Electronic Fundamentals: Understanding Resistors

Background Theory: What Does a Resistor Do?

Every single electronic device you own contains at least one resistor. A resistor is a passive electronic component that is designed to apply electrical resistance that reduces current flow through a circuit. Depending on how big or small the resistance value is, resistors regulate differing amounts of current. An excessive quantity of current leads to extreme heat that will risk fire or permanent damage.

Resistors To understand the behavior of a resistor, let's look at Ohm's Law. Ohm's Law is V = I x R. This states that the voltage (V) across a resistor is proportional to the current (I) multiplied by its resistance value (R). So how much R will any given resistor provide? You can use a multimeter but all resistors use a coding system to make them easy to read. See the Jameco resistor color chart.

Resistors can be connected in series or parallel. The value of resistance can be adjusted based on how the resistors are connected together. Resistors connected in series having one common shared node, are simply added together to find the total resistance. Resistors connected in parallel are a little more complex to find the total resistance. The formula is one over the added reciprocals of all resistors in parallel. Req = (1/[(1/R1+1/R2+...+1/Rn)] By choosing the right resistors and then designing them either in series or parallel, you can get close to the exact resistance your design requires.

Let's go hands on with a resistor educational project by building a power resistive decade load box.

The Load Box Project:

The Medium Power Resistive Decade Load Box is an intermediate kit that requires a lot of soldering and drill hole punching of the chassis. The final product is used in electronics labs to serve as a variable resistor for circuit creation and debugging, or as a high power load to test a circuit's ability to drive resistive loads.

You will need:

(4) Rotary Switch, SP, 12-Position
(1) Case, Metal, 7.5" x 9.8" x 3.2"
(7) 5 Watt 0.1Ω Resistor
(7) 5 Watt 1Ω Resistor
(7) 5 Watt 10Ω Resistor
(100) Flat Washer, #4, 9/32 OD
(100) Hex Nut, 4-40
(1) Solid Hook-Up Wire, 22AWG, Black, 100'
(4) Knob, 1/4" Shaft, JK-902A
(1) Insulated Banana Jack, Red
(1) Insulated Banana Jack, Black
(100) Pan Head Screw, 4-40 x 1/4"
Drill bits, 1/8" to 7/16"
Small crescent wrench
Phillips screwdriver
Soldering Iron
Metal file or nibbler
Allen Wrench
Wire stripper
5W Resistor 100Ω (Qty 7)
10W Resistor 0.1Ω (Qty 2)
10W Resistor 1Ω (Qty 2)
10W Resistor 10Ω (Qty 2)
10W Resistor 100Ω (Qty 2)

Resistors Take the base of the box and place all of the 5W or 10W resistors with the lowest value resistor on the right side of the box. Place the 0.1Ω resistor into a row evenly spaced along the right side of the base of the box. Then place the 1 ohm resistors in a straight line evenly spaced next to the 0.1Ω resistors without allowing them to touch. Next place the 10Ω in line and the 100Ω resistors in a line on the left side of the base of the box as shown below:

Carefully without disturbing the other resistors mark the inside of each hole in the aluminum cases (mounting holes) of each resistor with a felt tip pen. Remove the Resistors and using a hole-punch divot each marker mark. The hole-punch should be placed in the center of each mounting hole marked. Drill holes in the box at each divot location using 1/8" drill bit.

ResistorsDial Template (Click to Enlarge)

Use the dial template and poke a small hole in the center of each hole without tearing the paper. Take the box side wall with the 10W resistors, hold the dial picture over the outer side of the front side that has the clearance left for easier wiring. Holding the dial picture in the center of the side mark the center hole of each using a felt marker.

Be sure to keep the paper level so the switches will be mounted level with respect to each other and the box. Punch the marks using the center punch. Drill out the four holes on the front of the box incrementally increasing the drill size for the four switches and the label using no greater than a 7/16" drill bit.

Insert the Power switches partially with the small no-turn lug directly below the hole and mark the location of the turn stop (lug). Drill out one side of the no-turn lug area and use a nibbler or file to open a slit big enough for the no-turn lug to slide in, but small enough so that it doesn't allow play in the no-turn lug so that the switch won't turn when inserted into the box and operated after assembly.

Mount each resistor into place and solder pairs of resistors together with a wire to each pair alternating the side as you go down the lines (100, 10, 1, 0.1) as shown until a wire connects to each resistor on each row.

  • Measure the width of the box without the lid. Subtract 3/4" from the measurement and divide by two. This is the distance from each edge of the box that the banana jacks will be installed (length wise). Mark a vertical line at this distance from each side of the box.
  • Measure the height of the box without the lid. Divide by two. Mark this distance horizontally through the vertical lines drawn in the previous step.
  • Use the hole-punch to make divots where the lines intersect. The divots should be level and 3/4" apart in the center of the back side of the box.
  • Drill out the banana jack holes, insert the banana jacks and screw the bolts that hold each banana jack in place to the box wall.
Install the switches into the front of the box and begin wiring the switches into place following the schematic below (Tie pin 11 to pin 12 of each of the four switches):

Place an ohmmeter on Pin 1 of the switches and on pin 2 and cycle through each position and verify the resistance increments by the correct amount with each step.


Resistors Connect the wires to the banana jacks by adding the two wires shown in the schematic tied to the banana jack. The 100Ω row should tie to the black jack and the 0.1Ω row should tie to the red jack. Screw the lid onto the base of the box. Create curves for 0.1, 1, 10, and 100Ω resistor safe operating curve.

Cut 1/2" holes in the center of the resistance indicator paper in the center of each circle. Laminate the resistance indicator papers and cut out 7/16" holes within each half inch hole previously cut. Place the resistance indicator paper over each switch and mount to the box beneath the nuts that hold the switches in place. Add each knob to the switches and tighten them down. Now place the box into service.

Discussion Questions

What happens when the resistance is too high?
What are some advantages and disadvantages for using resistors in series or parallel?
How would the temperature of a resistor connected in a circuit affect the performance of a device?