Electronic Thermostat

Posted on Mar 2, 2013

Here is a circuit that implements phase-control of heaters, adjusting their power smoothly from zero to around 95% of the rated power, keeping the room temperature to within 1 or 2 degrees, while using few components. Notice that this is entirely an AC design. The TRIAC, DIAC, capacitor, resistor, potentiometer and thermistor are all AC devices. I did not have a suitable MOV at hand, so I used two Zener diodes in back-to-back fashion. The poorest aspect is that I don't know of the existence of any AC transistors, so I had to pair two complementary ones, joining the collectors through diodes, thus creating my AC transistor from 4 parts!

Click here to download the full size of the above Circuit.

It works well, but if the transistors are not well matched, this will result in the TRIAC firing earlier in one half cycle than in the other, superposing a DC component on the AC line, which in certain (few) cases can lead to trouble. Try to select nicely matched complimentary transistors! Let's suppose that a positive half-cycle starts. As the voltage rises, the 22k resistor will feed the circuit. The upper Zener diode will clamp the voltage at 42V, with the lower one just conducting. The 100k pot is adjusted in such a way that the base-emitter voltage is just enough to make the PNP transistor conduct. The capacitor will slowly charge up, reaching 32V sometime within the half cycle. The DIAC will then fire, dumping the capacitor's charge into the TRIAC, which will also fire, and conduct for the rest of the half cycle, keeping the control circuit powered down. For the negative half cycle, everything will be the same, except that the Zeners reverse roles, and the NPN transistor conducts. The unused transistor in any case stays off, with 0.6V inverse voltage across the base-emitter junction, and its collector insulated by the diode. If the room temperature rises just a little, the resistance of the NTC will drop a little bit. The small reduction in base-emitter voltage will make the transistors conduct dramatically less current, so that the TRIAC will fire much later in the cycle, strongly reducing the heating power....

Visit page

Leave Comment

characters left:

New Circuits

Zero consumption Anti-theft High power Code Switch for E-Bike Batteries

This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
Ultrasonic distance finder circuit

The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Simple Metal detector circuit with CD4030

Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Fully automatic watering circuit for flower pots

Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
Simple deep searching metal detector circuit

The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
Sub-Harmonic Bass Converter for Electric Guitars

The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
Echo Depth Sounding Sonar for Boats

In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
100W HiFi Audio Amplifier Circuit

This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
Plant-Pot Water Level Indicator Circuit

A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....

Create NOW your QR MENU for your Restaurant / Hotel / Bar / Caffe,

Comprehensive ordering/paying features. fully personalized to suit your Business.

Update Anytime! Easy and Simple!

Popular Circuits

Mains Frequency Monitor circuit and explanation

Here is a simple frequency counter designed to monitor the 240VAC mains supply. It as a frequency...

Baxandall Tone Control Circuits Using Two Transistor

This is design circuit of tone control circuit that is use very popular Baxandall configuration,...

Motion sensing LED Quantum Bits

The circuit works like this: Normally the LED is powered dimly through R1 and R3. SW1 is the...

Switch Timer For Bathroom Light

This 9-minute timer switch can be used to control the light in a toilet or bathroom. The timer is...

Geiger Counter

The basic charging circuit produces in excess of 500V+, depending on the inductor used, leakage...

Autodyne Circuit

An Autodyne circuit combines three functions normally found in different stages in an RF Receiver...

Niftymitter electronic schematics and parts list

Use the program to fit your needs at the woven tules of his roses in a single adobe 3 mac card....

Projects

This project was inspired by a sense of wanting to build something a little unusual rather than...

BA313 with ALC circuit diagram of sound recording

BA313 is a built-in automatic level .png">control circuit (ALC) of audio recording...