PTC Thermistor humidity switch.

VT91

Joined Mar 9, 2020
10
Hello.
I want to build a simple circuit that would control a humidifier in a terrarium.

I have 39 ohm 10 Watt resistors.

I have two kinds of PTC thermistors: "10P" and "16P" — I don't know their characteristics curve.
I was not able to find a good datasheet with the graph for "10P" and "16P" thermistors.

The big one, the "16P" can control a 12 volt, 40 ohm light bulb that is wired in series.

At 20 degrees C, the wet thermistor conducts enough current for a light bulb to burn bright, after two minutes, the thermistor gets hot and the light goes out.

I also have 5.1V, 5 watt 1n5338B zener diodes, 20pcs.

To control the load, I bought a 5-60 VDC Solid State Relay with a 3-32 VDC input.

Ideally, I would like this circuit to be sharp enough to use regular relays without causing contacts to buzz, as the voltage nears the low threshold.
Can zener diodes be used to solve this problem?
How did they solve this problem in the analog days?

What circuit should I use to control the humidifier load,
so when the water starts precipitating down and accumulating in the lowest spot in the system,
the humidifier circuit would be opened by the relay?

If I were to design a circuit as in this picture:

How do I calculate the values of R1 and R2 to get the sharpest corners on the graph on the oscilloscope?
The half-wave AC generator symbolizes the changing resistance of the thermistor.
Can I combine the thermistor with R1 or R2?

Thank you.

AnalogKid

Joined Aug 1, 2013
8,608
The resistance of a thermistor - any type of thermistor - does not depend on the relative or absolute humidity of the ambient air surrounding it. Its resistance depends only on temperature.

The circuit is nothing more than a simple signal clipper. I don't see how it relates to a control circuit for a humidifier.

The most simple way to control a humidifier is to buy a humidistat. Like a thermostat does with a heater, it turns on and off a humidifier based on the relative humidity of the air surrounding it. HVAC companies and distributors sell them for residential and commercial HVAC systems. Industrial distributors sell them for other less specialized applications.

ak

VT91

Joined Mar 9, 2020
10
I forgot to mention that I need to maintain an economical overcast condition.
Overcast drizzle, yet with the minimal water consumption.

A simple humidistat wouldn't do that, as it is not designed to maintain a perfect saturation of air with water.

In my system. the water accumulates at the lowest spot and cools the thermistor by evaporation.
There is a shallow pan, in which the water accumulates and cools the thermistor.
Water will eventually evaporate, exposing a thermistor, as if it was in the air.
This is the idea of how I want to maintain a steady overcast condition without running the humidifier indefinitely and consuming water, electrical power and humidifier life expectancy.

AnalogKid

Joined Aug 1, 2013
8,608
So in an indirect way, it is thermistor temperature that is activating the humidifier. You will get much more consistent (and adjustable) performance with a control circuit, such as a comparator, between the thermistor and the device. What are the voltage and current requirements for the humidifier, and what voltages are available to power the control circuit?

Also, what is your skill set for assembling a small circuit? If you are more of a software person, a small Arduino can replace the comparator circuit.

ak

VT91

Joined Mar 9, 2020
10
Thank you.

Everything runs on 12 volts.
The project uses a computer power supply, so the entire system uses 12v, 5v and can use 3.3v.

I am not good with writing code for Arduino.
I tried that approach many years ago to abandon it for good.
I can solder through-hole circuits.
I know that I can buy a humidistat module on Aliexpress, yet it won't exactly be what I want.

This is an 'overcast switch'.

A hot thermistor dries itself in a humid and corrosive environment.
This is why I like this design.

One of the circuits that I tried, used two thermistors.
One stayed out of the water and was used for comparison in a bridge circuit.

I would like to keep the circuit as simple as I can, while having the sharpest off-on "edge" that I can get with the least amount of simple components.

The goal is to have output that is not linear, yet sharply drops, off when a thermistor is submerged.

AnalogKid

Joined Aug 1, 2013
8,608
Quick grab off the innergoogle to show the concept:

Any of R1, R2, Rs can be made variable to adjust the trip point(s). And, the thermistor can be moved around for different output logic polarities.

Do you know the resistance at the points where you want the humidifier to turn on and off? Without more information, `you might be better off using something known and documented from Digi-Key.

Also, where did the circuit in post #1 come from?

ak

Last edited:

DickCappels

Joined Aug 21, 2008
6,593
@AnalogKid, am I missing something? That looks like a circuit for switching at a particular temperature. How does the circuit know about humidity?

AnalogKid

Joined Aug 1, 2013
8,608
You missed something. He says condensed excess humidity forms a puddle around the thermistor and cools it. I think the idea is that the thermistor (his part is a PTC overcurrent protector) self-heats and the puddle removes heat. I have doubts.

I threw up the schematic to give an idea of the complexity of a more traditional approach, not as a final solution.

ak

Last edited:

VT91

Joined Mar 9, 2020
10
I have a video of this simple experiment somewhere. Let me look....

Alec_t

Joined Sep 17, 2013
11,661
One of the circuits that I tried, used two thermistors.
One stayed out of the water and was used for comparison in a bridge circuit.
That's using the principle of the 'wet bulb hygrometer'. You might need some fan assistance for the evaporation.

DickCappels

Joined Aug 21, 2008
6,593
@Analog kid
I imagined something like that, though I do not see how a puddle would form. I accept the comparison of wet bulb compared to dry bulb. Thank goodness for freshman physics

Alec_t

Joined Sep 17, 2013
11,661

DickCappels

Joined Aug 21, 2008
6,593
Yeah, if you humidity reading is 100%....

jpanhalt

Joined Jan 18, 2008
10,269
Sorry to join late, but since the desire is effectively 100% relative humidity at whatever the current temperature is, why do you need to measure the temperature? Put another way, temperature of boiling water can be used as a standard for calibration thermometers, but not the other way around.

So, I would focus on building a humidifier or maybe ultrasonic nebulizer. That could be in a side chamber and exhaust to the main chamber with a low velocity fan. Drains in the main chamber would return condensed water to the humidifier/nebulizer chamber.

That is simply a modification of how we kept our microbiological incubators at a specified range. A baking dish of water did the trick. No control at all, but we monitored the relative humidity as usual. It is quite easy to maintain a steady state. No complicated apparatus to break or service either.

VT91

Joined Mar 9, 2020
10
I would not return the water back into the humidifier, as it is not clean and can contaminate the environment.
Yet the idea is to simply cool the thermistor by the puddle that accumulates at the bottom of the growing container so evaporation cools it and this switches off the humidifier.

So I want to build a simple circuit that would open the relay contacts when more current flows through the cold thermistor.
Close relay contacts once the thermistor is hot and less current flows through, to power the humidifier again.

Zener diode was introduced to keep relay contacts from buzzing, to make the circuit more discrete. This is really what I would like to know: how to make relay contacts not buzz, make this circuit output two sharp, discrete states, without using complicated comparator ICs and other components that would make this system more complicated to design/less reliable.

Thank you.

Alec_t

Joined Sep 17, 2013
11,661
without using complicated comparator ICs
They aren't complicated, and their use would arguably be simpler than replacing them with discrete components. One comparator should do the job, with hysteresis to prevent relay buzz.

AnalogKid

Joined Aug 1, 2013
8,608
Depending on the circuit configuration, the equations for a comparator with hysteresis (post #5) can appear complex. In fact they are pretty straightforward. Putting the thermistor in the right place among the input bridge components helps. Also, using an opamp instead of an open-collector-output comparator IC reduces the complexity. An LM358 will handle this job well; the second opamp can act as an output buffer to isolate whatever the circuit is driving from the hysteresis equations.

ak