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The evolution of the humble yet mighty resistor

10 Oct 2014  | David Ashton

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I think it's just right to say that the humble resistor is one of the first electronic components that most us would have come across. Certainly this was true in my day: Ohm's law was one of the first things in any electronics textbook. Even in these days of easy-to-use microcontrollers (MCUs), anyone who wants to drive an LED using an MCU pin needs to know about resistors and Ohm's law to avoid a dead LED or—worse still—letting the magic smoke out of the MCU. Pretty much anyone who is reading this will know about Ohm's law, so I'm not going to go into that here. (There are lots of tutorials on the Internet if you need to bone up on it.)

So, how have resistors evolved over the years? I was brought up and grew interested in electronics at the end of the valve (vacuum tube) era. Resistors at that time were huge in comparison to today's tiny components, and if there was a tolerance at all it was likely to be 10%. Resistors that went high or open circuit for no apparent reason were not uncommon—they were made of compressed carbon in a tube.

As an aside, early telephone microphones were made with carbon granules that were alternately compressed and released by a diaphragm, varying their resistance according to the impinging sound waves. These granules sometimes got "packed," which caused one's speech to sound truly horrible. Banging the headset on the table loosened them and gave a temporary respite, but we digress.

Some carbon resistors. The light coloured one second from the left is a (rather battered) carbon film resistor—a carbon track on a ceramic tube—and the one in plastic probably is, too. The rest are carbon composition resistors—basically just a rod of carbon inside a tube. Most of them have silver tolerance bands, which mark them as 10% types.

These days, resistors are more likely to be metal-film types. A metal film is printed or deposited—usually in a spiral—on a ceramic tube, which is then connected to wires, sealed with a tough coating, and marked. In the case of small surface-mount resistors, it is a zig-zag of film on a square block of ceramic. Laser trimming is sometimes used to get precise values. For higher-power types, resistance wire is wound on a tube, but more on these later.

Two very different types of resistor. Left: 1/4W 1% through-hole metal film resistors (3 x 1KΩ plus a 4.75KΩ). Right: part of a hybrid circuit (printed tracks and resistors and surface mounted devices on a ceramic substrate); note the laser cuts to the resistors, most clearly visible at middle top on the resistor between pins 7 and 8 of the LM431Vage reference IC.

More printed film resistors on a ceramic substrate. This is a 2 x 600Ω resistor I obtained from a telephone PABX board (the middle connection on the group of three pins is not connected). I arranged the reflection of the light source so that the laser trimming on the left-hand one can clearly be seen. The whole assembly is just under an inch wide. These resistors are very accurate—I saw no more than a 0.1Ω difference from 600Ω among six of them that I measured on my 4.5-digit multi-meter.

Showing some tolerance
As the late great Bob Pease would say: "What's all this tolerance stuff, anyhow?" In the early days, resistors could not be made to very precise values, so they were given a tolerance. If a resistor had a nominal value of 1,000Ω and a tolerance of 10%, it could be anywhere between 1,000Ω ± 10% of that value (i.e., 1,000Ω ± 100Ω, or anywhere between 900Ω and 1,100Ω).

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