v3 specific knowhow for ExhaustGasTemp
Even on v3.2 that is assembled to a very high level, the EGT parts are not always populated (- except the assembled controllers). The circuit itself is simple:
Short checklist (see table below for details):
- supply for AD597AR
- sensor (or compensational cable) wires connected (green+ to AD597pin1 and white- to AD597pin8). A(n optional) capacitor between them filters noise.
- make sure pins 1,3,4 of AD597 are connected, using the SMD pads and some wire (note: pins 8,3,4 would be just as fine). This provides a solid ground for the circuit input
- AD597 output (pins 5,6 connected) connected to MCP3208 via 2 resistor + 1 cap voltage divider and filter. This is onboard since v3.2 so requires little to no attention.
Table (colums) provide checklist for
- soldering (or checking, if the boardand has related parts)
- and measurements (only after all soldered, of course)
- report. Fill out a copy (print on paper) when completing. Fill in a digital copy if there are problems and asking for help.
No report can be evaluated that does not have all this completed.
AD597
| Reason | Check/Solder | Should measure | Actually measured |
| AD597 power | don't forget C37=220nF. L4 (1..10uH) (or 0..10 Ohm is just as fine, eg. solder a thin wire). L4 already shorted via trace on v3.0, Short C44 (and C55 for CH2), this is a remaint from a dual supply revision of the sch. Disregard anything saying that it should not be shorted. | pin7 vs pin4 is +8..15V | |
| feedback | pin5 and pin6 are connected | pin6 vs. pin5 is 0 Ohm (or 0mV in diode-check mode) | |
| common ground The K-Thermocouple sold in WebShop has no internal connection between the sensor pins and the sensor body. This is good. But the sensor must not 'drift' higher or lower then the AD597's supply. | Short SJ9 if your sensors body does not short the sensor to ground. (SJ8 for ch2) | pin4 vs pin3 is 0 Ohm | |
| No hysteresis | leave pin2 open | visually inspect | |
| EGT+ connection | green (+) wire from the sensor goes to pin1 | pin1 vs pin8 should measure 0mV in diode-check mode (upto +1mV if the sensor is connected and you heat the sensor tip 25C above the cold junction temperature) | |
| EGT- connection | white (-) wire from the sensor goes to pin8 | pin1 vs pin8 should measure 0mV in diode-check mode (upto +1mV if the sensor is connected and you heat the sensor tip 25C above the cold junction temperature) | |
| filtering cap | 100..220..1000..2200nF cap between EGT+ and EGT- (easiest on the board bottom side, right where the compensational cable is soldered to the header) | - | EGT reading becomes more stable |
| pin6 output voltage | If you don't have a K-thermocouple sensor at the time of these steps, short the EGT+ and EGT- inputs (eg. throughole pads that go to pin1 and pin8) | pin6 vs. pin4 is appr 100mV for 10C and 300mV for 30C |
If you are ready, the voltage on AD597 pin6 (vs. pin4, that is GND) depends on temperature, +10mV/K slope (higher voltage for higher temp)
- with sensor attached: temperature of K thermocouple tip
- or shorting wire inplace of sensor: depends on temperature of chip
AD597 to MCP3208 ADC
Default firmware release is compiled with MCP3208_EGT1=4 . This is valid for v3.2. However v3.0-v3.1 users will add MY_CONF += -D MCP3208_EGT1=5 to my_make and recompile (alternatively, it is OK to physically connect the EGT amplified output (with a small solder-blob) to both channel 4 and 5 (mcp3208 pin5 and pin6).
It doesn't make sense to care too much about EGT reading on LCD until the above steps are completed (pin6 voltage depends on temp - or EGT input).
Calibration
AD597 output (10V for 1000C) is divided with 2 resistors so the mcp3208 can measure it (max range is 5V). config.egt1_cal can be adjusted to show proper readings.
- v3.2 (R83=100k, R84=75k) since there is a cap on mcp3208 input that enabled us to use these high resistor values (little current consumption). Division is * 3/7.
- egt1_cal=0x49 (decimal 73) is most likely
- egt1_offs=00
- v3.0, v3.1 : (R83=2.7k, R84=2.7k) although division is * 1/2 , not same as for v3.2 (someone can find better matching values under 35k that are available in the rescue kit ?) might be 22k:33k closest. Some uses a piggyback cap that allows exact same (high value 100k and 75k) resistor values that are used since v3.2.
- egt1_cal=0x3F (or 0x3E decimal 63 or 62) is most likely, but do verify since different dividers were used
- egt1_offs=00
Calibration method
- connect input voltage to the EGT input. Eg. a 2*1.5V battery pack and voltage divider is perfect. Eg. 150k and 1k gives division of 3V/151=19.86mV.
- 10..30mV is best
- green (AD597pin1) is +
- white (AD597pin8) is -
Example
Let's say you measure 19.7mV on EGT input that you applied with the improvised voltage divider. If board temperature is 26C, the egt1_cal should be adjusted until EGT reading is appr. 26C + 19.7mV/(40uV/K)=26C + 492.5K = 518.5C In practice 510 .. 527C is perfect.
I usually start with egt1_cal=0xFF, read the value and adjust it in one step with a simple calculation. Or egt1_cal=0x49 and confirm the read value is perfect.
These seem to be old notes, everything has been standardized since and applied to the pick-n-place manufacturing
- R83=5k6, R84=4k7, (R83=100k, R84=75k on v3.2 schematic, C100=100nF makes that high impedance divisor possible without any loss of ADC-resolution )
- C44 (short, not capacitor. See explanation above)
- C90 (100nF)
- SJ8 - leave unconnected
- U16 AD597AR (not in rescue kit! Optional, see WebShop)
The recommended ratio of R83 to R84 is 4 to 3 (rails at 1100C ?), but anything above 1:1 will do, but ADC will rail at around 960C that way). On v3.1 and earlier boards, max recommended values are 20k. If you cannot calibrate, at least measure these resistors with DVM (before soldering).
Needed for EGT2:
- R81=4k7, R82=5k6 (R81=75k, R82=100k populated on v3.2, C101=100nF makes such high values possible)
- C55 (short, not capacitor. See explanation above)
- C91 (100nF)
- SJ8 - leave unconnected
- U25 AD597AR bottom side of board (not in rescue kit! Optional, see WebShop)
The best way of connecting the thermocouple to the board is special compensated connectors+wire,
[chassis connector], [wire connector] and something like this [compensated wire], these links are from a swedish supplier, the same products are available worldwide at better pricing...
[Farnell also has some cheap compensated thermocouple connectors]
If you solder standard wires for connecting the thermocouple, remember that the measured temperature will be the difference between the thermocouple and the joint between the thermocouple-wire and the standard wire
See also