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May 27, 2024

What Is There To Know About Resistors?

Resistor: A passive chunk of material that resists the flow of electrical current. A terminal is connected to each end you’re done. What could be simpler? It turns out it’s not so simple at all.

Resistor: A passive chunk of material that resists the flow of electrical current. A terminal is connected to each end you’re done. What could be simpler?

It turns out it’s not so simple at all. Temperature, capacitance, inductance and other factors all play a part in making the resistor a rather complex component after all. Even its uses in circuits are many, but here we’ll just focus on the different types of fixed-value resistors, how they’re made, and what makes them desirable for different applications.

Let’s start with a simple one, and one of the oldest.

These are often referred to as “old” resistors and were widely used in the 1960s but with the introduction of other types of resistors, and their relatively high cost, they’re used less now. They consist of a mix of ceramic powder and carbon bonded together using resin. Carbon is a good electrical conductor and the higher the concentration of carbon in the mix, the lower the resistance. Wires are attached to the ends. They’re then coated with paint or plastic as an insulator and different colored stripes are painted on to indicate the resistance value and tolerance.

The resistance of these carbon composition resistors can be permanently changed by long exposure to high humidity, being overstressed by voltage and by overheating when soldering. Tolerances are 5% or greater. Given that they’re basically just a solid cylinder they have good high-frequency characteristics. They also have a good ability to resist heat overloading comparable to their small size and so they’re still used in power supplies and welding controls.

However, their age didn’t stop me from using a bag of them I’d bought from a second-hand store in order to make up the different resistances I needed for a 555 timer music player. That’s my kludge you see in the photo above.

The Temperature Coefficient of Resistance (TCR) of carbon film resistors is typically around 200 and 500 ppm/C. 200 ppm/C means that for every 1C the resistance will not change by more than 200 ohms for every 1 Mohm of the resistor’s value. In terms of a percentage it’s a change of 0.02%/C. So for an 80C change in temperature, 200 ppm/C means a 1.6% change in resistance or 16 kilohms.

Carbon film resistors typically range from 1 ohm to 10 kilohms, have power ratings from 1/16W to 5W and can handle voltages in the kilovolts. Typical uses are for high voltage power supplies, x-rays, lasers and radar.

Metal film is made similarly to carbon film, by depositing a metal layer (often nickel chromium) on ceramic and then carving a helix from the metal. According to one document by manufacturer Vishay, after the terminals are attached, the helix was formerly trimmed by grinding or sandblasting but these days the trimming is done using lasers. The result is then coated in lacquer and labelled using color coding or actual text.

Metal film’s resistance change due to temperature are less than carbon film’s. The TCR of metal film is between 50 and 100 ppm/C, which for the 50 ppm/C amounts to 0.005%/C. Using the same example as for 1 Mohm carbon film above, for an 80C change in temperature, 50 ppm/C amounts to a 0.4% change or 4 kilohms.

Metal film also starts at a lower tolerance, 0.1%. They also have good noise characteristics, low non-linearity and good long-term stability plus a wide range of uses.

This is much the same a metal film resistors except that the metal is often tin oxide contaminated with antimony oxide for resistance. This gives it a better performance than either carbon film or metal film in terms of voltage rating, overloads, surges and high temperatures. While carbon film resistors are rated for roughly 200C and metal film, 250-300C, metal oxide works with 450C. However, they have inferior stability properties.

Wire wound resistors are made by winding a wire around a plastic, ceramic or fiberglass cylinder. Since the wire can be cut to a precise length, these can have a high precision resistance value with a tolerance of 0.1% or better. To get a high resistance the wire has to be very thin and very long. The wire can be thin for lower power ratings or thicker for higher power ratings. It can be made of a number of alloys including nickel-chromium, copper, silver, iron-chromium and tungsten.

They’re typically designed to withstand high temperatures depending on the wire material used, pure tungsten ones having a maximum temperature rating of 1700C, though silver ones can be in the 0-150C range. The TCR for precision wire wound resistors is around 5 ppm/C. For high power wire wound resistors the TCR is higher and this varies more.

High power wire wound resistors can range from 0.5W to 1000W and those in the hundreds of watts can be coated in a high temperature silicone or vitreous enamel. For highest heat dissipation there can even be an aluminum case that has fins to act as a heat sink though these seem to be in the 50W range.

Since the wound wire is basically a coil, they have sufficient inductance and capacitance to have poor properties at high frequencies. To reduce or eliminate this, other ways of winding it are done such as bifilar winding, winding on a flat former and Ayrton-Perry winding as shown in the illustration.

In the case of bifilar winding, induction is eliminated but capacitance is high. By winding on a very thin flat former the wires are close together and induction is lessened. And with Ayrton-Perry winding, since the windings with current in opposite directions are close to each other, self-induction is lessened and capacity is minimized as the potentials are the same at the intersections.

Potentiometers are often wire wound resistors due to the durability. Wire wound resistors are also often used in circuit breakers or fuses. And their induction can be enhanced and put to good use as current sensors by measuring the inductive reactance to determine the current flowing through it.

As you might guess, foil resistors use a foil, one that is several microns thick, usually a nickel chromium alloy with additives mounted on a ceramic carrier. They have the best stability and precision of all resistors despite having been around since the 1960s. The desired resistance value is obtained by photoetching a pattern in the foil. They have no inductance, low capacitance, good stability, and rapid thermal stabilization. Tolerance can be as low as 0.001%.

The TCR is around 1 ppm/C. Comparing with the 1 Mohm metal film above, for a temperature change of 80C, that’s a change of just 0.008% or 80 ohms. It’s interesting how this is achieved. As the temperature increases, the resistance of course increases. But the resistor is made in such a way that that increasing temperature causes compression in the foil resulting in a drop in resistance. The net effect is very little change in resistance.

With no inductance, foil resistors are good for audio applications where high frequencies are involved. They also lend themselves to applications requiring precision such as in electronic scales. And of course any location with large temperature swings can also use them.

Most Surface Mount Device (SMD) resistors are of this type. The film in thick film resistors are around 1000 times thicker than in thin film resistors and thick film resistors are the least costly resistors on the market. Thin film costs much more than thick film.

Thin film resistors are made by sputtering nickel chromium (usually) onto an insulating substrate. This is then etched using photo etching, abrasive or laser trimming. Thick film resistors are made using a screen and stencil printing process. The film is a mix of a binder, a carrier and a metal oxide. Final trimming is done by an abrasive or laser trimming.

Thin film tolerances are as good as 0.1% and TCR is 5 to 50 ppm/C. For thick film tolerances are as good as 1% with TCRs of 50 to 200 ppm/C. Thin film also has lower resistor noise than thick film.

Typical applications for thin film are wherever high precision is required. Thick film has application in pretty much any electrical device — some PCs contain over 1000 thick film SMD resistors.

There are also other types of fixed value resistors but the above are the ones most likely to be found in people’s resistor drawers. Are there any types that you find specially useful for some application? If so, please share with us in the comments below, along with any other types you frequently use. And if you want another run down on typical components you might find or want for your collection, consider checking out our article on capacitors.