# Strain Gauge Sensitivity

Discussion in 'The Projects Forum' started by ntwhee0, Jun 5, 2014.

1. ### ntwhee0 Thread Starter New Member

Jun 4, 2014
3
0
I'm a prosthetist (no EE background)
I have a Health O Meter bathroom scale, which has four strain gauges that have 3 wires each.
I'm trying to hack it to send S+ and S- to AD623an for amplification.
I'm told that S+,S- voltage needs to be around 2mV.
I'm getting about 355 mV at S+,S- and I don't understand why.

I've sending about 5 volts (4.99) into E+ ,E-

Resistance between E+ and S+ is 1048 Ohms, between E+ and S- is 1048 Ohms, between E- and S+ is 1048 Ohms, and. Between E- and S- is 1045 Ohms.

Here's my math to figure S+ and S-:
4.99*1048/(1048+1048)=2.495 V,
the voltage between S+ and E- should be
4,99*1045/(1048+1045)=2.4914 V,
the voltage between S+ and S- should be
2.495-2.4914=0.0036 V=3.6 mV

But i'm not getting 3.6 mV. I'm getting 355mV, and I don't know why. I want to get about 3.6 mV

Where should I begin looking to figure this out??

ps. (testing the voltage between each gauge's red and black wire (or red and white wire), I get about 640 ohms at each gauge.)

2. ### ericgibbs Senior Member

Jan 29, 2010
2,414
369
hi,
You should measure 3.583mV with those bridge resistance values.
[see image]

The value you measure 'I'm getting 355mV,' is numerically very close to 3.58mV expected, are you sure that you are reading and measuring the voltage correctly.?

E

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3. ### celticlord New Member

Jun 10, 2014
1
0
Could someone please give me a pinout and and help to hook this up to a Basic stamp. I dont need to know how to connect it just how this pinsout. All I could gather is it uses a diff amp for the strain gauges.

4. ### ntwhee0 Thread Starter New Member

Jun 4, 2014
3
0
Hi ericgibbs,

Thanks for checking the math.

I finally got the correct sense voltage.
I had not properly probed the OEM pcb to know the correct scheme for E+,E-,S-,S+.
I figured it out by randomly trying different schemes.

5. ### ntwhee0 Thread Starter New Member

Jun 4, 2014
3
0
He Celticlord,

As per your request, You'll notice the Health O Meter's circuit board has labels of S1, S2, S3, and S4 in very small text at the boarder of the board where each gauge's wires connect to the board. I soldered to the RED wire connection points only; Excite positive to S1, Sense positive to S2, Sense negative to S3, and Excite negative to S4. I used AD623an to amplify the signal. Then the signal goes to an STF4 discovery board.