Simple Analog temperature Controller for an incubator with Capacitor Power Supply

Published by MKDas on

For an incubator, a temperature controller is the main control unit. There are many temperature controllers in the market. But here is a simple one, I think the most simple one that you can make yourself. The circuit required no external power supply as it uses a simple capacitor power supply to run directly from AC. So let’s start our Simple Analog temperature Controller.

Disclaimer: Electricity is always dangerous. Skill required to work with electricity. Do work at your own risk. The author will not be responsible for any misuse or harmful act. This website contents are also copyright protected. Anything copied and directly posted on your website and claiming it as yours is prohibited and nonsensical. The author published all the articles as open source to help you to make your project and learning purpose only. Learn and make one for yourself. If need any help feel free to ask the author. The author will be helpful to you. Thanks.

Warning!: The circuit uses direct AC. Connect the phase and neutral as like the circuit. Also, the circuit may carry live voltage. Do work on your own responsibility. Always check the phase using a tester before touching any part directly.

The basic principle of a temperature controller?

A temperature controller can be of different types based on its control mechanism. But the basic is very simple. Sense the temperature, compare it with a reference value. Then turn the heater or cooler on/off based on that reference value. A simple comparator can work for this purpose.

Now, what are some advanced features a temperature controller may have?

  1. PID control
  2. Hysteresis control
  3. Fuzzy Logic control

Most of the available incubator controllers use either Fuzzy or PID to maintain the temperature and keeping a good correction speed. And simple ones use hysteresis control.

Here are some popular ones from different sources.

These are multi-functional. Also a little complex to make and easy to use. But here we are going to make one for ourselves. Let’s see what we can make.

You may find this helpful: Transformerless power supply design guide in detail with calculator

Circuit diagram:

Circuit diagram for our Simple Analog temperature Controller:

Analog Temp Controller with Capacitor power supply
Temperature controller circuit

Here, the red wire is the phase and the black wire is neutral. The top block of the circuit is a capacitor power supply. There is an article on this in my blog. You can read that too. This one is one of that kind with little modification as we need a common ground.

The C1 is the main capacitor here for this power supply. It should be good quality, better using X2 type but you can use as I used in my circuit. I got these capacitors from a Japanese company and bought thousands for my different products. Capacitor feedback is very good and satisfactory.

For BR1, you can use 1N4007 X 4. And rest of the components need no explanation. Just use a good Zener diode of 1W or larger. The power supply circuit supplies sufficient current for our circuit. The R1 (47R/2W) becomes hot so it’s better to place it keeping it a little high from the PCB surface.

The temperature control circuit is using a common Op-Amp LM358 in comparator mode. Resistor R6 (220K) is used for hysteresis. This is important. Otherwise, the relay will be turned on & off at the same point which will kill the relay as well as the loads. So a hysteresis is important. The thermistor used here is an NTC-based 10K thermistor. The rest of the part of the circuit needs no explanation I think.

Here keep in mind that, as this is a very simple circuit, there are no extra features added. A single circuit can be used for both the heater and the cooler. But for better heat regulation, I suggest using two circuits. One for heating purposes, the other for cooling purposes. By setting the temperature range little wide tuning the POT RV1, good heat regulation is possible.


I made this circuit in PCB. Here is the design:

Analog Temp Controller with Capacitor power supply PCB

You can follow this design or you can buy the Gerber file for $1.99 only from here.

Analog Temp Controller with Capacitor power supply
Transformer-less version circuit

And the transformer version:

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Test Result:

Circuit with 2 Relays:

Analog Temp Controller with Capacitor power supply

You can use two relays and set them for the cooler and heater individually. Keep a good hysteresis gap between the heater and cooler. For example, for incubator use, you can set the cooler on at 38.2’C and off at 37.5’C then set the heater at 36.5’C and off at 37’C. So a gap is kept between the two switches. Which will actually help in heat management.

Note: It totally depends on experience and expertise. But why you don’t try a bit?


The temperature controller circuit was very simple and I’ve demonstrated who it works. You can use this circuit on your own now. But don’t forget to give credit. I hope this article will help you. Thanks

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I'm Mithun K. Das; B.Sc. in EEE from KUET, Bangladesh. Blog: "First, electronics was my passion, then it was my education, and finally, electronics is now my profession." I run my own electronics lab, M's Lab ( Where I work with the creation of new products from ideas to something in real life. Besides this is my personal blog where I write for hobbyists and newcomers in the electronics arena. I also have a YouTube channel where I publish other helpful videos, you can find the link inside the articles. I always try to keep it simple so that it becomes easy to understand. I hope these will help them to learn electronics and apply the knowledge in their real life.


Joy · June 19, 2021 at 11:04 am

Good Job.

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