Maintaining circuit stability and accuracy to optimize performance is an essential requirement in virtually all designs. One often-overlooked factor that can influence circuit performance is the thermoelectric effect in resistors. This phenomenon can introduce unwanted voltages, which can disrupt the balance and functionality of sensitive applications. To give designers a better understanding of how to mitigate this potentially harmful occurrence in resistors, Bourns has developed an instructive application note, “Understanding Resistor Orientation and the Thermoelectric Effect for Balanced Circuit Design”.

What is the Thermoelectric Effect?

The thermoelectric effect, also known as the Seebeck effect, occurs when two dissimilar materials form a junction exposed to a temperature gradient. This temperature difference induces a voltage, or electromotive force (EMF), as electrons migrate from the warmer material to the cooler one. The magnitude of this voltage depends on the materials involved and can range from a few microvolts per degree Celsius (µV/°C) to several hundred µV/°C. While this effect is harnessed in devices like thermocouplers for temperature measurement, the unintended voltages produced can be particularly detrimental in precision resistors.

Impact on Different Resistor Types

Various resistor types exhibit differing sensitivities to the thermoelectric effect:

• Wirewound Resistors: These can generate a few µV/°C due to the junctions between the wire material and the resistor leads.
• Metal Film Resistors: These may produce tens of µV/°C, making them more susceptible to thermal EMF.
• Carbon Resistors: Certain non-precision carbon resistors can generate even higher voltages, sometimes reaching up to a few hundred µV/°C.

In high-gain systems or critically balanced circuits, even minor thermoelectric voltages can cause significant measurement errors. For instance, a 3 °C temperature gradient across a carbon resistor with a Seebeck coefficient of 100 µV/°C would possibly introduce up to 300 µV of noise, resulting in a 30% error in a 1 mV measurement.

Strategies to Minimize Thermoelectric Effects

To mitigate the adverse impacts of the thermoelectric effect in precision circuits, consider the following design practices:

1. Optimize Resistor Orientation: Position resistors to minimize natural convection effects. Vertical placement can create temperature gradients due to rising hot air, so horizontal placement is often preferable.
2. Maintain Uniform Temperature: Ensure that both the leads and the body of the resistor are at the same temperature. This can be achieved by placing precision components away from heat sources or cooling elements that could create temperature differentials.
3. Select Appropriate Resistor Types: Choose resistors with low Seebeck coefficients, such as wirewound types, which are less prone to generating thermoelectric voltages compared to metal film or carbon resistors.

By implementing these strategies, designers can significantly reduce unwanted thermoelectric EMF, leading to more accurate and reliable circuit performance.

Helping engineers achieve balanced circuit designs that meet performance goals calls for carefully considering resistor orientation, maintaining uniform temperatures, and selecting suitable resistor types. For more details on minimizing the adverse impacts of thermal EMF to help ensure the integrity and accuracy of sensitive electronic applications, you are invited to read Bourns’ helpful application note.