The Tech Blog

Over the last few posts, we’ve covered everything I wanted to write to you about resistors, what they are, how they work, and the different types based on construction. The series won’t be complete till I cover variable resistors and its following different types

Potentiometers are the most common type of variable resistor. They consist of a resistive track (usually made of carbon, cermet, or conductive plastic) with a movable wiper that slides along the track, adjusting the resistance between two terminals.
Use Case: Volume control in audio systems, brightness control in displays, and manual tuning applications.
Types: Linear (changes resistance uniformly) and logarithmic (better for human-perceived volume control). Digital potentiometers are also there which can be set to a particular resistance value via control signals to it.

Rheostats are essentially potentiometers used as two-terminal devices, where one terminal is connected to the wiper, and the other to one end of the resistive element. They are designed to handle higher currents and are often used in power control applications.
Use Case: Motor speed control, heater control, and lamp dimming.
Types: Rotary rheostats (knob-based) and slider rheostats (lever-based).

Trimmers(Trimpots, covered in great detail in older posts) are miniature, preset potentiometers used for fine adjustments during circuit calibration. Trimmers are usually set once and rarely changed unless recalibration is needed.
Use Case: Calibration of precision circuits, setting reference voltages, and fine-tuning oscillators.
Adjustment Methods: Screwdriver-slotted (single-turn) or multi-turn for finer adjustments.

Variable resistors offer flexibility where fixed resistors cannot and are an important tool in electronics.

And with that, we wrap up this Back to Basics: Resistor Series! Hope this helped you gain a new perspective on one of the most fundamental components. I will discuss something else from next week onwards.

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There are a few fundamental resistor parameters that folks often don’t consider while choosing a resistor. They choose a resistor value and size and call it a day. Please don’t do that. I am listing a few below.

First, every resistor has a Rated Power, the maximum power it can dissipate continuously at its Rated Ambient Temperature. This is what determines whether a resistor blows up or not.

Similarly, Rated Voltage defines the maximum DC or AC (RMS) voltage the resistor can handle without damage. This is the most common mistake I see in designs, using resistors and putting large voltage across them without checking voltage rating. Another one is the Maximum Overload Voltage, which is the max voltage capable of being applied to resistors for a short period in the overload test(Higher than the rated voltage).

Next, Resistor Tolerance indicates how much the actual resistance can deviate from its nominal value, typically expressed as a percentage at 25°C. Keep in mind that resistance also changes with applied voltage (voltage coefficient of resistance, VCR) and temperature (temperature coefficient of resistance, TCR). For example, a TCR of 100 ppm/°C means a 0.1% change per degree Celsius.

Reliability is another critical factor, defined as the probability a resistor will perform its function over time. This is often measured as the Mean Time Between Failures (MTBF) or a failure rate per 1000 hours. Testing at maximum rated conditions over thousands of hours helps predict how resistors will hold up in real-world applications, remember, reliability improves at lower power levels.

Another is Noise which is the unwanted AC signal generated by a resistor. It can harm low-level signals, charge & high-gain amplifiers, and other noise-sensitive applications.

Knowing these parameters helps you choose the right resistor for the job and design circuits that perform reliably under varying conditions. You will find a few of these mentioned in the datasheets.

PS: I should thank Vishay’s App Note for this one as there was a term that I didn’t even know was a considerable factor. 🙂

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