v3 specific knowhow for ExhaustGasTemp
Even on v3.2 that is assembled to a very high level, the EGT parts are not on (pads left empty). The circuit itself is simple:
but the
- probe
- AD597AR chip
- special connector (to eliminate the error caused by temperature difference between EC.. connector and AD597AR chip)
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) | |
| 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) | |
| 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
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).
AD597 output (10V for 1000C) is divided with 2 resistors so the mcp3208 can measure it (max range is 5V).
- 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.
- 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 ??) 22k:33k is closest prolly...
MCP3208 to LCD
In lcd_display.c channel 5 is written (from the valid real channel numbers 0..7). \n
mcp3208_data[5]; ... read_mcp3208(5);
The real channel 5 is confusingly (in eagle library or mcp3208 datasheet) named MCP3208_CH6 (from CH1..CH8). Anyway, 5 in the code reads pin6. (0 in the code reads mcp3208 pin1, ...etc... 7 in the code reads mcp3208 pin8)
Since EGT1 out goes (after divider) to mcp3208 pin5 on v3.2, this is the only change since v3.1.
To get good LCD reading you can
- change the code: write "4" instead of "5" to read mcp3208 pin5
- or connect the mcp3208 pin5 and pin6 (since you most likely not use pin6 for anything else anyway): the leftmost pins on the 2x2 througholes right-upper relative to AD597 are perfect for this
TODO:
- config.egt1_cal units, and calibration hints (verify with DVM)
- 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