Using an ESP32 with a load cell with 5 V recommended excitation voltage

Question

I am making an IoT device using an ESP32 that powers a load cell with 5 V recommended excitation voltage. I know the ESP32 output is 3.3 V, but I need it to be 5 V. Is there a way to do this with a level shifter or some other added component?

Answer 1

You can excite a load cell with less than the recommended excitation voltage. Load cells are usually specified in millivolts per volt excitation at full scale, or mV/V.

For example, a 10 lb load cell rated for 10 mV/V and excited with 3.3 V would output 33 mV when you put 10 lbs on it, or 16.5 mV when you put 5 lbs on it.

This is by far the easiest option.

However, electrical noise is a constant added to your actual output. Lower voltages mean the noise makes up a larger overall portion of your signal. With oversampling and averaging you can filter out some of the noise, but you'll get a better signal to noise ratio with a higher voltage.

That is, provided that you don't overheat the device. Load cells are voltage dividers and burn a constant power when excited, producing waste heat. Since \$P=\frac{V^2}{R}\$ and \$R\$ is constant, your power loss increases quadratically for increasing voltage. All resistors have some temperature dependence, so you may find that in your use case a lower voltage is necessary to prevent temperature-related drift.

If you still want to power a load cell with more than 3.3 V in your design, you will need a second power supply. Note that your load cell output will vary linearly with any power supply noise or ripple, so a very steady source is key. Filtering the spikes out of an SMPS is not easy; this may be a use case for a less-efficient linear regulator. (Keep its heat away from your load cells.)

Finally, many microcontrollers have an AREF or VREF pin to set the ADC maximum voltage. If you power this from your analog regulator, it helps to filter out any inaccuracies in its steady-state output. You'll probably have to use a voltage divider to make sure you don't exceed the maximum input voltage on your micro.