3 volts across the collector/emitter terminals (V CE(sat) see datasheet) when in saturation mode (fully on). What’s missing is the fact that both the LED and transistor - when turned on - have a voltage drop across their terminals and this must be accounted for in the formula.Ī general-purpose transistor will drop about. The formula is listed below assume we have +5V as our power supply (Vcc) and (I C(MAX)) = 15 mA.ĭid you calculate a value of 333.33 ohms for R C? You are correct! Okay, stop right there - we have a problem! The formula above is missing a couple of very important electrical parameters. Now, go ahead and use Ohm’s Law to calculate the value for the collector resistor (Rc). Let’s agree here to set the maximum current (I C(MAX)) flowing through our LED to 15 mA. A good starting point is somewhere between 5 and 15 mA - depending on how bright of an LED you need. Once the maximum LED rating is established, what do we do with that information? Well, it means we need to reduce the max rating of 20 mA to a safe current level so the LED isn’t destroyed. Most of the popular 5 mm diameter through-hole LEDs have a maximum current rating somewhere around 20 mA. You’ll notice on the LED datasheet a listing for the maximum forward current (I F). The first step is to Google the datasheets for both the LED and the transistor. Okay, now that you know the difference between a transistor amplifier and a switch, let’s use the transistor as a switch in order to flash an LED on and off. Simply put, the transistor is either on or off - amplification is immaterial. In turn, when the transistor is turned off (no base current), it goes into “cut-off” mode (fully off). Once a transistor is in saturation mode, it acts just like a closed SPST mechanical switch (see Figure 2). To use a transistor as a switch, all you have to do is increase the current at the base terminal to a certain level, and the transistor will go into a state commonly known as “saturation.” This is a state (mode of operation) where no matter how much additional current is pumped into the base terminal of the transistor, the collector current will not increase any further. In this state of operation, the transistor does not “amplify” the input voltage (V IN) in any way. On the other hand, when the transistor is used as a switch, it is operating in what is called the “digital” mode (on/off). Without going too deep into transistor theory, the active region entails a range of operating points (Google “transistor characteristic curves”) at which the transistor will amplify a signal without distortion. If you’re using a transistor to amplify a signal, the transistor is said to be operating in the “active” or “linear” region. Remember, transistors can operate either as an amplifier or a switch. How do you do that? Well, keep reading.īefore we start, it should be noted that we’ll be using the transistor as a simple switch and not as an amplifier. Of course, in order to make this circuit function properly, you have to calculate the correct resistor values for R C and R B. If you hook up +5 volts to Vcc in this circuit and pulse the input terminal (Vin) with +5V, 0V, +5V, etc., the LED will flash on and off accordingly. It shows a typical general-purpose (NPN) transistor/LED circuit. So, let’s begin! A TRANSISTOR AS A SWITCH Don’t worry, there’s nothing more challenging here than multiplication, division, addition, and subtraction. With that in mind, let’s take a look at one of those simple and ubiquitous circuits: the transistor switch. What should be important is presenting the subject of electronics in such a way that makes it easy for anyone to learn. Unfortunately, the answer to the blame question is beyond the scope of this article. Who do we blame for this abysmal dropout rate - students, teachers, or curriculum? Andrew Carnegie was once asked, "What's more important: labor, capital, or brains?" His reply was, "What's the most important leg on a three-legged stool?" Throw in a course on calculus and you'll witness a raging stampede for the exit door. If I remember correctly (back in the 17th century), half of the people in my electronics class dropped out after the first semester. Sometimes understanding how the simplest of electronic circuits work can be intimidating and/or confusing for the student, hobbyist, or novice.
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