how to decide which passive component to use [closed]

Question

i am not new to electronics and know the functions of a lot of basic components , but the problems arises when i have to use then because i dont know how to use them , when should i use a 1 ohm resistor or a 1M ohm resistor , 1 picofarad capacitor or a 1 farad capacitor , 1 Henry inductor or 1 microhenry inductor , even when do i use them , i cant design circuits on my own because of lack of this kind of knowledge , what should i do , get a good book , or there is a page good enough to clear all my doubts , or an expert ca explain me here in the answers ps : im a student in high school. i have example for problems i face , lets say there is a circuit where several resistors are used , but why do i use the specific resistors , i have a wild guess to match the voltage lower than the maxmimum voltage of the component , like to protect the ic , or bjt , am i correct

Answer 1

You need to think about the voltages and currents involved first. Those determine the coarse dimensioning of all the components.

For amplifiers and such, in a second step, you need to think about the signal path in the circuit, and determine the working points along that path. That gives you the fine dimensioning of the resistors.

Then you have to think about which frequencies are part of your signal, and which are noise, and add coils and capacitors in the correct dimensions for those frequencies.

The exact dimensioning method used depends on the kind of circuit. It's slightly different for digital circuits, and again different for power circuits. After doing the coarse dimensioning, the most simple approach is to look for chips that do most of the functions you need in the given voltage and current range, and see what their datasheets recommend.

Answer 2

The two most common ways to know what value components to use are:

1. Another component’s datasheet tells you what to use;
2. Ohm’s law.

For example, when you add a crystal to a circuit, it will recommend two 18pF capacitors. A good designer will factor in the capacitance of the actual circuit board, and use 15pF capacitors instead.

Or, you decide to use an RGB LED that has the following voltage drops: Red=1.6V; Green=1.9V; Blue=2.5V.

You will be using a 3.3V supply, and the LEDs are bright enough with 10mA through them. You need to add current limiting resistors, but what values? Ohm’s law tells you (R = V / I):

Red = (3.3-1.6) / 0.01 = 1.7 / 0.01 = 170 ohms

Green = (3.3-1.9) / 0.01 = 1.4 / 0.01 = 140 ohms

Blue = (3.3-2.5) / 0.01 = 0.8 / 0.01 = 80 ohms

Answer 3

The use of inductors and capacitors is most often for defining and controlling the speed of response, the rise and fall times, or phaseshifts, and bandwidth, such as 20 Hertz to 20,000 Hertz for audio.

In NTSC video with 4MHz bandwidth on the pixels, a 1Kohm resistor and a typical 20pF capacitor (sum of strays: Cob and Cbe) produce a 20 nanosecond time constant. Inverting this to 50 MegaRadians per second and then dividing by 2 * PI, we have 50,000000 / 6.28 ~~ 8MHz in which case we could add some extra discrete/lumped capacitor (say 18pF or 22pF), and have exactly 4MHz bandwidth.

However the typical values for device parasitic capacitance (I use 10pF on any node, just as a starting point with wire_to_wire or wire_to_ground or collector_to_base of low_power transistors) are not verified during the final manufacturing testing, so you should always allow an extra 50% just to ensure the necessary 4MHz bandwidth (which defines the line resolution on black_white TV acreens, just like line resolution on photographic film defines the sharpness of edges in an image) is achieved.

For resistors, we most often use them in voltage dividers (using 2 resistors) or to produce a well_defined current (one resistor, when the voltage is set). Low values in a voltage divider may be more accurate given any external load that requires a bit of extra ccurrent; you get to learn about ERROR prediction and as the designer, you get to chose how much variation you will tolerate.

A 1Kohm and 9Kohm resistor, as divider from 10 volts, will produce +1 volt. If your load is 1MegOhm resistor (approximately how we model an ADC, in sampling the input voltage before quantization), then your system has at least 0.1% error because of the 1MegOhm model of ADC input current, and using a 16 bit (1:65,000) ADC or 22 bit 1:4,000,000) ADC will be a waste of possible precision.

Notice YOU get to manage the ERRORS.

Thus a continual computation of simple ratios --- the error ratio --- is part of the design task.

To learn, get started. Read the book "Audio Power Amplifier Design" by Doug Self. There are numerous well_thought_out circuits, with lots of resistors and capacitors (few inductors).

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One method I've used for decades is HOW I VIEW A RESISTOR.

I view a 1Kohm resistor as 1 milliamp per volt. Thus 20 volts across that resistor should produce 20 milliAmps.

And a 33Kohm resistor is exactly 30 microAmps per volt. At 5 volts across the resistor --- just multiply 30uA * 5 ---- you can expect (mental multiply) 150 microAmps.

This method allows calculator-free circuit debugging.

How about a 47 ohm resistor? Round up to 50 ohms, and invert 1/50 = 0.02 amps per volt. Pick numbers that are easy to invert. 33 => 30, 25 => 40, 65 ~~ 66 => 16. Its your math. With about 10 of these pairings, you are no longer at mercy of having a calculator in your hand.

Is this precise? no. Can you perform a sanity check? yes