SCALING OF ANALOG CURRENT SIGNAL (4mA to 20mA) INTO TEMPERATURE(50°C to 400°C)

[firstoption]

Good day to all,
Please i need your support on how i can scale an analog current Signal with a range of 4mA to 20mA into temperature range of 50°C to 400°C. The 4mA corresponds to 50°C while 20mA correspond to 400°C. I tried to do the scaling with the formular below, while the 4mA result(50°C ) was okay,the subsequent values of the analog current were not correct.
I am working with this formular:

Temp in °C = [(Vout in mV) – 500] / 10 ( the voltage out is calculated from V=IR where R=250 Ohms).
So for example, if the current out is 4mA that means that the temperature is ((4mA x 250 Ohms) – 500) / 10) = 50 °C.
So if the current is 20mA, using the above formular, the result will be 450°C. I am looking for a way of adjusting the formular so that i can get exactly 400°C instead of 450°C . I would be glad if somebody could help me out. Once again thank you for the sdupport. Best regards.

 

[D.A.(Tony)Stewart]

First you convert mA to V and V to deg C

You can either convert in Analog for meter or in digital code if using ADC which would not use the full range of ADC but may be accurate enough.

To read temperature 50~400'C for 4~20mA input you need gain and offset using an Op Amp and precision reference voltage in the inverting input or use a negative programmable Zener to the fixed resistor with applied current.

Do you want 400'C= 4.0V ? or use an Op Amp and convert to full scale of ADC then convert full scale to 50~400'C

if Analog 50~400'C= 0.5~4V or 400'C/4000mV = 0.1'[C/mV] or 10 [mV/'C]
for 4~20mA for 50~350 'C, gain = = 350'C/16mA

then overall gain = 10 [mV/'C] * 350/16 ['C/mA] = 3500/16 [mV/mA] = 218.75 Ohms

and at 4mA * R , you need an offset of -0.875V

If using an Op Amp use precision reference or bandgap programmable zener to use + Vref into inverting gain and input current for + gain factor.

 

[firstoption]

Thank you very much for taking the time to go through my post, I really appreciate your Kind gesture. I am actually using the ADC of a microcontroller to convert the analog Signal to digital signal, so i did not not use any Op Amp in my circuit. My plan is to now convert the digital Signal from the ADC to the corresponding temperature through programming(some C codes).My voltage reference for the ADC is Vcc(5V) Will this still be possible without using the Op Amp. I will highly appreciate whatever suggestion you can render in this regard. Once again thank you for the Support.Best regards.

 

[FvM]

The calculation method is a linear transform

y = ax + b

E.g. x is adc value, y temperature reading. Choose constants a and b so that the equation is correct for two arbitrary points.

 

[vGoodtimes]

Assuming the transducer works with a 250 ohm load, it sounds reasonable.

An ideal current sensor would look similar to an inverting opamp without an input resistor -- effectively 0 ohm to gnd and with a voltage output. However, practical current outputs may allow a wide range of loads to be used.