How do I wire a 3 wire RTD thermometer to an MCP3008 ADC chip?

Thread Starter

Riskinit

Joined Jan 28, 2022
65
Good evening,

I have an RTD00575 with wiring diagram shown here.
but I don't understand which leg is which on the wiring diagram? I have 2 white ends and 1 red end.

I also have the MCP3008 ADC chip here.

So my thought is I need to connect the red wire on the RTD00575 to my 3.3v power supply, one white end to my ch0 input on the MCP3008 chip, and the last white end to ground???

Do I need to have all the resistors shown in the drawing? What values should all those resistors be approximately? And then is E0, the gap in the middle of the wiring diagram, supposed to be my ch0 input?

Thank you for any guidance on wiring this.
 

ericgibbs

Joined Jan 29, 2010
18,767
Hi R,
The RTD is designed to be used as part of a Wheatstone bridge, as the diagram shows.
The Vout is across +S and -S,
R1 value should be equivalent to L1/L3
R2 and R3 should be equal values, chosen so that the signal level point is close to 3.3v/2

Note: the Signal will be close to 3.3v/2 relative to 0V, Ideally a Differential or Instrument Amp would be used to amplify the signal and reference it to 0V.
Do you follow.?
E
EG 1514.gif
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
So the Wheatstone bridge outputs a voltage difference that can be measured? So is the thermistor and 3 resistors directly on the legs part of the device then? It wasn't obvious that was the case. The resistance on the RTD00575 device page is listed as 100 ohms. Is that saying the 3 resistors are all 100 ohms each? When I measure the resistance across the device I get anywhere from 110-120 ohms and can see it lower when I put it in water.

So I would assume I make R1 == R2 == 100 ohms and then put the device in ICE water and adjust R3 until the voltage difference is 0 v? Is that a correct assumption?
 

ericgibbs

Joined Jan 29, 2010
18,767
Hi R,
That would be my starting point, a bridge with 3 off 100R 0.1% resistor, the fourth arm of the bridge would the RTD.
Drive it with 3.3V [ same as the ADC supply.]
The bridge may be slightly off balance so that you have a Offset voltage across +S and -S.
One method to eliminate that Offset is to have a 98R resistor in one arm in series with a10R adjustable to trim out that offset.
The +S and -S will be at 3.3v/2 .
So the Signal could swing above or below the 3.3v/2

Do you have a sketch to post of your project.?
E
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
I do have a sketch but there isn't anything note worthy about it. I want to read the RTD value into a raspberry Pi ultimately which then turns on a fan at a certain temperature. I have the Raspberry Pi talking to the MCP3008 chip with a script but obviously I haven't been able to get channel 0 on the chip to read the analog value from the RTD sensor.

RTD sensor --> Wheatstone Bridge --> Amplifier --> MCP3008 chip --> Raspberry Pi --> Small DC Fan

The Wheatstone Bridge was exactly what I needed to read about to complete the project. I will report back with my findings today.
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
So I am finally reading values from the wheatstone bridge.

-0.006 volts at 34 degrees f
+0.247 volts at 209 degrees f

I want to add an amplifier at this stage but I am confused about what is needed. I want to turn the range listed above into something between 0 and 3.3 volts.

Would the LM358N suffice? It has a gain of 100 which would adjust the above values to -0.6vand 24.7v which I assume is out of spec for the MCP3008 chip. Should I be looking for an op amplifier with a gain of 10 then? Any suggestions?

Thanks for any assistance.
 

ericgibbs

Joined Jan 29, 2010
18,767
Hi R,
Usually, a W/Bridge output drives a Differential or Instrument Amplifier.
A LM358 could be used if it was powered with a dual supply, with a single supply voltage its output would not go to zero volts.
The gain of a LM358 can easily be set to *10.

I will post a circuit and LTspice simulation of one option.

E
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
Hi R,
Usually, a W/Bridge output drives a Differential or Instrument Amplifier.
A LM358 could be used if it was powered with a dual supply, with a single supply voltage its output would not go to zero volts.
The gain of a LM358 can easily be set to *10.

I will post a circuit and LTspice simulation of one option.

E
@ericgibbs Thank you for taking the time to produce these circuits. I could implement one of them but the theory behind them is currently beyond my knowledge level and I dislike "copying answers" without understanding them.

Regarding gif 2:
1) Why does the LM358 need 2 power supplies?
2) Is V4 a 3.3v power supply also like V3?
3) How does V1 and V2 play into the circuit?
4) How do you set the LM358 to a gain of 10 or any (reasonable) value X?
5) What differential amplifier would you recommend for my particular application?
6) Have you essentially created a differential amplifier using the LM358?
7) I located a pre-built circuit MIKROE-2815 that combines the [Wheatstone Bridge --> Amplifier --> MCP3008 chip] portion of the project into a single device. Are you aware of any similar devices?

Thank you for any information you can offer.
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
2) Yes
3) Powers the operational amplifier

New questions:
8) R5 and R6 are "4k3". Is that 4,300 ohms?
9) Is R1, R2, R3, and R4 supposed to be the W/Bridge?
 

Thread Starter

Riskinit

Joined Jan 28, 2022
65
One thing I noticed on the datasheet of the LM358 is there is a practical example for a DC differential amplifier.

1648821508042.png

This setup would appear to have a gain of 2? Is there a way to get a gain of 10? Sorry for bombarding you with questions.

Edit: I would have to increase R4 duhhh :)
 

ericgibbs

Joined Jan 29, 2010
18,767
hi R.
Regarding gif 2:
1) Why does the LM358 need 2 power supplies?

So that Vout can swing to Zero volts

2) Is V4 a 3.3v power supply also like V3?
V4 is a simulated RTD signal from -6mV to +247mV

3) How does V1 and V2 play into the circuit?
They are the +5V and -5V OPA supplies

4) How do you set the LM358 to a gain of 10 or any (reasonable) value X?
Vout= (R7/R4)*(Va-Vb)

5) What differential amplifier would you recommend for my particular application?
I would use a INA, instrumentation amplifier, a basic type is the AD623

6) Have you essentially created a differential amplifier using the LM358?
Yes

E
 

Attachments

ericgibbs

Joined Jan 29, 2010
18,767
hi R.
New questions:
8) R5 and R6 are "4k3". Is that 4,300 ohms? Yes
9) Is R1, R2, R3, and R4 supposed to be the W/Bridge? Yes
 
Top