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voltage follower?

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walshlg

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Hi all, I'm new to the forum and could really use some help. I learned a smattering of digital and analog electronics when I was a Physiologist but never used discrete components much - mainly simple A/D converters and op amp circuits. I haven't done anything much in electronics for 5 years but I'm getting drawn back in! Now of course I'm over my head, to be expected I suppose :)

Simple problem -> I have watercooled my computer and have 3 fans on the radiator. I'd like to control their speeds together by modifying their 12V DC voltage but fan controllers typically have too little curent to drive more than one fan. Computer fans effective voltage range is about 7-12 vDC, too low and they stall and you can simply monitor speed using the tach out (open collector 2x/ revolution) from one of the fans so no need for calibration, just need a way to modify the foltage but be able to provide a good 12 watts. (Is this called a voltage folower?)

Simple I thought, take the output from the fan controller and use that to drive a N-MOSFET (which I have never used before BTW). So I made the circuit:

Vcontroller -> Gate, +12VDC to Drain and Source to fan V+

well great, nice current but Vsource= Vout to fan max is about 9.5V with Vgate = fan controller output at about 12.1 VDC! So I'm dropping about 2.6 V across the drain to source??? I really need 12V to 7V as the total range. I don't need to put out voltages < 7V. Help me with a circuit design please, I'm over my head! TIA
Lyle
 
Most Mosfets need their gate voltage to be 10V higher than their source voltage to completely turn on. So for a source follower with a 12V supply the gate voltage must be 22V for the Mosfet to apply 12V to the fan. Some Mosfets need their gate voltage to be 5V higher than their source voltage to turn on pretty well. So Their max output voltage as a follower is 7V when the gate voltage is 12V. Some Mosfets are a little more sensitive and produce a max output voltage of only 9.5V like you have.

If you use a power transistor instead of a Mosfet then the base voltage must be about 0.8V higher than the fan's voltage. So with 12V in it produces 11.2v out to the fan.
 
Thanks for the quick reply! This may be a bad idea (remember I've never used discretes) but would it work running the current the other way - using the MOSFET to control the ground (sic, really V-low) of the fan:

have fan high to 12V, some sort of a voltage divider bewteen fan controller output and ground set to gate and Fan low to source and power supply ground to drain? Would Source voltage get to ground that way?
 
A Mosfet or transistor has voltage gain when it controls the negative wire of the fan. Then maybe the fan is off when the gate of the Mosfet is 4.1V but runs too fast when the gate is 4.3V. The voltage point changes when the temperature changes.

A transistor would have the fan turned off when its base voltage is 0.65V but the fan would start and speed up when the temperature rises. the fan would be at fulkl power when the base voltage is 0.7V or 0.8v depending on the temperature.
The temperature is of the transistor, not your computer.
 
fascinating - is there anything in particular I should look for in a power transistor, do you have one in mind that you would rec, aiming for 40 watts preferrably? (well maybe that is a bit more than i need :)

Is there anything else I need to add to the circuit or that I'm likely to overlook? or perhaps a site with advice on learning to use discretes? NpN vs PNP vs Darlington array?

Is it as simple a circuit as controller voltage to Base, +12V to Emitter and Collector to fan V+?

Above, when you mentioned that the base voltage must be about 0.8 V higher than the output to the fans voltage, what spec is that (so I can compare power transistors)? Is this Base-emitter saturation or On voltage? What dose the collector - emitter saturation voltage mean? So bottom line, looking for example at the 2n4921 with a Vc-e sat = 0.6V and a Vb-e on and saturation voltage of 1.3V and +12VDC at base and emitter, what voltage can I expect to see at the collector?

Sigh, one more idea - what about the low dropout voltage regulators, expensive but specs look interesting - any experience with them?

(I'm sorry for all the noob questions but looking at the array of stuff on the net and these pratical points seem to be left out or burried in mud)
 
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