4-20mA to Ohms converter

[ronsimpson]

Wait,

You show 10 to 38C=4 to 20mA. 1.75C/mA
I think you will get 0 to 100C=4 to 20mA. 5.5C/mA

This machine was built to use some temp probe. What probe? Are you certain the probe is linear?
OR Is the C/Volt linear in the data table above?

Crustchow's post #18. It will be hard to get down to 0.2V and fight the 2.2mA current source. Listen to Crustchow.

 

[JimB]

Can we just step back a minute and confirm my understanding.

In the original post the following data were presented:

(these resistances are what the panel recognises as the temperature)
10C - .229vdc - 99ohms
14C - .310vdc - 131.4ohms
18C - .399vdc - 170ohms
22C - .489vdc - 210ohms
24C - .523vdc - 226.9ohms
27C - .579vdc - 254ohms
30C - .653vdc - 287.5ohm
33C - .704vdc - 313.2ohm
36C - .755vdc - 338ohm
38C - .806vdc - 363.6ohm

Am I correct in understanding that these figures were produced by:
1 Connecting a resistor to the the terminals on the control panel
2 Measuring the voltage across the resistor
3 Reading the temperature display on the control panel
4 Measuring the value of the resistor when disconnected from the panel.

If this is not so, then my proposed system may not work - Shey Malcolm, please confirm or deny.

JimB

 

[ronsimpson]

Read post #7. I think he used a pot and a meter.

 

[JimB]

You show 10 to 38C=4 to 20mA. 1.75C/mA

Ron,

It does look like that in the drawing, but that was not my intention.
What I was trying to show, was that at that point in the system the signal was of 4 to 20mA form, although we are only using part of that range.
Before taking the discussion further, I would like to hear the OPs reply to my comments above in post #22.

JimB

 

[JimB]

Read post #7. I think he used a pot and a meter.

OK, pot or a box of odd resistors, the effect is the same.
It sounds like my idea is correct.
Let me kick some numbers around, and I will be back.

JimB

 

[ronsimpson]

I would like to know about the original probe. It looks like 0 ohms at 0C and 70C at about 500 ohms. (math in my head)
Where in the world would you get one of those?
Until we really understand the probe he does not have, how can we build a circuit that functions like it?

 

[Reloadron]

Along the lines of Jim's thinking here is something you can try. Using this concept with your temperature transmitter replace R1 below with around a 37 or so ohm resistor. Take the output and run it into your module.

Now we know you have an RTD and not a TC but you get the idea. Your temp transmitter is 0 to 100 C = 4 to 20 mA. We know or at least think your module has a fixed current out of around 2.3 mA for resistance measurements. So essentially we will have a fixed 2.3 mA source and a variable source in parallel with a 37 ohm resistor across them. I believe based on your numbers posted in your original post and assuming 2.3 mA fixed we should get 4 mA + 2.3 mA = 6.3 mA = 0 degrees C. Then 22.3 mA = 100 Degrees C. In keeping with your voltages as inputs using a 37 ohm resistor should work. The parallel currents should be additive forming the voltage drop across the 37 ohm resistor. Yes, 37 ohms is an odd resistance but try a pot or anything close to see if we get close.

You end up with a circuit that looks like the below image.

I1 is the internal reference current and I2 is your source current from your 4-20 mA temp transmitter. The drawing obviously isn't exact but you get the idea?

The plots look like the below attached thumbnail.



If anyone sees why this won't work please by all means correct me. I am not at all sure it will work but should be easy to try.

Ron

<EDIT> See post #30 </EDIT>

 

[Reloadron]

I would like to know about the original probe. It looks like 0 ohms at 0C and 70C at about 500 ohms. (math in my head)
Where in the world would you get one of those?
Until we really understand the probe he does not have, how can we build a circuit that functions like it?
_________________________
back to double posting.
------------------------------------------------

Ron, while I am not sure about this the PRT is a 0 to 450 degrees C PRT. It should use the standard PRT 100 chart. However, the temperature transmitter is a PRT type for a PRT 100 probe scaled for 0 to 100 degrees C = 4 to 20 mA. So the temperature transmitter will linearize the PRT curve (the ones I have do) and output 4-20 mA. So it really matters not at this point what the probe range is. That is my thinking on it anyway. All in all 0 to 100 C = 4 to 20 mA. All of the temperature transmitters like this I have worked with have done the same, be it a PRT input, T/C input or a host of other sensors.

Ron

 

[JimB]

Reloadron

I like your simplificaton, but there is a problem.
You have used 4 to 20mA for a temperature range of 0 to 100degC, the OP is looking for a temperature range of 10 to 38degC.
So to use your 36 Ohm resistor, the PT100 conditioning unit would have to be scaled for 4 to 20mA = 10 to 38 degC.

JimB

 

[Reloadron]

Reloadron

I like your simplificaton, but there is a problem.
You have used 4 to 20mA for a temperature range of 0 to 100degC, the OP is looking for a temperature range of 10 to 38degC.
So to use your 36 Ohm resistor, the PT100 conditioning unit would have to be scaled for 4 to 20mA = 10 to 38 degC.

JimB

Hi Jim and I understand what you are saying. You are absolutely correct. I scaled it all wrong. How about a 67 ohm resistor? Actually it would be nice to use a 100 ohm 10 turn pot and shove the probe in an ice bath and adjust the pot for 0 degrees C which should be around 67 ohms.

Thanks
Ron

 

[alec_t]

The constant current source in the instrument could be simply a nominal 10k resistor fed from 24V. Simulation shows 10.4k gives a pretty good match for the observed voltages/resistances.

 

[Reloadron]

The constant current source in the instrument could be simply a nominal 10k resistor fed from 24V. Simulation shows 10.4k gives a pretty good match for the observed voltages/resistances.
View attachment 87430

Makes sense. I wish we knew more about this thing.

Ron

 

[alec_t]

Fleshing out the above ideas a bit, here's where I've got so far on the assumption that 4-20mA corresponds to 0-100C.
R1 sets the scale factor V(in)/I(probe), and a current source subtracts a current offset.

 

[Reloadron]

Alec, as drawn that looks to be a very good solution. I like it because it only focuses on the desired range.

Ron

 

[alec_t]

Ok. Here's how I see it.


Edit: Q2 dissipates up to 450mW so should be a 2N2222 or similar rather than the 2N3904 shown.

 

[Reloadron]

I wonder how the original poster is doing. What they may have come up with or tried and what might have worked.

Ron