# Electronics for dummies - McComb G.

ISBN: 0-7645-7660-7

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Figure 18-1 shows a circuit made up of an LED, a resistor, and a battery (or other power source). You use this formula to calculate the value of R, the resistor.

Here’s what the alphabet soup of V, I, and R means:

V (also sometimes noted as E): The voltage through the LED. Because the voltage reduces when it goes through a diode, you have to subtract this voltage (about 1.2 volts for the typical LED) from the supply voltage. For example, V = 3.8 volts if the supply voltage is 5 volts and the drop through the LED is 1.2 volts.

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Chapter 18: Ten Electronics Formulas You Should Know 377

I: The current, in amps, that you want flowing through the LED. 20 mA is a reasonably safe value for almost any LED; a lower value makes for a dim light, and a higher value — much over 40 or 50 mA — may destroy the LED. Because you need to express I in amps, 20 mA becomes a fractional number: 0.020 amps.

R: The resistance needed, in ohms, to limit the current to the LED.

Figure 18-1:

Use Ohm’s Law to calculate the value of the current-limiting resistor that you need for an LED.

R

A A _

To continue the example for V and I we’ll plug in some real numbers in place of the V, I, and R (which you can also see called out in Figure 18-1):

jSjABE#

190 ohms =

3.8 volts 0.020 amps

See Chapter 1 and the Ohm’s Law table in the yellow Cheat Sheet in the front of this book for more about using Ohm’s Law.

Calculating Resistance

You can calculate the resistance of a single resistor in a circuit simply enough. But resistance changes when you add resistors in parallel or in series. For resistors in series, you add the resistance values together. For resistors in parallel, the result is a little less obvious.

Why bother with calculating resistance of multiple components? There are several good reasons:

You can find resistors in only a limited number of common values. Some circuits call for a specific value that you can create only by inserting two or more resistors in series or in parallel.

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378 Part VI: The Part of Tens

Resistors aren’t the only components that exhibit resistance. For example, the windings of a motor also have a certain resistance. For some special applications, you need to calculate the combined effect of having these various resistances in a single circuit.

Calculating resistors in series

The formula for calculating resistors in series is pretty simple — just add up the resistances. Here’s how it works:

Rt = R1 + R2 + R3 . . . _and any more, as needed)

In this case, R1, R2, R3, and so forth are the values of the resistors, and Rt is the total resistance.

For example, suppose you have two resistors rated at 1.2k ohms and 2.2k ohms. Add them together, and the resulting resistance is 3.4k ohms.

Calculating two resistors in parallel

Things are a little more complicated when you want to calculate two resistors in parallel. Here’s the formula you use:

Rt = R1 x R2 Rt R1 + R2

R1 and R2 are the values of the two resistors and Rt is the total resistance. Given a 1.2k (1200 ohms) and a 2.2k (2200 ohms) resistor:

77f„ 2640000 776.47 = "63400“

Now, to calculate three or more resistors in parallel:

Rt = -1-j-p . . . (and more as needed)

RT + R2 + R3

Here R1, R2, R3, and so forth are the values of the resistors. Rt is the total resistance.

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Chapter 18: Ten Electronics Formulas You Should Know 379

Calculating Capacitance

You can use the formulas in this section to calculate total capacitance in a circuit. Note that the formulas are basically the inverse of the formulas for resistors, described earlier in this chapter. And, like resistors, the same logic applies for why you’d ever want to calculate capacitance of two or more capacitors together.

Calculating capacitors in parallel

To calculate the value of a string of capacitors in parallel, just add ‘em up:

Ct = C1 + C2 + C3____

In this formula, C1, C2, C3, and so forth are the values of the capacitors; Ct is the total capacitance.

Calculating two capacitors in series

Use the following bit of math wizardry when you need to calculate the total capacitance of two capacitors wired up in series:

_ C1 x C2 Ct C1 + C2

In this formula, C1 and C2 are the values of the two capacitors; Ct is the total capacitance.

Calculating three or more capacitors in series

Got capacitors? Got lots of ‘em? Well, if you’re wiring them all up in series, you need to use a special formula to calculate the total capacitance:

Ct _ 1

_L + _L + _L

C1 + C2 + C3

C1, C2, C3, and so forth are the values of the capacitors. Ct is the total capacitance.

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380 Part VI: The Part of Tens

Now, why would you ever want to add a bunch of capacitors together this way? One common reason is to provide a specific capacitance value for which there is no standard component. You sometimes need to do this with very sensitive circuits, such as radio receivers.

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