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# How to connect this strain gauge?

#### bertus

Moderator
Nov 8, 2019
3,321
Hello,

Is the current for the complete sensor or for one section?
If it is for the complete sensor, the max voltage would be 25 mA X 120 Ohms = 3 Volts.
I found a couple of sites for the sensor, but no specs are given:
They are typical chinese sites whit hardly any info.

Bertus

#### danadak

Feb 19, 2021
767
No, I missed the module link. That module has a negative supply generator
on the board, and an opamp to do the offset.

All you do is connect the sensor to module, and adjust the G and Offset pots to get
the 0 - 5V (or 3.3V) range for the Arduino input.

Keep in mind the pots will interact in output V as you are changing the slope and offset
of the AD620 output.

So to get output swing of ~ 5V you need a supply of ~ 9V or more. Because of the AD620 output
swing limitations, ~ 2V off each rail.

With current limit of sensor 25 mA, if connected to 9V, then I = 9 / 240 ohms (2 sections in series ....?)
hence 37 mA. So connect - power lead in to ground, + to a R to limit current to 25 mA, other
side to 9V. So R = 9 / .025 = 360 ohms. So R = 360 ohms - 240 ohms = 120 ohms. Maybe use
some margin and make R + 120% = ~ 140 ohms.

As a caution as you are working this out use a series R from module output to Arduino input, say
4.7K, in case you overdrive it while adjusting pots. Over drive means above its + supply
or below ground, by ~ .5V. This will limit current and prevent damage to Arduino.

Regards, Dana.

Last edited:

#### kusspuss

Aug 25, 2023
32
Hello,

Is the current for the complete sensor or for one section?
If it is for the complete sensor, the max voltage would be 25 mA X 120 Ohms = 3 Volts.
I found a couple of sites for the sensor, but no specs are given:
They are typical chinese sites whit hardly any info.

Bertus
Yes 25mA for complete bridge , so i need 3V to supply , i have voltage regulator of 3-24V so how can i suppl power to it

#### kusspuss

Aug 25, 2023
32
No, I missed the module link. That module has a negative supply generator
on the board, and an opamp to do the offset.

All you do is connect the sensor to module, and adjust the G and Offset pots to get
the 0 - 5V (or 3.3V) range for the Arduino input.

Keep in mind the pots will interact in output V as you are changing the slope and offset
of the AD620 output.

So to get output swing of ~ 5V you need a supply of ~ 9V or more. Because of the AD620 output
swing limitations, ~ 2V off each rail.

With current limit of sensor 25 mA, if connected to 9V, then I = 9 / 240 ohms (2 sections in series ....?)
hence 37 mA. So connect - power lead in to ground, + to a R to limit current to 25 mA, other
side to 9V. So R = 9 / .025 = 360 ohms. So R = 360 ohms - 240 ohms = 120 ohms. Maybe use
some margin and make R + 120% = ~ 140 ohms.

As a caution as you are working this out use a series R from module output to Arduino input, say
4.7K, in case you overdrive it while adjusting pots. Over drive means above its + supply
or below ground, by ~ .5V. This will limit current and prevent damage to Arduino.

View attachment 60425

Regards, Dana.
thank you for reply
as you mentioned
" So to get output swing of ~ 5V you need a supply of ~ 9V or more. Because of the AD620 output
swing limitations, ~ 2V off each rail.
With current limit of sensor 25 mA, if connected to 9V, then I = 9 / 240 ohms (2 sections in series ....?)
hence 37 mA. So connect - power lead in to ground, + to a R to limit current to 25 mA, other
side to 9V. So R = 9 / .025 = 360 ohms. So R = 360 ohms - 240 ohms = 120 ohms. Maybe use
some margin and make R + 120% = ~ 140 ohms."

please can i have little more explaanation how to seteup the resistors ( i have 120 ohm resistors)

#### bertus

Moderator
Nov 8, 2019
3,321
Hello,

You could put an 120 Ohms resistor in each power line of the bridge:

Then you can use 9 Volts max on the circuit.

Bertus

#### kusspuss

Aug 25, 2023
32
Hello,

You could put an 120 Ohms resistor in each power line of the bridge:
View attachment 60426
Then you can use 9 Volts max on the circuit.

Bertus
please can you check this

#### Attachments

• Capture.jpg
192.7 KB · Views: 3

#### bertus

Moderator
Nov 8, 2019
3,321
Hello,

The outer resistors are not connected to the bridge.
I have "shifted" the resistors:

Bertus

#### kusspuss

Aug 25, 2023
32
Hello,

The outer resistors are not connected to the bridge.
I have "shifted" the resistors:
View attachment 60428

Bertus
i made one big mistake in my previous diagram , please can you check this

#### Attachments

• Capture.jpg
197.3 KB · Views: 6

#### bertus

Moderator
Nov 8, 2019
3,321
Hello,

In your set-up the strain gauge will get no power.
In my schematic the 4 resistors in a diamand are the gauge.
There is a resistor from Vin to the P+ of the gauge and a resistor from the P- to ground.

Bertus

#### kusspuss

Aug 25, 2023
32
Hello,

In your set-up the strain gauge will get no power.
In my schematic the 4 resistors in a diamand are the gauge.
There is a resistor from Vin to the P+ of the gauge and a resistor from the P- to ground.
View attachment 60430

Bertus
thank you for reply, i am very much confused that do i need external resistors (120 ohm) to make wheatstone bridge on breadboard , i am really much confused, stuck that how can i connect this strain gauge to breadboard

#### bertus

Moderator
Nov 8, 2019
3,321
Hello,

The straingauge IS the wheatstonebridge.
You do NOT need to make one on the breadboard.

Bertus

#### kusspuss

Aug 25, 2023
32
Hello,

The straingauge IS the wheatstonebridge.
You do NOT need to make one on the breadboard.

Bertus
thank you for reply , please can you see this

#### Attachments

• circuit.png
560.3 KB · Views: 3

#### bertus

Moderator
Nov 8, 2019
3,321
Hello,

Yes, that should be correct.
That way the sensor can be used upto 9 Volts.

Bertus

#### kusspuss

Aug 25, 2023
32
th
Hello,

Yes, that should be correct.
That way the sensor can be used upto 9 Volts.

Bertus
thank you very much , i have question can i supply only 3V to resistors and 5V to AD620 , i have two voltage adapters separately , have tried to see output values from AD620 using DMM , the values in mv jumping alot

DMM reading

#### Attachments

• nb.jpg
208.3 KB · Views: 10
Last edited:

#### danadak

Feb 19, 2021
767
i have question can i supply only 3V to resistors and 5V to AD620

Short answer is NO, you wont have enough range to make as good solution. The range
would only be ~ 1V due to the limited output swing of the AD620 running off 5V.

But the Arduino can switch its reference to ~ 1V, so a design could be done, but more
design and simulation work has to be done. I would advise staying with 9V.

There are micros out there that can do this all in 1 chip but you would have to be
experienced developer to do it. Keep this in your back pocket for the future. Its
an SOC (System on Chip), contains a ton of stuff including analog.

Here I show single chip, bridge driven by a onchip current source to give it regulated
power, and a 16 bit onchip A/D to do the measurmenrt, +/- 1V range, or 2 uV resolution
(I used amp in a/d front end with a G = 8.

So you dont need the AD620 or opamp amps, just this chip. I show it supports a character LCD
with I2C interface, and USBUART in case you want to talk to a PC with it. Also all onchip.

You can see in right hand window most of chip resouces left to do other stuff. Food for
thought for future work.....

Other stuff onchip, multiple copies in many cases :

Regards, Dana.

Last edited:

#### kusspuss

Aug 25, 2023
32
Short answer is NO, you wont have enough range to make as good solution. The range
would only be ~ 1V due to the limited output swing of the AD620 running off 5V.

But the Arduino can switch its reference to ~ 1V, so a design could be done, but more
design and simulation work has to be done. I would advise staying with 9V.

Regards, Dana.
so the both AD620 and strain gauge i have to supply 9V

Last edited:

#### danadak

Feb 19, 2021
767
Your jumping values, do you have a scope ? I suspect what you see is the power
supply noise from the adapter. With a scope on infinite persistence you can see what
pk-pk noise is on a power supply rail. 100 - 200 mV of noise OK in a 5V system, most
designs. Depends on what you are doing.

Regards, Dana.

#### kusspuss

Aug 25, 2023
32
Your jumping values, do you have a scope ? I suspect what you see is the power
supply noise from the adapter. With a scope on infinite persistence you can see what
pk-pk noise is on a power supply rail.

Regards, Dana.
i am not sure i can find or not here , but the values are jumping a lot , never stable , please need your guide to solve this jumping values

this device?

#### Attachments

• sc.jpg
237.9 KB · Views: 3
Last edited:

#### danadak

Feb 19, 2021
767
so the both AD620 and strain gauge i have to supply 9V

I would run the strain gauge off 5V, the AD620 off 9V. That would put
40 mA thru strain gauge, what was the max allowed current ?

If thats too much current power off 9V with a series R to limit current
to spec. Or use a low voltage zener. This all has to be simed out to see
what range you get into A/D. Ideally A/D would see 0 - 5V or something
like that. Depends on how unbalanced the starin gauge is in application.

Is your Arduino running off 3.3 or 5V ?

Regards, Dana.

Last edited:

#### kusspuss

Aug 25, 2023
32
Your jumping values, do you have a scope ? I suspect what you see is the power
supply noise from the adapter. With a scope on infinite persistence you can see what
pk-pk noise is on a power supply rail. 100 - 200 mV of noise OK in a 5V system, most
designs. Depends on what you are doing.

View attachment 60450
Regards, Dana.
i have this device here

#### Attachments

• sc.jpg
237.9 KB · Views: 3

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