Electronics Puzzler Solution: Emergency DIY Resistor

By Forrest M. Mims III
Puzzler 19

How Do You Make an Emergency DIY Resistor?

While Tony Watts wondered how to quickly find a 1 megohm surface mount resistor, he suddenly realized that his boss' yellow number 2 pencil suggested a solution. "May I see your pencil?" Watts asked his boss. Watts took the pencil and drew a thick line back and forth across the back of a business card.

Watts then touched the line with the probes of a multimeter. When the probes were several millimeters apart, the meter indicated a resistance very close to 1 megohm. While his curious boss watched, Watts then rubbed the pencil point between and over the two contacts of the missing resistor on the SMT board.

After several tries, Watts was able to create a SMT resistor with a resistance of 999,643 ohms. The circuit was ready for demonstration when the client arrived an hour later. The visit and demonstration were so successful that Watts' boss gave him a raise!

Going Further

You can easily experiment with this method for making a temporary resistor of a megohm or so. If you use a PC board, the method works best with a pair of bare copper pads having no solder coating.

Begin by rubbing the sharpened point of a standard number 2 pencil back and forth between the two pads. Do this until a multimeter connected across the two pads indicates the approximate resistance target. If you overshoot, erase some of the marked region and try again. Use a softer pencil lead for lower resistances. If the PC board does not readily accept the pencil marks, try buffing the area with a fine grade of sand paper.

This method provides a temporary resistor, but you might be able to extend its life by covering the pencil trace with a small dot of enamel or other protective substance. Be sure to measure the resistance after the coating has dried – it may alter the final resistance.

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