In EEVBlog #102, Dave Jones presented the idea of using multiple 1-Ohm 1/4 Watt 1% resistors to create a DIY current shunt. This page will explore that idea a bit.

1 Ohm Shunts

By combining series-parallel combinations of 1-ohm resistors, we can increase the overall power handling of the shunt without changing the overall resistance.

Fractional Ohm Shunts

Alternatively, we can simply parallel the resistors. This increases power handling as well but also decreases the overall resistance of the shunt. This means the signal (the voltage drop) will be smaller.

Temperature-Induced Drift

As large currents flow through the shunt, it will dissipate more power, and its temperature will start to rise. As this happens, its resistance value will start to drift, according to the resistors temperature coefficient.

To calculate the worst-case drift, we need to know the temperature coefficient of the resistors and the thermal resistance (to ambient) of a 1/4 Watt package.

Temperature Coefficient

A quick search on Digikey.com reveals that a typical 1/4 Watt 1% metal film resistor has a temperature coefficient of 100ppm/C:

Thermal Resistance to Ambient

According to the Analog Devices Wiki:

For a 1/4-watt resistor of typical size, the thermal resistance is about 125°C/W.

Calculating Temperature-Induced Drift

Let's calculate the drift at max load (1/4 Watt).

At 250mW, the resistor will rise by 31.25°C (0.25W * 125°C/W), which will cause a drift of 3125ppm (31.25°C * 100ppm/°C), which is 0.3125%, resulting in a resistance of 0.996875 to 1.003125 Ohms.

Combining this with the initial tolerance of 1%, we get a worst-case drift of 1.013125 to 0.986875 Ohms.

Here is a google spreadsheet which calculates the drift for some of shunts described above: