If you&#;re into electronics or DIY projects, you&#;ve probably heard of variable resistors, and potentiometers. These essential components control the amount of current flowing through a circuit. By adjusting the resistance of a variable resistor, you can regulate the voltage, current, and signal strength in your circuit. Since you know the variable resistors&#; importance, let&#;s talk about their market price. As per the recent survey by Markwide Research, by , the market price of variable resistors is expected to reach around $1.14 billion.
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But with so many types of variable resistors available in the market, choosing the right one can take time and effort. That&#;s why it&#;s important to consider some crucial factors before purchasing. Here is a list of some factors you need to consider before you buy a variable resistor for yourself.
The resistance range is one of the first factors you should consider when buying variable resistors. This refers to the minimum and maximum levels of resistance that a resistor can achieve. The resistance range is crucial because it determines the range of values you can adjust your circuit&#;s current flow. For instance, if you need to control the volume of an audio amplifier, you need a variable resistor that can adjust the resistance from a few ohms to several kilohms.
The resistance range of variable resistors can vary greatly, depending on the type and design of the resistor. For example, some variable resistors have a logarithmic resistance curve, which means they are better suited for audio applications. In contrast, others have a linear curve, making them ideal for adjusting voltage levels. Therefore, when choosing a variable resistor based on resistance range, it&#;s important to consider the specific needs of your project and choose a resistor that can deliver the required range of resistance.
The power rating of a variable resistor refers to the maximum power it can dissipate before overheating or failing. This factor is crucial, especially when working with high-power circuits, such as amplifiers or power supplies. Choosing a resistor with a lower power rating can lead to overheating, damaging the resistor or other components in the circuit. On the other hand, choosing a resistor with a higher power rating than needed can increase the size and cost of the resistor.
When selecting a variable resistor based on power rating, it&#;s important to consider the maximum power that your circuit will draw and choose a resistor with a power rating that&#;s slightly higher than that. This will ensure the resistor can handle the maximum power without overheating or failing.
The tolerance of a variable resistor refers to the maximum deviation from the nominal resistance value. Tolerance is an important factor to consider because it determines the accuracy of the resistor in regulating the current flow in the circuit. Therefore, a variable resistor with a low tolerance value will provide more accurate regulation than one with a high tolerance value.
Tolerance is typically expressed as a percentage, with the most common values being 5%, 10%, and 20%. For instance, a variable resistor with a nominal resistance value of 100 ohms and a 5% tolerance means the actual resistance value can range from 95 to 105 ohms. Therefore, when selecting a variable resistor based on tolerance, it&#;s important to consider the required accuracy of your circuit and choose a resistor with a tolerance that can provide the necessary level of accuracy.
The temperature coefficient of a variable resistor refers to how much the resistance value changes with temperature variations. This factor is crucial, especially with circuits operating at different temperature ranges. A variable resistor with a high-temperature coefficient can significantly change the resistance value, resulting in inaccurate current flow regulation.
Temperature coefficient is typically expressed in parts per million (ppm) per degree Celsius (°C), with the most common values being around 50 ppm/°C. When selecting a variable resistor based on temperature coefficient, it&#;s important to consider the temperature range your circuit will operate in and choose a resistor with a temperature coefficient compatible with that range. Choosing a resistor with a low-temperature coefficient can ensure the resistance value remains stable, even when the temperature changes.
The mechanical design of a variable resistor is another important factor to consider when buying one. The design determines how the resistor can be mounted, the physical size, and the level of precision in adjusting the resistance value. For example, some variable resistors have a rotary design, which allows for precise control over the resistance value, while others have a linear design better suited for quick adjustments.
The physical size of the resistor is also a crucial factor, especially when working with space-constrained circuits. Choosing a resistor that&#;s too large can make it challenging to fit in the circuit while choosing one that&#;s too small can lead to overheating or poor regulation. The mounting style of the resistor is also important to consider, as it determines how the resistor can be attached to the circuit board or enclosure.
When selecting a variable resistor based on mechanical design, it&#;s important to consider the specific needs of your project and choose a resistor compatible with those needs. Consider factors such as the required level of precision, the physical space available, and the mounting options available.
Choosing the right variable resistor can be the difference between the success and failure of your electronic circuit. By considering factors such as resistance range, power rating, tolerance, temperature coefficient, and mechanical design, you can ensure that you choose a resistor that&#;s compatible with the specific needs of your project. Remember that different resistors have different properties, and the best choice depends on the application and the specific requirements of the circuit. By following the guidelines outlined in this blog post, you can make an informed decision when buying variable resistors and ensure the success of your electronic project.
Ultimate Electronics: Practical Circuit Design and Analysis
Standard resistor values, packages, matching, and accuracy versus cost tradeoffs.
4 min read
Real resistors are not sold as exactly 100. Ω with 8 significant digits of precision. They&#;re sold as, for example, (100±5%) Ω . Both a 95 Ω and a 105 Ω resistor would be acceptable under this specification.
Manufacturing tolerance has to do with manufacturing cost. If you want ±0.1% precision resistors, you can get them, but they cost more. In some parts of your circuit design this will matter, and in others it won&#;t. Simulation is a useful tool for rapidly making sure your circuit performs as expected for all possible extremes of each component.
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It would be hard to test a wide range of component values (sometimes called &#;process corners&#;) in the real world, but it&#;s relatively easy in simulation.
Let&#;s look at the LED plus resistor circuit we discussed briefly in the Power section, but we&#;ll modify the simulation to repeat itself for values of the resistor from 95 Ω, 100 Ω, and 105 Ω :
Exercise Click the circuit, then click &#;Simulate,&#; and &#;Run DC Sweep.&#; We&#;ve used the second sweep parameter to adjust the parameter &#;R1.R&#;, which causes the simulator to repeat the simulation for each specified resistance.
This provides a quick way, for example, to verify that the current doesn&#;t exceed a certain limit, or that the efficiency is still reasonable, over the expected component variation. And this task takes a small fraction of a second in simulation.
Some resistor values are common. Others are not. You can buy a 100 Ω resistor, but you can&#;t easily buy a 104.357 Ω resistor.
If you truly needed a 104.357 Ω resistor in your circuit, you would probably have to make one by combining multiple resistors in series and parallel and/or using an adjustable resistor called a potentiometer.
Just as in any commodity part market, common size standards have emerged for compatibility and cost savings.
Some common 5% tolerance resistor values are: 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91. These are then multiplied by a power-of-ten scale, generally ranging from 10&#;1 to 105 . So you could easily purchase a 18×10&#;1=1.8 Ω resistor, or a 47×105=4.7 MΩ resistor.
Something interesting is that you&#;ll find that if you look ±5% from any of these values, you&#;ll find that you are within the ±5% band of the next nearest value! This means that all possible resistors can be assigned to one bin or another. This eliminates any manufacturing waste from a &#;bad batch,&#; as the parts can be measured, sorted, and labeled after manufacturing.
We haven&#;t yet covered Electronics Packaging, but most resistors are sold either as axial through-hole components (with a wire lead sticking out of either end) or as a surface mount component, like a tiny rectangle the size of a grain of rice.
Axial through-hole resistors typically have color bands. You can look these up to decode the color code. It may be useful to memorize the color code, but there is never any harm in simply double-checking with a multimeter before you use a resistor.
Surface mount resistors have printed codes with a sequence of numbers and letters. Beware, though: there are at least two different sequences used for coding surface mount resistors. In one system, the code might be &#;753&#; to indicate a 75×103 Ω resistor. In the other, the exact same resistance might be labeled &#;85C&#;. To make things worse, it&#;s easy to confuse a surface mount capacitor with a surface mount resistor. When in doubt, just use a multimeter to measure!
Sometimes, it&#;s more important that two resistors are matched closely to each other than that their absolute resistance is any particular number.
When necessary, matching can be achieved with even more precision than finding a single precision resistor alone. When you think you want precision, think carefully: sometimes you really just want matching.
You can do decent matching with a bag of resistors by hand: pick 10-20 resistors, measure all of them, and then pick the closest two. It&#;s not hard to take a bag of same-valued ±5% tolerance resistors and end up with a matched pair that&#;s ±0.2% or so with just an ordinary multimeter.
You can do very good matching by adding a potentiometer in series with one of the resistors and adjusting it until the two legs match in resistance.
You can do even better matching with a bridge circuit, but we&#;ll leave that for later.
In the next section, Practical Resistors: Power Rating (Wattage), we&#;ll talk about the fact that resistors turn electrical energy into heat, and that heat has to go somewhere before the resistors melts!
Robbins, Michael F. Ultimate Electronics: Practical Circuit Design and Analysis. CircuitLab, Inc., , ultimateelectronicsbook.com. Accessed . (Copyright © CircuitLab, Inc.)
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