Hi all,
I would like to simulate using arduino a PT100 temp sensor. Basically I need only between -10 to 100 C.
I wasn t able to identify a digital potentiometre to have such a small steps like I found in this table they should be

Any idea, hints of how can I achieve this?
Thanks in advance

 

hi david,
Its possible using LTSpice and a PT100 resistance table.
E

 

I don t think I was clear in my question. I want that instead pt100 sensor to put my arduino to a device that usually read temperature from pt100.
What I found that is at 1k digital pot there are only 256 steps maximum which allow me to move like 3.9 ohms per step. But I am thinking if I put 2 of these in parallel then I can have about 32k possible values...I am still calculating if I can achieve like this....

 

To emulate a Pt 100 you need to build a resistor running from 80 to 200 Ohm ( check values in table).
This not simple however;
The current flowing is a measure resulting in a current circuit allowing you to change it.
The current is always lower then 0,5 mA this to avoid heating the Pt100.
Your current limiting device should run from 0 to 0,5mA.
Start with measuring the open voltage (No Pt100 connected) this allows you to calculate the current @ 100Ohm.
Next step is to connect your adjustable current limiter .
All simple electronics but calibrating is the real issue.

Picbuster

 

My PT100 is 2 wire model.

 

Hello,

For -10 °C the value is about 96 Ohms.
For 100 °C the value is about 139 Ohms.
When you put a resistor of 180 Ohm parallel with the potmeter,
you would need to set the potmeter on about 610 Ohms for 100 °C
and about 205 Ohms for -10 °C.

With a E48 resistor of 162 Ohms, you would have more range of the pot.
For - 10 °C the pot needs to be about 235 Ohms.
For 100 °C the pot needs to be about 980 Ohms.

There are also digital potmeters with more steps (even upto 1024).

Bertus

 

I haven t found any 1k digital pot with more than 256 steps. What I found is a quad 1k digital pot with 256 steps and I am thinking using that and put all 4 outputs in parallel...

 

Hello,

With the use of a 20K 1024 steps digital pot and a E48 resistor of 140 Ohms, you would need to set the pot and about the following values:
For -10 °C at about 305 Ohms.
For 100 °C at about 19460 Ohms.

Here are a couple at Mouser:
https://eu.mouser.com/Semiconductor...s/_/N-4c498?P=1yzekj2Z1z0wljoZ1yzxt5nZ1yzv6lw

I have selected a pot with a tolerance of 1%.
There are pots with tolerances that are much higher, even over 20%.

Bertus

 

See

 

 

Hello,

@Bordodynov , The TS wants to do a hardware simulation.
You are showing a software simulation.

Bertus

 

Some years ago I had some thoughts about simulated resistors for the Pt100 amplifier test.
The principle is as the following schematic:

A reference resistor R2, which should be slightly less than the lowest resistor value to be simulated.
P1 selects how much resistance is to be added to the reference R2.
A2 and Q1 form the fourth resistor in the bridge between the two part of P1 and R2.
A1 prevents P1 from loading the simulated resistor.
R1 gives bias to A1 if the output of the simulator has no connection to the output.

I have not considered what is required for supply to the operational amplifiers, but the amplifiers are likely to be some chopper-stabilized types.

The transistor Q1 is relatively uncritical: Low power, up to 5 Ohm on resistance and logic level of gate/source voltage if the supply of the amplifier is to be kept at 5 Volt.

It must also be said that the relationship between the P1 setting and the resulting resistance is a reciprocal function.

 

Some years ago I had some thoughts about simulated resistors for the Pt100 amplifier test.
The principle is as the following schematic:

I think that's an excellent approach!

I have not considered what is required for supply to the operational amplifiers, but the amplifiers are likely to be some chopper-stabilized types.

Another thing to consider is loop stability and avoiding high-frequency oscillations, especially in A2. My guess is that inserting a resistor (1 kΩ or thereabouts) between A2's output and the gate of Q1, and a small capacitor between A2's output and its inverting input, would do the trick.

 

See

 

Kjeldgaard.
You beat me (I was stuck with modeling). I now came up with the same idea as you (the ideas are hovering in the air). But the priority is yours. I bow before you. I slightly complicated my scheme. Now I give a simplified version.

 

Thank you for your cooperation on this idea, @Bordodynov

I think I can see a solution with one of the classic R/2R DACs without amplifier, but with the matching feedback resistor.

Of course, you can't get a direct ADC input code to temperature function, but maybe get rid of the reciprocal part.

 

Hi @Bordodynov ,

I am new to LTSpice tool. Could you please give me some guidance on your circuit?

 

Yeah, I don't mind, but ask specific questions.

 

Now I give a simplified version.

Your DAC shows a current output into the op amp input non-inverting input, which won't work.
But you can operate a multiplying DAC in reverse to act as a digitally-controlled variable voltage attenuator by using its output as a voltage input, and the Ref input as the output (see this).

 

I used the power supply in conjunction with a 1 Ohm parallel resistor and capacitor. I did this instead of a voltage source for better convergence.