Keras
National Hazard
Posts: 772
Registered: 20-8-2018
Location: (48, 2)
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Tinkering with a heating mantle feedback loop
Folks,
I'm pretty unhappy with the way my heating mantle (model MS-E012, one of those OEM models you'll find branded under many different trademarks) works.
Now, I'm clearly pernickety: the thing works. Temperature control is a bit off, and I think the feedback loop overshoots too much. I shouldn't
complain: it is quite cheap and does the job.
So anyway, I decided to open the thing, removed the PCB and tried to figure out the schematics from the board. The temp sensor is a type-K
thermocouple. There's a dual op-amp in it: the first cell boosts the thermocouple voltage, which gets lower as the temperature rises (well, no, it
gets higher, but the - wire is connected to the op amp, not the + one). The second cell is a bit more of a mystery (I guess I have the schematics
wrong, I'll have to double check) but roughly, the negative voltage output by the first cell is offset at the + input by a positive voltage generated
by the temp control dial. When the sum of both voltages is positive, the element is turned on (which make the first voltage drop). As the sum nears
zero, there is some sort of oscillation (like a rough PWM control of the heating element). When the sum is clearly under zero, the heating element is
shut off. I'm not sure how the PWM control is achieved, there is probably some sort of Schmitt trigger magic at the - input, I'll have to investigate
further.
Never mind. The thing is controlled by two trimpots. The first one generates an offset voltage at the + wire of the thermocouple (probably sets the
temp curve 0°C voltage), and the other one is placed in series with the temp control dial (which itself is a 500 Ω log pot) – this one clearly
sets the dynamic range of the temp control dial.
My problem is with the first trimpot: Is there a standard offset at which a K-type thermocouple works best? Also, the values of the voltage I read at
the thermocouple connector seem very low (in the order of hundreds of µV at room temp).
Has anyone already tinkered with a heating mantle? Any suggestions? If you have the schematics, btw, I'd gladly take it.
Thanks,
V.
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