System turns on alarm once temp. dips below 10.5°C

[adamaero]

Homework Statement

Homework Equations

(Above given. I think it's a hint.)

The Attempt at a Solution

How is delta T = 0.25??
Where is the 10kΩ coming from?

We were given a solution for this homework, since it's not collected. It just seems out of order.

 

[mfb]

How is delta T = 0.25??

This looks like an arbitrary choice to keep the heating of the thermistor small compared to the monitored temperature changes.

 

[rude man]

This requires detailed circuit analysis including the hysteresis requirement (on at 10.5C on the way up., off at 8C on the way down). Thefone parameter you can ignore is the 5mW/C "dissipation constant". Unfortunately the presented solution seem to have taken off with it ... I find the given solution totally irrelevant and incomprehensible.

 

[Tom.G]

Actually that 5mW/°C and ΔT=0.25C is important in practice. The problem statement gives an overall 0.5°C tolerance so the usual approach is to limit the thermistor self heating to half (or less) of the allowed error band, leaving room for tolerance buildup of the other components. "I", the maximum thermistor current, is correctly calculated as 0.354mA

The 10k is defined in the problem statement as the thermistor resistance at 10°C.

I haven't evaluated the rest of the circuit in detail but it doesn't look right. For instance the equivalent resistance the thermistor sees should be ≥4.2kΩ to limit the thermistor current, but the circuit shows 4.2k and 9k in parallel.

The hysteresis still needs calculating.

 

[rude man]

OK, but that number is not characteristic of the thermistor itself. It depends on the degree of heat-sinking to the measured ambient. So if the given number is somehow determined in situ then I guess that number can be incorporated into the calculations.

 

[mfb]

but the circuit shows 4.2k and 9k in parallel.

They are not in parallel. There shouldn't be a relevant current through the 9 kOhm resistor around the nominal temperature. The negative side of the comparator is held at the same voltage we have between thermistor and 4.2 kOhm resistor at this temperature.

 

[Tom.G]

They are not in parallel. There shouldn't be a relevant current through the 9 kOhm resistor around the nominal temperature. The negative side of the comparator is held at the same voltage we have between thermistor and 4.2 kOhm resistor at this temperature.

You're right. Thanks. I had stopped my evaluation before getting there.