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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. 🙂

#BackToBasics #Electronics #Resistors

This series won’t be complete without resistor configuration and on how to use them in circuits.

Series: When resistors are placed end-to-end, their values simply add. If you have two resistors R1 & R2 in series, Reff=R1+R2. Req is greater than any single one.
Parallel: When resistors share both connection points(meaning current splits at the point), the reciprocal of the total is the sum of reciprocals: 1/Reff=1/R1+1/R2. This makes Req lower than any single one.

These are the core fundamentals that all of you know by heart from school. Now I want to discuss the cases where it’s useful in complex designs.

Imagine building a precision measurement instrument with op-amps where setting exact gains is crucial. Typically, you’d use highly precise resistors with tight tolerances. However, in practice, variations from the ICs can prevent these resistors from delivering the exact values needed. This is where a combination approach shines. For instance, if you need an effective resistance of 10kΩ for a specific gain, instead of using a single resistor, you can combine multiple resistors to achieve the desired value.

Approach 1: To reach a maximum of 11kΩ, pair a 10kΩ resistor with a 1kΩ resistor and a finely tuned potentiometer. This method works well for increasing resistance. But what if you need to lower it?
Approach 2: Use a three-resistor setup by selecting a larger resistor in one arm and combining a resistor with a potentiometer in the other. This allows you to fine-tune and reduce the overall resistance precisely.
Approach 3: For the flexibility to both increase and decrease resistance, use four resistors, pairing a resistor and potentiometer in each arm. Adjusting each pot lets you finely control the resistance up or down as needed.

These combination techniques are essential for device calibration, allowing folks to compensate for unit variations accurately. Once device calibration is complete, the potentiometers are typically fixed in place with hot glue, ensuring the spec remains stable and reliable.

You can create elegant and precise engineering solutions, by just mastering the simple resistor combinations.

#BackToBasics #Electronics #Resistors