The Tech Blog

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

Continuing on the types of resistors and where each one shines, let’s look at a few more variations of resistors.

Metal Film Resistor: They are similar to carbon film resistors except a metal film is deposited instead of carbon on a ceramic core. The usual material is nickel chromium. Similar to carbon film they have great precision. Where it shines is in the thermal stability over wide temperature ranges.

Thick Film Resistor: Created by applying a thick paste of conductive material onto a ceramic substrate using the same techniques we use in making T-shirt prints via screen printing. These are the most common resistors you find in SMD resistors and are dirt cheap. The coating thickness would be around 100um(Hence “Thick” film). The only drawback is that you might not get very low resistance values.

Thin Film Resistor: Similar to thick film resistors but use a thinner layer(0.1um) of conductive material. They provide higher precision and lower resistance ranges in manufacturing. The thin layer formed on the ceramic core is created by using vapour deposition. Laser trimming is done to adjust the values post-deposition.

Metal Oxide Resistor: Made by coating a ceramic rod with a layer of metal oxide (typically tin oxide), these resistors are highly durable and can handle high temperatures. Useful in industrial and high-power applications.

Metal Foil Resistor: They are the most precise and stable types of resistors available in the electronics industry. The core element of a metal foil resistor is an extremely thin layer of resistive metal alloy. Metal foil is etched into a specific resistive pattern using photolithography and then laser trimmed to get precision. They are known to get tolerances of ±0.001% range. They have a fast thermal response to changes in temperature and is known for minimal drift in value even after years of use. But they are expensive.

These are the resistor types I found. As a designer, relying on datasheets is often enough to select the resistor, but understanding resistor evolution helps you see why & where each type excels.