10V Load Cell supplied in the field?

[Dodger6600]

I have a load cell rated for 25000 lbs and such requires an excitation voltage of 10V. The DAQ board that came with it can only supply 5V and all this testing will be done in a remote location meaning I'll only have a truck as a power supply and any batteries I may need. How can I meet the 10V requirement so that I can accurately test. My circuits knowledge is quite rusty, if I invert the power from the truck to run a surge protector so that I can charge the laptop, can I run power to the load cell as well? Will I need to regulate the input into the cell from the surge protector? My other option is run straight from a separate battery but that would require a resistor in series, correct? The simpler the better.

 

[danw2]

Isn't the load cell output x.xx mV/volt, where the excitation voltage is the volt term?

If that's the case, if you use 5V excitation, you'll get half the output than if you used 10V for excitation.

My impression is that many load cells are designed for nominal 10V excitation but will run at other voltages. Any excitatation fluctuation from nominal will be a direct proportional error though.

I suspect the load cell will run fine 5V. Why not call the rep/mfg and check?

The DAQ card scaling needs to take the excitation difference into account.

 

[BrianE22]

I agree with DanW. Your load cell will run fine on a lower excitation voltage. However, you need tight voltage regulation. Either that or you'll need source sensing.

 

[OperaHouse]

With 5V excitation you will get half the accuracy for the same setup as 10V. Another problem is the excitation may need to float or be isolated from normal power ground. There are isolated power supplies (most are not isolated) that can take 5V to 10V or 12V to 10V. A 7805 regulator can be adjusted up to 10V with two resistors and a small value pot so you can get exactly 10V from a battery. Problem is that setup will need more than 12V (15-24V).

 

[Dodger6600]

So with the 5V from the laptop, it's common to get a voltage drop through the USB (hence the tight voltage regulation)? Without the regulation, will any nominal fluctuation be apparent in my numbers (will it show up in my graph)? I'm using TracerDAQ Pro and if I set my range to +/-5V should I just assume that without the regulation, my voltage wont be exactly 5V regardless?

 

[danw2]

Your original statement was, "The DAQ board that came with it can only supply 5V". I'm confused about where USB came from. Is the 5V excitation from USB or from your DAQ instrumentation card? I would think a DAQ card would be designed to supply a stable excitation output but what do I know?

The point being that the load cell output is "mV per excitation volt". If the excitation jitters, the output jitters. Stable source voltage is assumed for this type of measurement.

I have no idea how well USB voltage is regulated or filtered. Maybe others do.

 

[Dodger6600]

Correct, the DAQ board is connected and powered via USB port to the laptop which then in turn supplies power to the load cell. Each USB port supplies a voltage of 5V. I was under the impression that the DAQ board supplies a steady source voltage but Ive read that the output from a USB can drop due to other sources in the laptop drawing starting and drawing power. The DAQ board pin has a max output of 5V so I can either power the load cell via the DAQ board or another external source, hence my dilemma.

 

[ScottyUK]

Every chance that the DAQ board has a stabilised power supply on board to provide power for its own circuitry and to provide a reference for the cell. If the board has a reference voltage available I'd definitely use it.

 

[IRstuff]

The only question about the DAQ reference voltage is how much power it can supply. Generally, reference voltage sources output 1mA, maybe 10mA? You might need a 1:1 buffer amp

TTFN
faq731-376