You may have heard someone say something like “…nothing in life is perfect” before. This is definitely true for resistors, which, like everything else in life, are imperfect.
When we need a resistor, most of us dip our hand into the appropriate drawer in the parts bin and pull one with the right color bands without giving it much thought. A few of us may consider things like tolerance and power rating, but other important considerations often fall victim to neglect.
Sometimes this is okay, other times this can cause problems and headaches if you don’t pick the right resistor for your application.
If you want to know more about specific resistor types and their pros/cons the post Resistor Types can help.
In this post, I’ll be covering often overlooked but still important attributes you should consider when choosing a resistor.
There are many different types of resistors, each having its own set of limitations and suitable applications. So, a resistor that is good for one application may not be so great for another.
What follows here is a rundown of some of the important specifications you need to consider when selecting resistors. As always, you can find more detailed specs on a resistor by checking the manufacturer’s data sheet.
The 7 Resistor Characteristics You NEED to Consider
1) Power rating
This should be a no-brainer, but sometimes the best of us overlook it. All resistors have a temperature limit. This is defined in terms of the maximum power they can handle, measured in watts.
Standard power ratings range from 1/16th of a watt to 300 watts. Many of the resistors in your parts bin are likely ¼ watt resistors.
Here’s a good rule of thumb for picking an appropriate power rating: start with Ohm’s Law: P = IV (or some other variation depending on the variables you have) to calculate the power your application requires. Then, pick a resistor with two to four times that rating. And remember, other factors such as whether the resistors are in an enclosure (such as a box) or how tight their spacing is can matter when it comes to heat dissipation.
Manufacturers often provide derating curves which specify the power handling capability of a resistor operating at ambient temperatures above a certain threshold. As you may have guessed, the hotter the environment, the harder it is for a resistor (or anything) to shed heat. This means you may need a resistor with a higher power rating for hot environments. Figure 1 shows an example of a derating curve.
Figure 1: example of a resistor derating curve.
Like the first item on our list, this one is sort of obvious, but it’s surprising how many people barely give this one a thought.
The fact remains, tolerance matters. Often, the last band on a given through-hole resistor gives the tolerance. Tolerances range from 20% to a fraction of one percent. In fact, precision wirewound resistors can sport a tolerance as low as 0.005%.
Carbon composition resistors — the type that you most likely have an abundance of in your parts bin — usually have the worst tolerance.
Carbon film resistors have a range of about 1 to 5 percent and metal film resistors have about 1 percent tolerance. Precision metal film resistors boast tolerances as low as 0.1 percent.
Then there’s the relatively new foil resistors, which can achieve a tolerance of 0.0005 percent (that’s 5 ten-thousandths of a percent!). For many of us, a tolerance of 5 percent will do.
Tolerance is expressed as the deviation in resistance from the nominal value, at 25⁰ C with no load.
Why is this a big deal?
Let’s say you pick a 100 Ohm resistor with a 10% tolerance. The actual value of that resistor can be as low as 90 Ohms or as high as 110 Ohms. Depending on your circuit, this may or may not be a problem.
For more information about resistor color code and tolerance see An Introduction to Resistor Color Code.
3) Voltage rating
What happens when you attempt to drop 1,000 V across a resistor rated for 250 V?
You liberate the magic smoke.
This is because voltage and current are usually proportional, and power is related to both (Ohm’s Law is back to haunt us again). In other words, the voltage rating and power rating are related.
Needless to say, if you’re working with high voltages you really need to pay attention to this spec.
4) Temperature coefficient
Or temp co for short, this spec gives the amount of resistance change that occurs when the temperature of the resistor changes. Temp co, or TC values are typically given as parts per million or ppm for each degree C from some reference temperature, usually room temperature or 25⁰ C.
Ohm’s Law tells us that the more current we ram through a component, the more power that component dissipates; this results in a rise in the temperature of the component itself. In the case of resistors, this can change the value of the resistor. Then there’s the whole ambient temperature thing that you need to take into account.
A positive TC means that an increase in temperature gives rise to an increase in resistance.
A negative TC means that an increase in temperature gives rise to a decrease in resistance.
There are a wide range of TC values available from plus/minus 1 ppm/⁰C to plus/minus 6700 ppm/ ⁰C.
Carbon composition and carbon film resistors have a much higher TC than metal film resistors.
Carbon film resistors are also the only resistor with a negative TC.
This phenomenon manifests itself in the form of small AC voltage fluctuations when DC voltage is applied. While very difficult to measure accurately, noise can have a devastating effect on low-level signals, digital amplifiers, high gain amps, and more.
Noise in a resistor is dependent on the material that composes it, the applied voltage, and physical dimensions of the resistor.
The three main types of noise are: thermal, contact noise, and shot noise.
As you may suspect, thermal noise is mainly dependent on temperature, but also can be dependent on bandwidth and resistance of the resistor. Temperature dependent thermal noise often carries different names, like Johnson noise or white noise. White noise is noise whose level is the same at all frequencies.
Current noise is a function of the amount of current flowing through the resistor and the value of the resistor. It varies inversely with frequency, thus becoming less dominant at higher frequencies than Johnson noise.
Interestingly enough, thermal noise is not dependent on the type of resistor (i.e. carbon comp, metal film etc.), but as mentioned earlier, the value of the resistor. The only way to reduce it is to lower the resistance.
Shot noise is dependent on bandwidth and average DC current. The higher the average DC current the higher the noise. Keep DC current levels low to fight this type of noise.
Contact noise depends on average DC current, bandwidth, geometry, and type of material. Only resistors that are made from carbon particles exhibit this noise, therefore wirewound resistors do not experience contact noise.
Contact noise increases as the current increases, so for low noise circuits, keep the current low.
The worst offenders for noise in general are carbon composition resistors. The best are precision wirewound resistors, followed by precision film resistors, then metal oxide resistors. Carbon film resistors are the second worst offenders next to carbon comp resistors.
6) Frequency response
Resistors sport inductive and capacitive features in addition to their resistance. These features, though often small, can alter the devices electrical impedance, especially at higher frequencies. Because of this, a resistor can act like an RC circuit, filter, or inductor.
Due to their nature, wirewound resistors have terrible frequency response. It’s easy to see why this is the case since any coil exhibits inductance.
Frequency response can also suffer from capacitances in composition resistors. This is due to the many conducting particles and the dielectric binder which holds them together.
The best resistors for high frequency operation are film resistors.
Stability is formally defined as the repeatability of the resistance of a resistor when measured at a reference temperature and subjected to various operating and environmental conditions.
It’s a difficult spec to measure.
As usual, composition resistors score low in this department with wirewound and metal designs coming in at the top.
This spec can be important because environmental factors can alter the resistance of a resistor. For example, humidity can cause the insulation of the resistor to swell, which applies pressure to the resistive element and can also cause cracking over time once the resistor dries out.
Ignoring Resistor Characteristics is Futile
You now know about seven of the most common resistor attributes you should take into account when designing your projects.
Next time, before you stick your hand in your parts bin and pull out a ¼ watt carbon composition resistor with the appropriate value, take a second to consider these seven things.
Taking a few minutes to think it through can save you hours of troubleshooting “mysterious” errors because a certain resistor’s value is out of spec or doesn’t perform well for whatever application your considering.
Ask yourself what tolerance is acceptable, how much power you expect it to absorb, what sort of frequencies you’ll be dealing with, what kind of voltages it will encounter and what are the other environmental factors like temperature and humidity the resistor will see.
You just may be glad you did.
Until next time, leave a comment and tell us about the most unusual or odd type of resistor you’ve found yourself using. Or you can comment about whatever you like, as long as it has to do with resistors 😉