Boy, talk about over-engineering. You would think the OP asked how to design an interstellar spacecraft from the responses here.
1) get a temperature sensor.
2) connect it to a comparator with hysteresis.
3) connect the output of the comparator to a relay
4)done
Hi,
Why vague? It's just mathematics. All input values should be known or (in doubt) measurable.
Klaus
Hi,
Believe it or not, in the past 20 years of electronics developmemt I did not use a pot this way, I calculated the part values.
It is faster and cheaper.
If it does not work with the calculated values, then you know you did something wrong.
Klaus
Hi,
Please do some calculations in your own.
What are the LM35 temperatures at your expected switching points?
Klaus
below 37.7 °C the relay should be switched ON (the relay turns a heater ON or OFF), when temperature reaches to 37.7 °C the relay goes to OFF-state; when drops to 37 °C, the relay should be ON again...
the incubator's temperature for incubation should be 37.7C. the shown relay in the circuit will switch a heater ON or OFF. we assume the incubator's temperature=T.This is not a good specification. You must provide some interval. For example, the heater on if the temp <37.7C and heater off if the temp is >37.8C.
If you reduce the interval to zero, the relay will chatter incessantly. Just think this like logic levels: it is zero (say) if the voltage is <3V and it is one if the voltage is >5. The undefined range between these two limits constitute hysteresis.
You have given two values when the heater should turn on: below 37.7C and also 37C; this is inconsistent and cannot be implemented.
Hi,
I kindly ask for some effort on your own and for "stable" requirements, not 1°C ... 0.7°C ... 0.5°C hysteresis change with every post.
Klaus
Dr Mo said "I provided a box which the heating source (40W light bulb), a little DC motor for rotating eggs and a thermostat embedded in.
the temperature in the box should be around 37°C and the accuracy is not too important. I want when the temperature reaches to 37°C the light bulb be turned OFF and once the temperature goes below 37°C the light bulb be turned ON."
The critical question is how variation is there between the egg surface and air temperature surrounding it and the where the temperature is sensed? Given that this heat is a combination of radiant and convection, the temperature difference will most likely exceed your "not too important" spec. Therefore more distributed bulbs with same total heat and/or a fan to reduce temperature swing is necessary. A solid state switch is necessary for bulb unless you use an inrush current limiter (ICL) NTC component for reliability. Then you can use 0 hysteresis.
yes you're right. but after doing some researches & simulations...
Good.
Now you also need to provide some "margins" for the settings. As you have given an exact value for the set temp (37.7C) can I assume that it can be 37.7+/- 0.1C?
So the relay gets off at 37.7+/- 0.1C and it again turns on 37.2+/-0.1C, right?
I assume that the environment is cooler than 37C so that only need an heater to keep the temp.
I see that you are using an LM35 as the temp sensor. It is good, cheap and simple. But it does not provide 0.1C accuracy. According to the datasheet, it is around 0.5C.
It just means that if you set the temp at 37.7C, it can be anything from 37.2 to 38.2C (with 99% confidence).
I see that you are using a LM358 as the comparator. It is a good and classic way you have made this circuit.
Now I do not see how you are setting the temp (obviously by RV1 but how do you ensure that the temp is the correct setting?).
I do not see (or understand) how you are controlling the hysteresis?
The temperature differences observed with an on-off or "bang-bang" controller due to process transfer function are not hysteresis, but they are unavoidable, see discussion in post #7. The OP seems to accept these effects by requiring a controller with 0.7 or 1K hysteresis. I believe it's more instructive to realize the concept than to continue endless discussions about the dos and don'ts.Depending on the physical setup an location of temp sensor vis-a-vis heating element, there will actually be a substantial hysteresis already in-built.
Please refer to the diagram in post 49.
As per the datasheet, the output of the sensor is 10mV /C; hence to get an accuracy of 0.1C, the reference voltage must be stable to 1mV. The datasheet also says that the sensor is accurate to +/-0.5C. So the reference stability is important.
As you are using a simple voltage divider for the reference voltage, the power supply voltage must be very stable; 5V supply must have very little noise (less than 10-20mV). Better to use a zener and use that for reference.
As the relay is driven from the same supply, the noise in the power supply cannot be fully eliminated. You need to use capacitors to filter the noise.
My suggestion: use a regulator to get the 5V to the circuit; power the relay from the unregulated but filtered supply.
Put the reference voltage to the non-inverting input. Calculate the voltage divider to get 370mV and use a trimmer (low noise) to calibrate (+/- 2C).
To introduce some hysteresis, use another voltage divider to connect the output to the non-inverting input so that when the output is high (say 4V) the reference voltage is lifted by 10mV.
Use a small capacitor to filter noise and prevent the relay from chattering.
for power supply, I used full-wave rectifier with 470uf capacitor which a 7805 Regulator fixes the output voltage to 5V. But where the Zener should be connected? what are it's specifications? (voltage and type and ...).
do you mean I connect the output of LM35 to non-inverting input of LM358? and where did 370mv come from?
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