage adding up to under the supply voltage. In this case, the max supply would be 12V - the 0.7v of the transistor. so 11.3 volts, and with 2.2V forward voltage, that means a max of 5 LEDs in each series string. (adds up to 11V, 5 X 2.2V) 6 would take you over the supply voltage, and wouldn't work in theory.

I hope that gives you an idea how to work out how to connect the LEDs, and how to work out the resistor value required.

Edited by latelesley - 11/7/13 at 3:39am
post #17 of 28
Thread Starter 
Thanks, this is even more then I could dream off biggrin.gif

So basicly using your calculation, I can put 5 LEDS in series with a 330Ohm resistor, I suppose I can add another 5LEDS+resistor in parallel or am I wrong here? Or do i have to add the Ampere here and make the calculation all over again?

I have these transistors from another project I did some time ago, I think they would do the trick: http://rfkits.com/parts/BD139-16.pdf
Edited by St0p - 11/7/13 at 8:22am
post #18 of 28
For 5 LED's the resistor would be different. The 330 ohm resistor was for 2 LEDs in the example. for 5 LEDs it would be -

Supply V - Transistor V - LED1 V - LED2 V -LED3 V - LED4 V - LED5 V
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Forward current in Amps


- - - - - - - - - - - - - - - - - - - - - - - - - -


- - -

= 15 ohms

If you change the number of LEDs, it changes the calculation, and thus the resistor value.

And yes, you can do strings of 5 LEDs in series with a 15 ohm resistor, and then put each string in parallel. Just remember, each string adds to the current drawn on the transistor, and it needs to be able to handle it. two strings would be 40mA, 3 would be 60mA, 4 would be 80mA etc etc.

As you increase the current drawn, you increase the power which is needed to be dissipated by the transistor - so you may need to spec a transistor far more powerful to cope with the heat. or add a heatsink to it. You'll find that data on the transistor datasheet.

As an example of the heat dissipated, 3 strings at 60mA, the power dissipated by the transistor will be P=I * V, = 60mA * 0.7V (voltage drop across the transistor) = 0.060 * 0.7 = 0.042 Watts, or 42 milliwatts. Not a lot really, so a small transistor should be ok. Just remember this increases with more strings.

Actually, I just noticed the transistors you linked to, these will be more than fine, you should get away with running multiple strings off of them. smile.gif

PS - oh and remember you need a resistor for each string, you cant just connect them to the one resistor, or it changes the calculation all over again!
Edited by latelesley - 11/7/13 at 2:51pm
post #19 of 28
Thread Starter 
Great smile.gif thanks allot for the info.i'm not sure were I got this 5 from.. I'm sorry, but I'm sure now how itr works, no further questions. biggrin.gif I'm gonna try to make it as soon as I can. I might just try to use RGB leds and a switch to select the color. each string of leds will need 3 resistors then, One for each color. I'm surez it will work smile.gif

Thanks for all the help, I really appreciate it!
post #20 of 28
smile.gif not a problem.

Oh and a wee idea, if you're only going to be picking one color at a time you might get away with one resistor. just put the switch between the resistor, and each string of LEDs. (I'm assuming a 3 position switch?)

Or if you are using 3 switches, one per string, you will need 3 resistors, but you can choose more colors that way, by having more than one string on at once.
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