Howdy all,

I've been vexing the brain but i'm hoping that the solution is just something I neglected to consider.

I am building a temperature controlled fan using an 8051, an adc0809, and a lm35.

Everything works properly and as the temperature passes the threshold, my output goes high (4.5V), but will not drive a fan requiring 150mA.

I currently am trying to use the output as a switch into the base of an npn transistor and have a separate power supply feeding the board that the transistor and fan are tied to. The 4.5V show up when I get to the right temp, but when I hook up the fan, it doesn't turn on and the voltage plummets. This leads me to believe that there isn't enough current, but with a separate power supply providing proper voltage, shouldn't it work? When the fan is plugged directly into vcc, it operates fine. Any suggestions on the hookup to drive this darn thing?

Many thanks

 

Show your schematic and we'll see if you used the popular internet circuit with a 100k ohm resistor in series with the base of the transistor.

 

I have a fritzing diagram at home that I can pop up after I get off work. Not sure about the internet, it was a suggestion from my professor to use a 10k from the output to the base, ground the emitter and feed the fan with the collector, though he was never able to show us a working example so there's that...

 

Yeah, 10K is too big too. You need the base current to be about 1/10th the collector-emitter current, or in your case about 15mA. With 4.5V and applying ohms law: 4.5 = 0.015•R or R=300Ω. OK, well technically the base voltage will be about 0.7V, so the ∆V to calculate with is really about 3.8V.

 

Okay, so I was mobile and a little off. I have power getting to the fan, just not enough. I am using a 300 ohm resistor from the output of a pin on my 8051 to connect to the base of a npn transistor. I have Vcc (5v) coming into the collector and I have the fan connected to the emitter.

Once my temperature passes the threshhold, the fan starts to turn on and when pushed, it actually keeps spinning until I cool off the lm35. I just need a little more power and have messed with the darlington pair, but the schematics I found have me grounding the emitter using a resistor AND attaching the load (fan) to the emitter (both to the second transistor obviously). This didn't seem to work and to me seems counterintuitive being that I am grounding out my power? Perhaps I am thinking of it wrong.

I have a decent amount of typical parts lying around, but nothing too fancy. Perhaps an lm 741, but my attempts seem to just fall short. I am so close and the fan can spin on its own, I simply need mo powa!

The help is greatly appreciated. I don't have the ability to post a schematic where I'm at, but really, it's all connected and working, just needs more juice.

as a note, the 8051 is separately powered and the adc0809 and lm 35 share the same 5v incoming power that I am trying to use for the fan, but I can't imagine a measly 150mA NOT being left from the draw of those two miniscule ICs.

 

You're supposed to put the fan in the collector circuit. The driver transistor can actually use up less voltage from collector to emitter than it does from base to emitter.