Wednesday, September 5, 2012

How to Read SMD, Memorize Color Coded and Solve PUZZLE Resistors?

Infinite Series Parallel and Cubic Puzzle Resistor

Resistance is the property of material to resist the flow of charge or electric current. It also provide voltage drop in a circuit. I made this article with ECE board examiners in mind and when reviewing on technical Job interview for a tricky and puzzle resistors/resistance questions.

Resistor is a 2 terminal passive electrical component used to limit or exhibits resistance to the flow of electric current. Also, it has series inductance and small parallel capacitance.

How to Read Surface Mounted (SMD) Resistors? 

Surface Mounted Resistor
SMD - Surface Mounted or Surface mount technology resistors are printed with numerical values on top of component. Standard tolerance are usually mounted with 3 digit codes and precision resistors are marked with 4 digit codes.

To read 3 digit code SMD resistor with values 152 and 272, the first 2 digits are significant values and the third is the is the power of ten. Here are 2 examples.

152 = 15 x 102 ohms= 1.5 kilohms
272 =  27 x 102 ohms = 2.7 kilohms

For precision resistors, the first 3 digits are significant values and the 4th one is the power of ten.

1001 = 100 x 101 ohms = 1 kilohm
4992 = 499 x 102 ohms = 49.9 kilohms

How to Memorize Color Coded Resistors using Your Hands?

Cell phone Charger Circuit
Unlike SMD which hare numerically  mark, Axial Resistors are  use pattern of colored code stripes to indicate resistance. Available color coded resistors includes 4 band and high precision 5 band, the first 2 color code are significant values, the 3rd color is the power of ten or multiplier and tolerance for the the last color band. For high precision color coded resistor, the first 3 color band are significant digits, the 4th multiplier and the 5th band is tolerance.

To easily memorize Color code resistors and tolerance, just memorize this phrase, "Bad Boy Rape Our Young Girl But Violeta Give Willingly" on your two hands. The first letter of this phrase resembles the first letter of the resistor color code. When you are taking ECE board exam or for long term memory, this technique is effective and very helpful.

Use your two Hands to Easily Memorize Color Coded Resistor
Black - Brown -Red -Orange - Yellow - Green - Blue - Violet - Gray - White - 9

Example of Color Code Resistor question:

Q: What is the value of a resistor with colors from left, Orange, Blue, Gold and Silver?

How to Solve Series Parallel Puzzle Resistors?

There are many kinds of puzzle resistors ranging from simple series parallel, cubic and infinite puzzle resistors. Solving this kind of problem is very challenging and sometimes given during technical Job interview.

Knowledge of Ohm's Law, Kirchhoff's Current and Voltage Law and Wye-Delta Transformation is really helpful to find the solution of this kind of problem.

A. Simple Series and Parallel Puzzle Resistor
Simple Series Parallel Puzzle Resistor

This is simple series parallel resistor puzzle. You can solve it by taking it's equivalent circuit and solving for the total resistance.

Another way is using Kirchhoff's Current and Voltage Law (KCL and KVL). The total resistance is equal to 1.333 ohms.

B. Infinite Series Parallel Puzzle Resistor
How to Solve for Infinite Series Parallel Puzzle Resistor

The circuit can be solved by letting R for the infinite circuit of 1 ohm resistors. The total resistance is equal to 2.732050808 ohms.

If you solve a finite circuit of this kind, you will you will arrived at R = 2.7321 which is very close up to 3 decimal points.

C. Cubic Puzzle Resistor
Equivalent Series and Parallel Connection of Cubic Puzzle Resistors

There are 3 ways to solve this cubic puzzle resistor

1. Find the equivalent circuit
2. Using Kirchhoff's Current and Voltage Law (KCL and KVL)
3. Wye-Delta Transformation

From the above figure, R = A//B//C + G//H//I//J//K//L + D//E//F so, the total resistance of 1 ohm cubic puzzle resistor is equal to 5/6 ohms.

Wikipedia Resistor
How to Solve Cube Resistance Problem

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