ok, i have drawn up a schematic to control a fans speed... logic 1 would give 7v, logic 0 would give 12v, i think... i'd like someone to verify this. i have been told, having talked to some people, that using MOSFETs would be much better, but having never used a MOSFET in my life, i wouldnt know how to do this, so would appreciate it if someone would draw me up a schematic, with suitible components for this, if indeed it would be better Thanks in advance \\RiPz//
luckily google can explain to you what a mosfet is. a most excelent site below: http://www.play-hookey.com/semiconductors/
i'm new to transistors, and i've not yet even made a working transistor circuit, thats why i'm asking people to verify things will work before i waste more money. and that webby is dead.
no its up i asure you, try the google cache if it still dosen't work, its mostly text. its a complete intro duction to diodes and transistors, then FETs. so if u've never touched them before, and want to verify stuff, try learning it, otherwise how do you know people on forums are actually right in the advice!
i dont know they are right, but in communities like this there are things called experience and trust can anyone even verify that my schematic will work, or not ?
http://forums.bit-tech.net/showpost.php?p=650633&postcount=9 You need the transistors to work as switches... There's no real need for mosfets here - you don't need their speed or power handling plus the more expensive ones made for logic-level switching would be needed. They're nice with 12V CMOS as the gate current is negligible. But I'd up the transistors to 2N2222A for a bit more current handling than a 2N3904, and reduce the base resistors to about 470R. Play-hookey works fine for me, a good site.
Thanks cpemma, always a star will modify y circuit and upload a new screeny, see if its still correct Thanks again \\RiPz//
aargh, i dont know how i can do this... i dont even think its possible doing it with the transistors switching... if it is, i dont have a clue how... cpemma... my brain hurts, help!
ok, i guess i'm going to have to use PNP transistors if i want to do it switching, rather than NPN, i wish my brain was fast, hehe
One problem is if you try to do the 7-volt trick with 12V-5V. Easier to ignore that and reduce 12V to 7V with a 5V 1.3W zener diode in series with the fan. I'll have to draw this in the morning, my head's tired.
i've drawn this up... i was just planning on using 2 resistors as a potential divider to get the +12v and ~+7v, doesnt have to be exactly +7v, just as long as its enough i assue if i do this i will have to keep the resistor values low enough as to not limit current too much ?
The moment Q1 turns on the emitter-base junktion of Q2 will be reverse biased by close to 12V. - End of Q2 (see datasheet EB breakdown 6V), end of parallel port, end of mobo. Luckily neither Q1 nor Q2 will turn on. The fan is connected as an emitter follower. That means the drive voltage at the base has to be higher than the voltage at the load. If you want 12V at the fan you need 12.7V at the transistor. The logic circuit won't do that. Suggestion (sorry no picture, but it is simple): * Connect the fan (+) to +12V. * Put a resistor in series with fan (-) that connects the fan to ground. Select the resistor so the fan gets its 7V. * Put a "logic level fet" (google that) with drain and source parallel to the resistor. That way the fet can short out the resistor and put 12V across the fan. * Control the fet directly from the digital output at the gate. It is made for that purpose, and since the fet source contact is now always at ground the digital output can do that. Edit: I just saw that cpemma already mentioned logic level fets as an expensive alternative. Check with www.digikey.com, right now they have FDP6030L in stock for $0.89 each. Note: Above version of the circuit won't work either. Both transistors will happily turn on, with logic 0 (0V) or logic 1 (5V at best).
The second transistor shorts out the zener so fan (BL1) gets full power. Ignore the zener code, a 1.3W is needed, say BZX-85 type. Pick a value for the 'low' voltage you fancy, the transistors will lose about 0.2V so the 4.7V zener will leave the fan with 7.1V. A 1.3W zener will handle fans to around 250mA (at 4.7V), any more and you need to up the wattage or use a resistor.
