If you go a couple of volts above the threshold, that’s usually more than enough for low-current things like turning on an LED. How much higher depends on how much current you’d like to have flowing (and you’ll find that info in the datasheet). So to turn the transistor properly on, you need a voltage a bit higher than that. The threshold voltage of a MOSFET is actually the voltage where it turns off. (You’ll find this info in the datasheet). For example, the BS170 has a gate-source threshold voltage of 2.1V. To turn a MOSFET transistor on, you need a voltage between gate and source that is higher than the threshold voltage of your transistor. Example: How To Turn ON a MOSFETīelow is an example circuit for turning on a MOSFET. In the MOSFET transistor, the voltage between gate and source decides how much current can flow from drain to source. In the BJT transistor, the current from base to emitter decides how much current can flow from collector to emitter. You can use Ohm’s law to find the resistor value:Ī MOSFET works similar to the BJT transistor, but with one important difference: If the battery is 9V, and the base-to-emitter of the transistor grabs 0.7V, then there’s 8.3V left across the resistor. What resistor value do you need for R1 to get 0.1mA flowing? That means that if you have 0.1 mA flowing from the base to the emitter, you can have 10 mA (100 times more) flowing from collector to emitter. This is called the gain of the transistor.įor a general-purpose transistor, such as the BC547 or 2N3904, this could be around 100. There is a connection between the sizes of the two currents. When a current flows from the base to the emitter, the transistor turns on so that a larger current can flow from the collector to the emitter. To choose the component values, there’s one more thing you need to know about how transistors work: If you also add a pushbutton, you can control the transistor, and thereby the LED, ON and OFF with a button: Choosing Component Values This is the same principle you use to limit the current through an LED to make sure it doesn’t blow up. So you’ll automatically get around 0.7V by adding a resistor. If you add a resistor in series, the rest of the voltage drops across the resistor. A diode has a forward voltage that it “grabs” from the available voltage. Since most of us don’t have a 0.7V battery, how do we turn on the transistor?Įasy! The base-to-emitter part of a transistor works like a diode. If you had a 0.7V battery, you could have connected it between the base and emitter, and the transistor would have turned ON. To turn the transistor ON, you need a voltage of about 0.7V between the base and the emitter.

Transistor keyboard or controller