[Explained] Can Resistors Change Value Over Time?

Yes, resistors can change in value over time due to various factors. But for the majority of actual applications, the change in resistance is typically minimal and frequently insignificant.

The Reasons Why Resistor Value Change Over Time

The deterioration of the resistor’s material is a typical cause of a change in its value. The resistance of the resistor may wander over time as a result of changes in environmental conditions such as humidity, temperature, and others. 

We refer to this phenomenon as drift or aging. The performance of resistors can be harmed by an atmosphere with a lot of moisture. The rate at which a resistor’s value changes depends on factors such as its construction, materials used, and operating conditions.

There isn’t a single universal equation that can accurately describe the aging process of all resistors. However, a simplified equation that represents a general trend of resistance change over time:

ΔR = R₀ * β * t

Where:

ΔR = Change in resistance

R₀ = initial resistance at time t = 0

β = aging rate or coefficient, indicating the rate at which the resistance changes over time

t = Time elapsed since the resistor was put into operation 

If resistors age differently due to temperature differences, the scaling factor of a voltage divider, for example, will change over its service life. Resistance steadily rises over time as a result of normal aging, with the pace of this increase varying depending on the environment, electrical bias, resistivity, and the origin of the manufacturing process. 

Due to the physical characteristics of the materials, they are comprised of, resistors can alter in resistance as a function of temperature which is known as the temperature coefficient of resistance (TCR). The following equation can be used to represent the relationship between resistance and temperature:

R₂ = R₁ * (1 + α * (T₂ – T₁))

Here,

T₁ = Temperature across the resistor R₁

T₂ =Temperature across the resistor R₂

α =Temperature coefficient

Ohm per degree Celsius (Ω/°C) or parts per million per degree Celsius (ppm/°C) are the units used to quantify the temperature coefficient of resistance. It shows the resistance’s percentage change per degree Celsius.

It is possible to generalize the drift seen in most resistors to forecast how things will evolve over time in typical situations.

The resistor’s tendency to self-heat is another element that might alter resistance. Due to the power lost while a resistor carries current, heat is produced. One characteristic that all resistors have in common is that they can change their electrical characteristics in response to mechanical or thermal stresses. These stresses in circuits are brought on by temperature and power dissipation. 

Whenever subjected to mechanical stress, some resistant materials show a change in resistance. The piezoresistive effect is the name ascribed to this phenomenon.  The change in resistance, ΔR, can be expressed as

ΔR = R₀ * (GF * ε)

Here,

ΔR= change in resistance

R₀ = initial resistance

GF= gauge factor, a material-specific parameter indicating the sensitivity of resistance to strain

ε = strain applied to the resistor

A resistor produces heat when current flows through it, and the thermal reaction creates mechanical changes by expanding the various materials. As a result, over the course of an applied electrical load, the resistance value will fluctuate, occasionally at different rates.

The equation to calculate the heat produced by a resistor is given by Joule’s Law, which states that the heat (Q) produced by a resistor is equal to the product of the square of the current (I) passing through the resistor, the resistance (R) of the resistor, and the duration of time (t) for which the current flows:

Q = I² * R * t

Where:

Q= is the heat produced (in joules),

I= is the current flowing through the resistor (in amperes),

R= is the resistance of the resistor (in ohms),

t= is the duration of time for which the current flows (in seconds).

If such aspects as shape, length, or diameter are changed by mechanical or other means, the electrical parameters also change. The degree of change and of its predictability varies substantially with the resistor technology, i.e., the materials used to build the resistive element and the terminations.

Does the Change in Resistance Value Over Time Really Affect Operation?

In the majority of applications, resistance increases brought on by age or self-heating are insignificant and have no impact on the performance of the circuit. But for some sensitive or exact applications, such as precision measurement systems, periodic calibration or temperature compensation may be required to guarantee accurate findings.

FAQs – Frequently Asked Questions and Answers

Can resistors get old?

A resistor’s lifespan is unpredictable. Each type of resistor will differ because there are numerous varieties of them. There is no reason to anticipate a resistor failing if we use it at a power and voltage that is less than half of what they are rated for.

Excessive power or voltage ratings can cause failure in electronic circuits. Small resistors with 1/8 ratings are commonly used, with a few hundred volts and 1/4, 1/2, or 1 watt required. Combinations in series or parallel can improve power, but not too closely. Stringing up identical resistors in series can raise the voltage.

Can a resistor lose resistance?

No, a resistor cannot “lose” resistance in the sense that its underlying characteristics alter with time. The resistance of a resistor is determined by its material and physical characteristics, such as its length, cross-sectional area, and resistivity. As long as the resistor is in good shape and not damaged by harsh conditions, these characteristics hold true. 

However, there are times when a resistor may seem to have less resistance as a result of outside causes or malfunctions such as overheating, physical damage, contamination, or manufacturing defects. 

Which type of resistor cannot change its value?

Precision resistors, specifically those with fixed resistance values, are designed to have very stable and consistent resistance values that do not change over time. Two common types of precision resistors that fall into this category are,

• Metal Film Resistors and, 
• Wirewound resistors 

Both Wirewound and metal film resistors reduce resistance variations due to temperature, mechanical stress, and aging, used in high accuracy, stability, precision analog circuits, and measuring equipment.

To conclude

External factors like overheating, extreme environmental conditions, physical damage, and contamination can deviate resistance values over time. To optimize performance and resistance changes, considering operating conditions, power ratings, and appropriate types is crucial. Regular inspections and quality control measures help identify potential issues early on.