# Strain Gauge Load Cell Questions

1. Aug 10, 2014

### FinanclEngr

I am not a (real) engineer, but have undertaken an ambitious project which contains some technical intricacies I have to learn on the fly. My first question (of many to come no doubt) is about strain gauge load cell technology - specifically powering the device.

My project requires a battery powered weighing system, load range 0 to 500lbs, split between two contact points with required accuracy of .2% (1lb). I need to calculate the power draw of prospective load cell types so that I can select the best battery (cost vs. capacity) for my application.

Question 1: is the load cell's output resistance figure the best proxy for total device resistance?
Question 2: if yes to above, do I derive total Amp usage by dividing excitation voltage by output resistance?
Question 3: Is the listed excitation voltage an absolute requirement or can I power the device with a lower voltage battery? (ex if specs. state 15v, can I power with a 5v or 10v, perhaps this is dependent on the MV/V rating?)

Here is one particular cell I am considering: http://www.flintec.com/type_pb_planar_beam_load_cell.html

2. Aug 12, 2014

### Baluncore

Welcome to PF.
Yes, this is the right place.
A1. The symmetrical bridge will have similar input and output resistances. Use the input resistance.
A2. Current consumption will be excitation voltage divided by input resistance.
A3. Output is the product of excitation voltage and bridge distortion due to load.
If you lower excitation voltage then you proportionally reduce sensitivity and lower the resolution.
You will then need more gain in the amplifier to reach FSD. Noise may be proportionally greater.

3. Aug 13, 2014

### FinanclEngr

Thanks Baluncore

With the aforementioned sensitivity in mind, is there any way to increase resistance of the sensor to lower total current draw? It looks like most single point sensors in my weight capacity range are ~350-400 ohms. I need this type of sensor but with >1000ohm resistance...any ideas? If I drive 2+ sensors with the same battery does overall circuit resistance increase? Will excitation voltage subsequently drop/vary between sensors? Can I weigh a load with a series of sensors connected in a single series or parallel circuit?

4. Aug 14, 2014

### Baluncore

How you minimise current will be determined by the electronics you use with the sensor.
You only need to power the sensors when a reading is needed.

An accuracy of 0.2% is 1 part in 500 so it will require at least a 9 bit converter.
The FSD output from the load cell bridge is probably one part in 1000, = 1mV per 1V of excitation. With 3V excitation you will only get 3mV signal. You could possibly use the two sensors in series, probably running on between 3V and 5V, but then you will have only half that signal.

Strain gauges are ideally suited to sigma-delta converters. They convert to between 16 and 24 bits.

I would consider ground referencing and summing the outputs by using an LTC6943.
http://cds.linear.com/docs/en/datasheet/6943f.pdf
Figure 1. Differential to Single-Ended Converter.