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So far, I've dealt with only devices like resistors, capacitors and inductors. Calculating the current drawn by these devices is very simply by using the formulas I read in my Physics textbook (e.g. V = IR, Q = CV etc). But how do we calculate the current in case of a connected diode, LED, transistor or a microchip. Sometimes some examples refer to a forward voltage or some direct value of current and do the calculation, which confuses me completely. How can I find out these values of voltage and currents so I can solve the circuits just like I was able to when I dealt with circuits with R, L and C (individual or in combination).

J J
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  • For ICs, you need to consult the datasheet. The current may (probably will) vary depending on what the IC is doing, and/or what current anything the IC is driving requires. For LEDs, you control the current by selecting the series resistor. – Peter Bennett Jun 25 '20 at 04:12
  • I'm assuming you don't mean linearized about an operating point, but instead the global non-linear behavior. If so, then for a diode, use the Shockley equation. For a BJT use the [Ebers-Moll model](https://electronics.stackexchange.com/a/252199/38098). You will definitely want to learn about the product-log (aka LambertW) function. – jonk Jun 25 '20 at 05:22
  • Hi J J! This question is overly broad, as AnalogKid's answer points out. At your current point of understanding (not quite sure how a BJT works, it seems), you really need to take to take a step back and look at *one* simple type of device. And then look at complexer devices. "Physics textbook" might indicate you're still in high school, which is fine with me, but please understand that this is kind of an engineering site, and you'll be met with expectations made towards engineers or at least engineering students. – Marcus Müller Jun 25 '20 at 08:33
  • You might feel overwhelmed at points – that's normal, even experienced engineers get that feeling regularly. What we've learned, however, is that you can't easily understand a topic just by asking a lot of questions on the surface. Whenever possible, learning from the ground up is *easier*. So get an introduction to electronics! – Marcus Müller Jun 25 '20 at 08:34
  • Hi Marcus. I am an Enginner myself but my field of study was CSE. I only had chance to learn high-school Physics and not advanced electronics afterwards, but I am trying to learn them to improvise my embedded projects with an aim to design my own circuits myself someday. Thank you for your understanding and explanation. – J J Jun 25 '20 at 08:36
  • Thank you for your answers Marcus, Peter and Jonk. – J J Jun 25 '20 at 09:04
  • LEDs are approximately constant-voltage and the current goes to infinity when the voltage goes higher than that, which is why you put a resistor in series so the resistor can "take" extra voltage. Transistors have their own equations, which are more tricky, but still usable. For microchips, anything goes. You look at the explanation in the chip's data sheet. And you usually can't solve the current because it depends on what the chip is doing. The datasheet says "may use up to 100mA" for example and you plan it so your circuit works from 0 up to 100mA. – user253751 Jun 25 '20 at 17:07
  • I got it now. Thank you for explaining clearly user – J J Jun 26 '20 at 00:29

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As you move up the food chain from passive devices to active ones (LEDs, transistors, etc.), power calculations vary from a little bit more complex (LEDs) to very complex (multi-core microprocessors). Start with the simple step, LEDs. Learn what the forward voltage is and how it affects the most basic circuit (battery, resistor, LED) calculations.

Electronics is no different than any other complex discipline - the hard parts are built up on many layers of increasingly complex ideas and concepts. Start small, draw a basic schematic, try to figure it out, then post the schematic and your questions.

AnalogKid
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Get any edition of Grey and Meyer "Analog Integrated Circuits".

Read up on "bias generators", which often use some version of "band gaps".

In ananlog/RF ICs, you'll see the "vout" of the bias_generator running all around the IC, to control the currents by using the "matched diode" assumptions. Which in an IC with internal heat generation, will not be true.

analogsystemsrf
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