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4-20mA to Ohms converter

Shey Malcolm

New Member
Hi everyone, im after some help with integrating a PT100 sensor into my alarm panel (its a pretty smart alarm panel, just not quite smart enough to take a 4-20mA reading and convert it to temperature)
Basically I have a PT100 probe, and a 4-20mA converter for it, what I need to do is accurately convert the 4-20mA signal back to a known resistance change so the alarm panel can read it, preferably with the following increments (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
This is the probe, and the transmitter -
http://www.kiatronics.com/temperature-sensor/temperature-probe-pt100-stainless-steel.html
http://www.kiatronics.com/temperature-sensor/temperature-sensor-4-20ma-transmitter-code-70692.html
Thank you all for any help & ideas you might have.
Cheers
Shey
 

ronsimpson

Well-Known Member
Most Helpful Member
Where did you get the table? It must have come from your alarm panel manual.
Can you measure voltage?
What is you alarm?
Do you have the manual for the temp to 4-20mA module?
 

Reloadron

Well-Known Member
Most Helpful Member
Where did the resistance values you posted come from as they do not look like PT100 resistances? Also, where did the voltages come from?

Generally you would connect the PRT100 to your temperature sensor using the 3 wire input. Your sensor list a range of 0 to 100 degrees C so 0 to 100 degrees C = 4 to 20 mA. Now if your alarm panel doesn't have a 4 to 20 mA input what does it have? If it has a voltage input you place a resistor in your current loop. For example with a 4 to 20 mA loop if I insert a 250 ohm resistor my 4-20 mA = 1-5 volts, using a 500 ohm resistor gets you 2 to 10 volts for the temperature span. What input is the alarm panel looking for?

Ron

Edit: Morning Ron. Ron got it posted before me. :) That's what I get for drinking and refilling coffee. :)
 

ronsimpson

Well-Known Member
Most Helpful Member
The Rons agree. (more or less) It is time for liquid caffeine. I take mine cold in the form of Dr Pepper.

Many alarms can handle the 1 to 5 volts from the 250 ohm resistor with a 4-20mA current loop.
BUT
You will need a separate power supply to power the 2-20mA box.
 

crutschow

Well-Known Member
Most Helpful Member
It's difficult to generate an accurate resistance proportional to a voltage. So as an alternate, put a 500Ω pot across the alarm panel input and measure the voltage across the pot for various settings of the pot (set the pot resistance to the values in your table). From that you can determine what voltage inputs the alarm panels needs for the various temperatures. Then just amplify/attenuate the sensor voltages to correspond to those voltages and feed it to the alarm panel.
 

Shey Malcolm

New Member
Where did you get the table? It must have come from your alarm panel manual.
Can you measure voltage?
What is you alarm?
Do you have the manual for the temp to 4-20mA module?
Good morning every one, my alarm is a Inner Range Integriti panel, while it says it has analog inputs built in, after much arguing about the use of them i have been informed that it cannot use 4-20mA or 5-10v as that requires an additional board (crazy expensive) - it can however, read temperature based on the change of resistance over its zone input/common terminals. The list of temp/ volts / ohms is what I measured one night with a variable resistor, I already had the PT100 probe and transmitter, but was stuck as its a 4-20mA output. Unfortunately the transmitter did not come with a manual, nor anyone who could provide more than a price and a riugh idea how to operate a till.

After feeling a bit scammed both by the company we buy alarm & cctv gear through, and having a 4-5k reef tank whose temperature I want to be able to monitor, and control various heaters/chillers etc plus enable "shits hit the fan" monitoring, i decided that the best thibg I could do was try and make some kind of converter, unfortunately for me my electronics knowledge is limited, a few years in high school and my daily alarm technican role is a out all i have to draw on, lucky for me theres Google and some smart people around :)

Sorry for the ramble, its barely 7am here, and i havnt had my caffiene in any way shape or form yet :)
 

Shey Malcolm

New Member
It's difficult to generate an accurate resistance proportional to a voltage. So as an alternate, put a 500Ω pot across the alarm panel input and measure the voltage across the pot for various settings of the pot (set the pot resistance to the values in your table). From that you can determine what voltage inputs the alarm panels needs for the various temperatures. Then just amplify/attenuate the sensor voltages to correspond to those voltages and feed it to the alarm panel.
Thats exactly what ive done to get the data, my problem is amplifying/attenuate-ing? To get the voltages required
 

Shey Malcolm

New Member
The Rons agree. (more or less) It is time for liquid caffeine. I take mine cold in the form of Dr Pepper.

Many alarms can handle the 1 to 5 volts from the 250 ohm resistor with a 4-20mA current loop.
BUT
You will need a separate power supply to power the 2-20mA box.
Maybe thats where I was going wrong, I was trying to power the transmitter from the alarm supply too. Lucky for me I have a spare psu in the alarm cabinet :)
 

Shey Malcolm

New Member
Thanks Ron, where exactly do I connect the resistor? We break the negative lead to read the current through it, does the resistor go in this break and the measure voltage over it? Cheers!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Can you get the schematic diagram for the unit?, specifically the analogue input section you're trying to make a converter for - it's VERY difficult to make a variable resistance, particularly when we've no idea if it's grounded at one end, connected to HT, or entirely floating. The schematic would make this clear, and show if you 'could' simply input a voltage.

For example, of one where you COULDN'T input a voltage, an analogue joystick port, where it measures resistance by using the resistance to charge/discharge a capacitor. The resistor being part of a bridge would be another problematic design.
 

Shey Malcolm

New Member
Can you get the schematic diagram for the unit?, specifically the analogue input section you're trying to make a converter for - it's VERY difficult to make a variable resistance, particularly when we've no idea if it's grounded at one end, connected to HT, or entirely floating. The schematic would make this clear, and show if you 'could' simply input a voltage.

For example, of one where you COULDN'T input a voltage, an analogue joystick port, where it measures resistance by using the resistance to charge/discharge a capacitor. The resistor being part of a bridge would be another problematic design.
Hi Nigel, I cannot get a schematic for the zone input on the alarm panel - ive looked. The alarm panel is a Inner Range Integriti, but it does not take 4-20mA or the 1-5V analog inputs, it turns out I need to cleanly provide it the above resistances, when my 4-20mA loop provides the right current to the device..
 

JimB

Super Moderator
Most Helpful Member
Looking at the figures in the original post, it looks as though the "panel" has a constant current generator inside which is giving 2.3mA, the measured voltage is that current through the external resistance.
Plot it out on a graph and it gives a nice straight line (give or take the errors in the measurements).

To give the required results, I thought that this may provide a solution:

Insert a low value fixed resistor, say 47 ohms, to keep the constant current generator happy.
Then use the 4 to 20mA converter to drive a voltage source which will give the required voltage across the 47 Ohm resistor.

At 10degC the current source will be giving it usual 2.3mA to the 47R resistor, the voltage source will give the extra 2.5mA, giving a total of 4.8mA through a 47R resistor, which gives the 0.229 volts required by the panel.


Let me just re-state this waffle!

The panel is actually reading the voltage across the resistor by passing a constant current (0.23mA) through it.
Replace the resistor by a "stiff" voltage source, which is not affected by the 0.23mA constant current.
Drive the stiff voltage source by the 4-20mA from the PT100 converter, and scale the voltage souce to be 0.229v at 10degC and 0.806v at 38degC.

Does this almost make sense?

JimB
 

Reloadron

Well-Known Member
Most Helpful Member
Thanks Ron, where exactly do I connect the resistor? We break the negative lead to read the current through it, does the resistor go in this break and the measure voltage over it? Cheers!
Your drawing is about what you want. A few years ago I sent some temperature transmitters to another forum member. They look pretty much like what you have:

Temp Tran1.jpg

There are a few versions but they all do about the same thing. They convert a signal from a thermocouple, RTD or other temperature sensing device to a current which is generally 4 to 20 mA. Below is an image of what a basic setup would look like:

Temp Tran CKT.gif

Resistor R1 in this illustration is what I was mentioning earlier. In this case the 4 to 20 mA in the loop is converted to a voltage drop across the resistor.

I have a rough idea of what you have for this "alarm panel". They are called alarm panels, supervisory panels and a host of other names. Most use input cards so you can set the device up for various inputs. Doing what you want to do won't be easy. I don't know of an easy way to get from the 4 to 20 mA current loop you have to a resistance. Depending on how this alarm panel input is configured you may be able to run directly in from your PRT probe? Then write the software routine so the panel can derive a temperature from the resistance input. Unfortunately there is no simple magic box. I know cards for these panels can get expensive. It sounds like when everything was purchased there was some lack of communication.

I like Jim's approach above.

Ron
 

Shey Malcolm

New Member
Looking at the figures in the original post, it looks as though the "panel" has a constant current generator inside which is giving 2.3mA, the measured voltage is that current through the external resistance.
Plot it out on a graph and it gives a nice straight line (give or take the errors in the measurements).

To give the required results, I thought that this may provide a solution:

Insert a low value fixed resistor, say 47 ohms, to keep the constant current generator happy.
Then use the 4 to 20mA converter to drive a voltage source which will give the required voltage across the 47 Ohm resistor.

At 10degC the current source will be giving it usual 2.3mA to the 47R resistor, the voltage source will give the extra 2.5mA, giving a total of 4.8mA through a 47R resistor, which gives the 0.229 volts required by the panel.


Let me just re-state this waffle!

The panel is actually reading the voltage across the resistor by passing a constant current (0.23mA) through it.
Replace the resistor by a "stiff" voltage source, which is not affected by the 0.23mA constant current.
Drive the stiff voltage source by the 4-20mA from the PT100 converter, and scale the voltage souce to be 0.229v at 10degC and 0.806v at 38degC.

Does this almost make sense?

JimB
Thanks, it sounds like it makes sense, but i still cant quite see the idea in fruition, any chance of drawing a diagram to explain what you mean? Replace the 47r resistor with something that can be driven by the 4-20mA source, which in turn puts out the desired resistance. I call that device a magic box to solve all problems, is there a slightly more, "real" term/device/circuit? Haha thanks so much everyone :)
 

Shey Malcolm

New Member
Your drawing is about what you want. A few years ago I sent some temperature transmitters to another forum member. They look pretty much like what you have:

View attachment 87398

There are a few versions but they all do about the same thing. They convert a signal from a thermocouple, RTD or other temperature sensing device to a current which is generally 4 to 20 mA. Below is an image of what a basic setup would look like:

View attachment 87399

Resistor R1 in this illustration is what I was mentioning earlier. In this case the 4 to 20 mA in the loop is converted to a voltage drop across the resistor.

I have a rough idea of what you have for this "alarm panel". They are called alarm panels, supervisory panels and a host of other names. Most use input cards so you can set the device up for various inputs. Doing what you want to do won't be easy. I don't know of an easy way to get from the 4 to 20 mA current loop you have to a resistance. Depending on how this alarm panel input is configured you may be able to run directly in from your PRT probe? Then write the software routine so the panel can derive a temperature from the resistance input. Unfortunately there is no simple magic box. I know cards for these panels can get expensive. It sounds like when everything was purchased there was some lack of communication.

I like Jim's approach above.

Ron
Thanks Ron, Ive tried running the probe straight into the panel, there are a few problems with this, the first is that with the resistances being so different its almost impossible to make the alarm panel understand what on earth is connected to it, it reads about 80-100 degrees depending on the gain/offset details I tell it, and it goes up/down inversely, as the temperature increases the temperature on the panel goes down, etc. The other problem is that for a 10+ degree temperature change, it only changes a tiny amount, fractions of degrees. So its hard to get anything accurate.
 

crutschow

Well-Known Member
Most Helpful Member
..............................
To give the required results, I thought that this may provide a solution:

Insert a low value fixed resistor, say 47 ohms, to keep the constant current generator happy.
Then use the 4 to 20mA converter to drive a voltage source which will give the required voltage across the 47 Ohm resistor.

At 10degC the current source will be giving it usual 2.3mA to the 47R resistor, the voltage source will give the extra 2.5mA, giving a total of 4.8mA through a 47R resistor, which gives the 0.229 volts required by the panel.
..............................
I agree with the concept but you could use a higher value resistor to perhaps allow the use of a standard op amp to drive it. For example a 220Ω resistor would give an output of about 0.5V with no added current. Then the op amp sinking 2mA would give an output of 66mV and the op amp sourcing 2mA would give an output of 946mV.
 

JimB

Super Moderator
Most Helpful Member
Thanks, it sounds like it makes sense, but i still cant quite see the idea in fruition, any chance of drawing a diagram to explain what you mean? Replace the 47r resistor with something that can be driven by the 4-20mA source, which in turn puts out the desired resistance. I call that device a magic box to solve all problems, is there a slightly more, "real" term/device/circuit?
Something like this:

Current to resistance converter.JPG

JimB
 

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