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# Making my own fan controller - Page 3

Quote:
Originally Posted by St0pÂ

Thanks Artikbot, I really have a better idea how this works now, I wasn't sure how to combine 2 555 to control my fans. I'm really just a beginner at this. I also edited my previous post with the component values on it.

I have a couple of questions:
- How did you calculate the frequentie? The problem is that i have only a couple of capacitors at my disposal, resistors shouldn't be a problem doh.
- The output:
What do you meen if fed at 5V?
How is the fan connected (it's a 3-wire fan), this output has to go to the tacho signal wire? (yellow mostly)
And do i have to put a 100Ohm resistor between the fan and this output? (or does it go to the 12V input of the fan? I guess not)
-The potentiometer:
You mention a 100Ohm resistor again, were do I put this 1 exactly? Between the output of the pot meter and the mod input of the 555?
If you connect the pot meter like this, wouldn't it generate heat?

Thanks again for you assistance.

I see you took a change of plans, but still, I'm going to answer these questions should you need them further on

1) Frequency is calculated using the formulae in the datasheet. Although there's plenty of calculators around the net where you just plug in those three values and get what would you read on the output signal. This one is my favorite: http://www.ohmslawcalculator.com/555_astable.php

Don't worry about the capacitor. They cost a few cents, and 4.7nF is a very common value for a lentil type capacitor. Odds are you have some scrap electronics laying around that use a handful of them.

2) I mean, if the 555 is powered with 5V at its input. I just use the voltage to determine an output current.
In this case, if you were to supply the 555s with +5V, by Ohm's law you'd get 5=I*100 -> I=0.05A -> 50mA. 50mA*5V=0.25W, half of what the power dissipation of a 555 is rated at.

If you powered it straight from 12V though, you'd get 120mA, which if you multiply by 12V (again Ohm's law) results in 1.44W at the output, 3 times more than what the 555 output driver can supply (basically, you'd fry it after a few minutes of usage, perhaps even less).
In this case you would want a 330Ohm resistor at the output, to bring current down to roughly 25mA. 0.025*12=0.3W, you've got a considerable power margin.

To sum up, the resistor is just to protect the 555 from delivering excessive current.

About how to feed this signal to a 3-wire fan. 3 wire fans do not possess a PWMIN pin (fourth wire, blue one usually), so you have to sort of 'convert' them to. Be advised that this can potentially shorten the life of some fans, although nobody has done the testing (it would be very time consuming). Many people run them this way without any problem, so odds are it doesn't shorten it noticeably.

What you want to do in this case is buy a fairly common N-channel MOSFET like the IRFZ44 (which is immensely overkill, but it's also super cheap), or any other general purpose N-channel MOSFET you can make use of, and:

Solder the PWMOUT from the 555 pair to the MOSFET gate with the matching resistor,
Solder the Drain pin to the +12V supply of the PSU
Solder the Source pin to the positive terminal of the fan you're controlling.

This way you're feeding a high power PWM signal to the fan directly, which is basically what a PWM fan does (only, it does it at the fan controller level instead of at power level).

About the potentiometer, yes, of course it will generate heat, as will the 100Ohm resistor. But you're looking at fractions of a watt, negligible in your operation. If this was to be wristwatch battery operated, we would resort to other methods. But it's not needed.

And last, to catch up with the ongoing LED control.
The same way you wired the MOSFET to power the fan also serves you to control an array of LEDs.
Just be sure to use matching resistors for each LED, to wire them in parallel, and to feed them from 5V as to not shorten their lifespan unnecessarily (yes, LEDs work with current, but higher voltage also puts a higher strain on the junction, which means less lifespan).
Edited by Artikbot - 11/8/13 at 1:54am
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Thanks for the reaction Artikbot, I think I'm gonna try to combine the 2 projects. One controller for 2 fans and another 1 for a couple of bright RGB LEDs.

That calculater is awesome, I didn't know they had stuff like that, I learn somethign new every day thanks allot for that.
You did an excelent writeout.

Why does it have to be a MOSFET doh? Is a power transistor acceptable aswel, I never reallyt did get the difference between these components.

Quote:
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.

I think I'm gonna go for the multiple color selection ;D Thanks for the tip!
Quote:
Originally Posted by St0pÂ

Thanks for the reaction Artikbot, I think I'm gonna try to combine the 2 projects. One controller for 2 fans and another 1 for a couple of bright RGB LEDs.

That calculater is awesome, I didn't know they had stuff like that, I learn somethign new every day thanks allot for that.
You did an excelent writeout.

Why does it have to be a MOSFET doh? Is a power transistor acceptable aswel, I never reallyt did get the difference between these components.

You're welcome

I used a MOSFET because they have super sharp switching curves, and their on state resistance (namely RdsOn) is usually orders of magnitude lower than that of a general purpose NPN transistor working in switching mode. This means much less heat output and much better frequency response. Which is not particularly relevant on low frequencies such as these, but since the price difference is negligible, it's not crazy to use them.

The main difference between a FET and a transistor is in the way they work. Put simply, a transistor charges its gate with current, which determines how much it is partially on or off. A FET charges its gate with voltage, as if it were a capacitor.

As a general rule, if you're using it as a switching element and not an amplifier, a FET is generally the part to choose.

If you do require this linearity between base current and collector current a transistor has, you use a transistor. Simply because a FET is horrible in that aspect.
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Thanks for the info, I nderstand what they do now.

Well I'm using scrap or leftover components atm, and I wanted to try building this without having to go shopping, I'm just gonna try it and if it doesn't work as wel as intended I'm gonna buy me some MOSFETs.
Quote:
Originally Posted by St0pÂ

Thanks for the info, I nderstand what they do now.

Well I'm using scrap or leftover components atm, and I wanted to try building this without having to go shopping, I'm just gonna try it and if it doesn't work as wel as intended I'm gonna buy me some MOSFETs.

I don't believe any of your leftover electronics don't have a single MOSFET on them

Read all the three-legged components' names, odds are you'll find at least a handful of N-Channel MOSFETs. Doesn't really matter the specs, they will 99.995% assuredly fit the bill
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I'll look for them but most devices I have use SMD components so

But thanks for the help, if I finish my project I'm gonna post some pics of it
Beehn testing some stuff and I got it to work with the LEDs, so I thought it would be easy to get it to work with my fan. Here's where the problems start.

When I test the fans with a power transistor attached, I can see that when I switch the transistor on, the fan starts going in full speed, when I do this quickly, I see that I can regulate trhe speed of the fan. But not as much as I'd like. When I switch the transistor off, I get a voltage drop over it from 6V, not the total 12V. result that the fan keeps running but at a lower speed, so I can't put it out completly. I tried to add some resistors but that the fan couldn't run at full speed, or the fan still tries to start but it can't, making this squiky sound. I there a solution to this or?

Also I have a feeling the transistor is getting a bit hot :s

Thanks for the help!
The duty cycle and the frequency vary together, and that cannot be changed with a single 555.
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