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I have a boiler controller with a failure at this Fuel temp. sensor. The type of sensor is OM04-K130-5 2K. In one of the controller is showing a fix value at LCD and when I heat the sensor no changes on it. At another one is showing 0ºC. I have replaced the OP07C and after that both still failing, the fix value changed but it stays fixed and the other one is showing 500ºC.
At both the R10 was hot because it´s black but on both the value is correct, 237K. In one of them the IC was physically damaged (burnt).
When I tried to measure at a good unit to compare values with scope at PIC pin 18 I see a not measurable signal, it´s a waveform. I see how value is changing at LCD but no visible changes at input of microcontroller. I opened the R27 line to measure mA just in case it was a 4-20mA input but it showed 0mA even with the value changing at LCD.
I´m not sure how to deal with this because I don´t understand the theory of operation...
Hi 2PAC,
Hope you got your boat sorted out.
As said in a previous post, I think, with the information you have posted, that I have established how your thermocouple circuit works, but first about the carbon deposits/ burning you mentioned.
At both the R10 was hot because it´s black but on both the value is correct, 237K. In one of them the IC was physically damaged (burnt).
It sounds like your board has overheated which is not unusual with boilers. Check that the PCB is not carbonised. A carbonised PCB and carbon (soot) will cause a short circuit. (This was very common on the small PCB on the neck of the tube in the old CRT TVs). Give the board a good clean if necessary.
Circuit Function
Pin 18 of the PIC18F87J60 can be configured as one of the channels connecting to the internal 10 bit, analogue to digital converter (ADC), and this must be the way that it is configured in the thermocouple circuit. The ADC samples voltage, not current as you suspected, probably in the range 0V to 1.8V or 0V to 3V. (Table 27.26, Page 446 of PIC datasheet attached below).
As far as I can tell, commercial thermocouples generate a small positive voltage proportional to absolute temperature (0 deg K). This voltage is increased by the shunt feedback (virtual earth) inverting amplifier, U8 which has a voltage gain of -247: -1*(R10+R11)/R12. This means that the voltage output from the amplifier goes
down as the temperature increases.
RV(10K), R20(10K), R46(100K), R47 (100K), R49 (100K), R48 (15K) are involved in generating a negative offset current at pin2, the virtual earth point of the opamp. This current is adjusted by RV to calibrate the circuit against temperature. The National standard is to calibrate at 0 deg C, but the calibration temperature for a boiler application could well be different.
If what I think is correct, the output voltage from the opamp will be highest at the lowest temperature the system is intended to handle. This means that the ADC will read a binary maximum at the lowest temperature which is back to front, but I suspect a conversion is done by processing in the PIC.
On the other hand it is possible for the thermocouple to be connected to produce a negative voltage with increasing temperature, but this would mean that the offset circuit would need to connect to +8V instead of -8V. Also I am not sure if, for practical reasons, commercially available thermocouples can be reverse connected.
Resistors R21 (2K), R23(100R), R22(5K1, R27 (330R) are concerned with protecting the input to the PIC and presenting it with an impedance of 2K5 or less; 2K13 is actually achieved, which is OK
The capacitors play no part in the basic function of temperature measurement. They are just for filtering to keep hum, pickup, and general hash from affecting the DC signal from the thermocouple; there is probably plenty of noise in a boiler environment. C23 and C24 form 2 passive low pass filters and the unidentified capacitor forms an active low pass filter.
I assum that, if I am right about the circuit function, you would be happy to locate the fault/faults, but if you would like a fault-finding procedure just ask and I will post one. Good luck.
PIC DATASHEET
https://ww1.microchip.com/downloads/en/DeviceDoc/39762f.pdf
OP07 DATASHEET
https://www.analog.com/media/en/technical-documentation/data-sheets/OP07.pdf