Pressure sensor implementation

Thread Starter

geoffers

Joined Oct 25, 2010
475
Hi all,

I'm using a TE pressure sensor based as most are I suppose on a Wheatstone bridge.
In the data sheet it shows thisScreenshot_20221207-131247.png
I've used a ina122 instead of the circuit shown, my question is about the symbol shown as a current source on the left. Does this mean it will draw 1.5mA or I should only let it draw 1.5mA with a resistor?

I'm using 5v supply and the resistance across the bridge is 6k ohms which would give me 0.8mA draw.

This should be fine in my head but the first sensor has lost one strain gauge (could be down to mishandling) but I wanted to rule out it being damaged by the circuit driving it as they are a little bit dear!

Cheers Geoff
 

ericgibbs

Joined Jan 29, 2010
18,766
hi geoff,
The symbol is for a constant current source of 1.5mA to power the bridge.
If you use a 5V source and a series resistor, you may not get the constant current.?

Do you have a datasheet for the sensor.?
E


Update: @geoffers
Downloaded the d/s, it states, see image,,so using a voltage source and resistor set for a 1.5mA current will be OK.

As it does show not the actual resistance of the bridge, if you start with a 3k3 resistor from 5V and measure the current, you can determine the bridge resistance.
Then calculate the actual value of the series resistor.OK.?


EG57_ 331.png
 
Last edited:

Jerry-Hat-Trick

Joined Aug 31, 2022
544
Datasheet attached. Whilst the example "application schematic" diagram shows 1.5mA input current the specification table shows 0.5mA to 2.0mA which "Guarantees output/input ratiometricity" (See note 6). But the input resistance can vary from 2.0k to 6.5K (depending also on the range of your sensor) So 5V could give you from 2.5mA to 0.77mA. If you measured 6K the 5V would be fine, although if you've lost a leg (open circuit) you'll see roughly twice the resistance you'd expect from a complete bridge. With a complete sensor, subject to the measured resistance being within the values to ensure a current of 0.5 to 2.0mA I think just using a 5V supply would be fine and if the resistance is too low a series resistor to target 1.5mA would be do. For accurate measurements I'd be checking the supply voltage with A/D alongside the output A/D.

If you can live with approximate readings (for example, for proof of concept) you could use the sensor with open circuit leg by connecting a resistor of half the value of the resistance measured between pins 2 and 3 across the missing leg - which should roughly halve the resistance between 2 and 3.
 

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