How are your 2 thermistors wired, plz?
In my ageing box for references one NTC is on the PCB in the middle of all voltage references.
(left in the middle of the PCB)
The other is in the middle of the top heater foil (with the red wires).
The bottom heater foil (below the PCB) is in series with the top heater foil so it needs no extra NTC.
The heater is controlled by a simple P-Controller (fast) from the heat spreader NTC.
The slow contol loop with the NTC on the PCB is a I-Controller with some anti windup limitations when the current temperature is far away from setpoint. The I-Controller output gives a Offset which is added to the setpoint for the P-Controller.
Since there is some self-heating due to many references the Offset is negative (I-Out in deg C)
here a log of the heat-up phase:
Time T-PCB T-HTR 14V 17V 24V I-Out P-Out (hex from 0x0000 to 0x2400 or 0..10000 dez)
08:21:01;D7 0 0 9999.9 H9; 38.21; 31.79; 13881; 16974; 24006; -4.18; P2400;
08:22:01;D7 0 12 0.0 D7; 39.34; 39.41; 13881; 16981; 24012; -4.18; P2400;
08:23:01;D7@ 1 12 0.0 D7; 40.87; 45.15; 13874; 16974; 24033; -4.18; P2400;
08:24:01;D7@ 2 12 0.0 D7; 42.55; 49.58; 13867; 16988; 24053; -4.18; P2400;
08:25:01;D7@ 3 12 0.0 D7; 44.15; 51.91; 13860; 17021; 24593; -4.18; P2400;
08:26:01;D7@ 4 12 0.0 D7; 45.24; 51.88; 13860; 16981; 24087; -4.18; P2400;
08:27:01;D7@ 5 12 0.0 D7; 46.16; 51.91; 13847; 17028; 24600; -4.18; P2400;
08:28:01;D7@ 6 12 0.0 D7; 47.02; 51.91; 13847; 17028; 24600; -4.18; P2400;
08:29:01;D7@ 7 12 0.0 D7; 47.77; 51.88; 13840; 17028; 24094; -4.18; P2400;
08:30:01;D7@ 8 12 0.0 D7; 48.45; 51.91; 13827; 17028; 24600; -4.18; P2400;
08:31:01;D7@ 9 12 0.0 D7; 49.06; 51.79; 13827; 17028; 24600; -4.18; P0000;
08:32:01;D7@ 10 12 0.0 D7; 49.44; 50.55; 13820; 17028; 24600; -4.18; P0000;
08:33:01;D7@ 11 12 0.0 D7; 49.71; 49.49; 13820; 17028; 24600; -4.18; P0000;
08:34:01;D7@ 12 12 0.0 D7; 49.85; 48.49; 13813; 17028; 24600; -4.18; P0000;
08:35:01;A4 0 0 0.1 A4; 49.98; 47.63; 13820; 17028; 24600; -4.18; P0000;
08:36:01;A5 0 0 0.1 A5; 50.01; 46.86; 13813; 17028; 24593; -4.18; P0000;
08:37:01;A6 0 0 0.1 A6; 50.03; 46.12; 13820; 17035; 24593; -4.18; P0000;
08:38:01;A7 0 0 0.1 A7; 49.96; 45.49; 13813; 17035; 24593; -4.18; P0133;
08:39:01;B4 0 0 0.1 B4; 49.94; 45.17; 13820; 17035; 24600; -4.18; P0257;
08:40:01;B5 0 0 0.1 B5; 49.92; 45.06; 13820; 17035; 24600; -4.15; P02bf;
08:41:01;B6 0 0 0.1 B6; 49.92; 45.01; 13806; 17035; 24593; -4.13; P0302;
08:42:01;B7 0 0 0.1 B7; 49.87; 44.99; 13820; 17035; 24593; -4.08; P0330;
08:43:01;C4 0 0 0.1 C4; 49.85; 45.01; 13813; 17028; 24593; -4.03; P0343;
08:44:01;C5 0 0 0.1 C5; 49.87; 45.03; 13820; 17028; 24593; -3.99; P035c;
08:45:01;C6 0 0 0.1 C6; 49.85; 45.03; 13813; 17028; 24593; -3.94; P0383;
08:46:01;C7 0 0 0.1 C7; 49.87; 45.10; 13813; 17028; 24600; -3.90; P0373;
08:47:01;D4 0 0 0.1 D4; 49.87; 45.10; 13813; 17035; 24600; -3.86; P039a;
08:48:01;D5 0 0 0.1 D5; 49.87; 45.15; 13813; 17035; 24600; -3.81; P0397;
08:49:01;D6 0 0 0.1 D6; 49.87; 45.19; 13813; 17021; 24600; -3.77; P0395;
08:50:01;D7 0 0 0.1 D7; 49.89; 45.21; 13820; 17035; 24593; -3.73; P03a7;
08:51:01;E4 0 0 0.1 E4; 49.92; 45.26; 13806; 17028; 24593; -3.71; P039e;
08:52:01;E5 0 0 0.1 E5; 49.92; 45.28; 13813; 17028; 24600; -3.68; P03a2;
08:53:01;E6 0 0 0.1 E6; 49.94; 45.31; 13813; 17035; 24593; -3.66; P03a7;
08:54:01;E7 0 0 0.1 E7; 49.96; 45.31; 13813; 17035; 24600; -3.65; P03b9;
08:55:01;F4 0 0 0.1 F4; 49.96; 45.37; 13813; 17035; 24587; -3.63; P0387;
08:56:01;F5 0 0 0.1 F5; 50.01; 45.35; 13813; 17035; 24607; -3.64; P03a7;
08:57:01;F6 0 0 0.1 F6; 50.01; 45.40; 13813; 17021; 24600; -3.64; P037b;
08:58:01;F7 0 0 0.1 F7; 50.05; 45.37; 13806; 17035; 24593; -3.66; P038e;
08:59:01;G4 0 0 0.1 G4; 50.03; 45.37; 13813; 17028; 24593; -3.67; P037e;
09:00:01;G5 0 0 0.1 G5; 50.05; 45.35; 13813; 17035; 24600; -3.68; P038a;
09:01:01;G6 0 0 0.1 G6; 50.07; 45.37; 13813; 17035; 24593; -3.71; P0366;
09:02:01;G7 0 0 0.1 G7; 50.07; 45.35; 13813; 17035; 24593; -3.73; P0364;
09:03:01;H4 0 0 0.1 H4; 50.10; 45.33; 13813; 17028; 24600; -3.76; P0362;
09:04:01;H5 0 0 0.1 H5; 50.10; 45.31; 13813; 17028; 24600; -3.80; P0359;
09:05:01;H6 0 0 0.1 H6; 50.07; 45.28; 13813; 17035; 24600; -3.82; P0350;
With best regards
Andreas