The short answer is that a resistor may be constructed using nearly any conductor or semiconductor material. There are so many options ranging from a simple piece of wire all the way to a pool of mercury. In this article we explore a luminous example. In the process we will better understand the properties that make a good resistor. This article is centered around Video 1 which shows a cherry hot pencil lead functioning as a resistor.
Video 1: The makeshift pencil lead resistor smokes and consumes itself before breaking near the end of the video.
Pencil lead as a makeshift resistor
Figure 1 and Video 1 show how a piece of 0.7 mm pencil lead may be used as a resistor. In this example the lead (graphite) is placed between the green wires. You will notice that two B&K Precision power supplies are operating in parallel to obtain the desired cherry red color.
Using Ohm’s Law, we can calculate the resistance of the makeshift resistor using the values obtained from Figure 1. Together, the power supplies are providing approximately 7.6 VDC with a combined current of approximately 7 A.
R = \dfrac{7.6 \ VDC}{7 \ A} \approx 1 \ \Omega
As seen in the video, pencil lead is not a particularly good resistor. It consumes itself as the high heat in open atmosphere burns away the carbon. In fact, if you look closely at the end of the video you can see that the lead tapered down to nearly nothing before it broke.
Tech Tip: There was a time, not so long ago, when the world’s best engineers were exploring ways to prevent this self-carbon consumption. The end result of those experiments was a reliable electrical light bulb and the closely related vacuum tube. The history of the vacuum tube is fascinating as the diode was a happy discovery made in an attempt to eliminate the black carbon deposits on the inside of the glass bulb. It was a simple question, what if we installed a second element inside the light bulb to attract the carbon? The rest is history.
Figure 1: The makeshift pencil lead resistor is cherry red as it dissipates 53 W.
Properties of the ideal resistor
This experiment yields a few important clues for the design of a good quality resistor:
- The resistor must survive the elements. The temperature must remain relatively low, or the resistor will consume itself. This may not be a design requirement so much as an operating point. Clearly, we were making a point by pushing the makeshift resistor beyond a reasonable operating point.
- The material must be stable for the chosen temperature. Perhaps you noticed the smoke in the video. This is highly undesirable as it stinks. It’s also a good reason to perform the experiment outside of a public institution or risk setting off the fire alarm.
- The material composition of the resistor must be consistent. Using a 0.5 mm diameter lead would change the parameters. Likewise, changing to a different composition would change the resistance. You could test this hypothesis by comparing lead grades with different H (Hardness) and B (Blackness) all the way from 4H to 4B.
- While not shown in the video, the resistance of the material must be stable throughout the decade’s long lifetime of the resistor. In this example, the resistance is expected to increase as the resistive element consumes itself.
- We should also add that the material must be inexpensive, easy to work with, safe for humans, and have a low environmental impact. With this stipulation pools of mercury are not viable.
Real-world resistors
In practice, real-world resistors aren’t that different from our simple pencil lead example. Classic components such as carbon composition resistors could almost be constructed on the same production lines as pencil lead. The difference is that the quality control personnel would focus on the electrical and longevity aspects as opposed to H and B. Also, the final packaging is different, as a wooden covering isn’t an effective solution for the bulk carbon element. Instead, the ideal resistor covering is fireproof (no smoke), is a perfect electrical insulator, and has zero thermal resistance thereby allowing the inner heat to be transferred to the environment. The perfect covering is also hermetically sealed to isolate the bulk resistive elements from being damaged by the environment. All the while, the perfect covering is low in cost and meets all the Environmental Health and Safety (EHS) considerations.
Meeting all these considerations is no easy task. It requires a good deal of compromise. This tendency to compromise is linked to the resistor application. There are different resistor types for different applications. As a short example consider some of the possibilities:
- In the past carbon composition resistors were common. As described, they aren’t so different that this pencil lead.
- Today, carbon film resistors can be manufactured at low cost. With this design carbon is sintered on a ceramic mandrel.
- High power resistors may be wound with metal wire. Once again, the resistive element is wound around a thermally stable ceramic substrate.
- A surprising number of resistors are constructed with silicon. For instance, consider the classic 555 timer. The timer’s internal comparator circuit consists of three on-chip resistors that provide voltage references at 2/3 and 1/3 of the input voltage.
There are certainly more resistor types to discover. To learn more be sure to visit and then bookmark this authoritative page that describes resistors in a much greater detail. You are also invited to join the DigiKey TechForum for additional information about components and application.
Best Wishes,
APDahlen