mattjd2699 Posted December 21, 2004 Report Share Posted December 21, 2004 What kind of transistor acts most like a switch contact fully on or fully off??? I have a notebook and want to turn on the cooling fan when it gets warm and back off when it is cool......... Quote Link to comment Share on other sites More sharing options...
Guest Yevgenip Posted December 21, 2004 Report Share Posted December 21, 2004 I think it is called a shmit-trigger. There is no "perfect" Transistor. You can always consider a relay. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 21, 2004 Report Share Posted December 21, 2004 A transistor switches between two voltages. Since it is a current controlled device it can't actually act like a switch. You have to set the current with the voltage at the emitter base junction. However many circuits do utilize the cutoff and saturation feature. I have operated a transistor in the saturation region where the voltage across the transistor remains constant for a given amount of current. This is the closest approximation I have for a switch. Cutoff is easy, you just reverse the emitter base junction. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 22, 2004 Report Share Posted December 22, 2004 Cutoff is easy, you just reverse the emitter base junction.Hi Kevin,It might be complicated to reverse the junction.Didn't you know that a transistor will be cutoff if you simply stop its base current? You could do it by shorting its base to its emitter with something like another transistor, or if its emitter is grounded, drive its base feed also to ground.It is dangerous to have a reverse bias voltage on a silicon transistor's base-emitter junction. The absolute maximum voltage rating for the reverse-biased junction is only 6V to 8V and exceeding it will cause the junction to breakdown with damage.Hi Matt,Sure you can use a transistor as a switch for your fan. Simply provide base current to turn it on and stop the base current (like above) to turn it off. A Mosfet is better (and more expensive) but you won't notice the difference unless your fan is enormous.You can make a simple Pulse-Width-Modulation circuit to control the speed of the fan, controlled by a temperature sensor. It would operate smoothly and quietly instead of suddenly switching. Quote Link to comment Share on other sites More sharing options...
MP Posted December 27, 2004 Report Share Posted December 27, 2004 Matt, there is a temperature controlled computer fan project in our projects section if this is what you are looking for.MP Quote Link to comment Share on other sites More sharing options...
hotwaterwizard Posted January 3, 2005 Report Share Posted January 3, 2005 An SCR works great as a Latching switch and a Triac Works Best as a toggle Quote Link to comment Share on other sites More sharing options...
ante Posted January 3, 2005 Report Share Posted January 3, 2005 Hi John,I think S2 should be a NC not NO as drawn here. Quote Link to comment Share on other sites More sharing options...
EnigmaOne Posted January 4, 2005 Report Share Posted January 4, 2005 Actually, a MOSFET would be an excellent choice for what you are considering here. Quote Link to comment Share on other sites More sharing options...
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