Categories: Practical Electronics, Electrician Secrets
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DIY do-it-yourself thermostat

Unusual use of the adjustable zener diode TL431. Simple temperature controller. Description and scheme

Anyone who has ever been involved in repairs of modern computer power supplies or various chargers - for cell phones, for charging “finger” AAA and AA batteries, a small detail is well known TL431. This is the so-called adjustable zener diode (domestic analogue of KR142EN19A). Here it truly can be said: "Small spool, yes dear."

The logic of the Zener diode is as follows: when the voltage on the control electrode exceeds 2.5 V (set by the internal reference voltage), the Zener diode, which is essentially a microcircuit, is open.

In this state, current flows through it and the load. If this voltage becomes slightly less than the specified threshold, the zener diode closes and disconnects the load.

When such a zener diode is used in power sources, the emitting LED of the optocoupler controlling the power transistor is most often used as a load.

This is in cases where galvanic isolation of the primary and secondary circuits is necessary. If such isolation is not required, then the zener diode can directly control the power transistor.

The output power of the zener diode microcircuit is such that, with its help, it is possible to control a low-power relay. This is what allowed it to be used in the construction of a temperature regulator.

In the proposed design, the zener diode is used as a comparator. At the same time, it has only one input: a second input is not required for supplying the reference voltage, since it is generated inside this microcircuit.

This solution allows you to simplify the design and reduce the number of parts. Now, as in the description of any design, a few words should be said about the details and actually about the principle of operation of this thermostat.

Simple tremoregulator circuit

The voltage at the control electrode 1 is set using the divider R1, R2 and R4. As R4 is used thermistor with negative TCR, therefore, when heated, its resistance decreases. When pin 1 voltage above 2.5V chip is open, the relay is on.

Relay contacts include triac D2, which includes the load. With increasing temperature, the resistance of the thermistor drops, due to which the voltage at terminal 1 becomes lower than 2.5V - the relay is turned off, the load is turned off.

Using a variable resistor R1, the temperature of the thermostat is set.

The temperature sensor should be located in the temperature measuring area: if it is, for example, electric boiler, then the sensor must be fixed to the pipe leaving the boiler.

The inclusion of a triac using a relay provides galvanic isolation of the thermistor from the network.

Thermistor type KMT, MMT, CT1. As a relay, it is possible to use RES-55A with a winding of 10 ... 12V. The KU208G triac allows you to turn on the load up to 1.5 kW. If the load is not more than 200W, the triac can operate without the use of a radiator.

Boris Aladyshkin

See also at e.imadeself.com:

Indicators and signaling devices on an adjustable zener diode TL431

Electronic thermostat for oil cooler

How to protect against voltage fluctuations

The easiest twilight switch (photo relay)

Thermostat for electric boiler

Comments:

# 1 wrote: | [quote]

You have a beautiful site, everything is written in a clear language and the schemes are very simple (let's hope that they are reliable), which is especially pleasing.

Regarding this circuit, the question is: diode D1 indicated on the circuit is not indicated in the circuit description. Which one should I take?

I was able to find on the old power supply a microchip in the same case as TL431 but az 431 is marked. Somewhere in the net I read that it is one and the same. Is it so?

Why does the relay turn on the triac? Is it possible to simply connect 220v to the relay if the load is permissible within 200 watts?

Comments:

# 2 wrote: andy78 | [quote]

Diode - any with a reverse voltage of at least 30 volts.

AZ431 - the same adjustable zener diode, only from another manufacturer, analogue of TL431.

RES55 - reed relay. The switched power is very small - 7.5 watts (there is another 15 watts). Without a triac, it won’t work. The triac in the circuit performs the role of a switching element, a key that commutes the load circuit. The maximum load of 200 W in the article is mentioned in the sense that below this power you can use a triac without a radiator, but the presence of a triac is mandatory.

Comments:

# 3 wrote: | [quote]

The scheme is too small, the values are not visible. Please make it larger.

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# 4 wrote: andy78 | [quote]

Here is a link to the temperature regulator circuit on the adjustable zener diode TL431 on a larger scale: https://e.imadeself.com/en/termoregul.png

Comments:

# 5 wrote: | [quote]

I bought some Chinese relay. It is written 12VDC (it is on incl. Winding).

5A 250VAC changeover. At what switching power is the relay enough? Need about 200W

By the way, I’m trying to assemble a heater for an aquarium for fish from MLT-2 resistors (I read it somewhere on the forum) and old fumigators, but it’s easier than this regulator to be found anywhere, so you can add a useful article with heaters :)

Comments:

# 6 wrote: | [quote]

And what goes to the entrance? (1 circuit 12 volts) how to turn it on? consistently ??

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# 7 wrote: andy78 | [quote]

On the left - the control circuit of a reed relay 12 V. The adjustable zener diode TL431 is connected in series with the relay. On the right is the power part of the circuit. The relay trips the triac, and it controls the load.

Comments:

# 8 wrote: | [quote]

What is the oversight of this regulator? What temperature range does it regulate? For example, the error is + -0.5 degrees, the range is from -5 to +40 degrees

Comments:

# 9 wrote: | [quote]

Is it possible to increase the range, for example, to 70 degrees?

Comments:

# 10 wrote: | [quote]

Thanks for the circuit. This is the simplest scheme that I could only find.

The only thing I would like to simplify the circuit is to come up with how to control the triac directly, without a reed relay, as indicated on the circuit or opto-isolation. The only thing I do not know is if the TL431 has enough power to open the thyristor. For this, 50-100mA is needed. And you also need to come up with a simple power supply for the control circuit, for example, a voltage divider on resistors or capacitors, which reduces the voltage to 20 volts + diode bridge + Krenka, issuing 12 volts. (such as this "Power supply on the bank for 10 minutes"

The circuit is without digital elements, so I think the lack of isolation will not be very bad.

I think you can connect the control circuit directly to the triac like this: +12 volts to any power terminal of the triac, the output of the control circuit (terminal number 3 TL431) to the unlock terminal of the triac through a resistor.

Instead of a thermistor, I want to use the 1N4148 diode as a temperature sensor, because it is common and cheap. And he has a good range, I need from 100 to 300 degrees.

Comments:

# 11 wrote: Glory | [quote]

The control electrode in the triac that is included in 220? Also did not understand the purpose of the diode D1? And in my opinion it would be necessary to put some kind of limiting resistor for this adjustable zener diode, and not immediately supply power to it.

Comments:

# 12 wrote: | [quote]

Tell me the diagram of the thermostat for the garage cellar. It is necessary that when the temperature drops to +2, the heater is turned on. Thanks in advance.

Comments:

# 13 wrote: | [quote]

bounce relay on and off

Comments:

# 14 wrote: | [quote]

Did the circuit. It is necessary to apply stabilized power to the roll or 7812lm, and in parallel to the thermoresistor Conder 0.1 microns. If bounce continues, then increase.

Comments:

# 15 wrote: | [quote]

I have a thermistor MMT-4 1.5kOhm. Is it possible to use it in this scheme and how?

Comments:

# 16 wrote: Max | [quote]

Relay bounce can be eliminated by connecting a capacitor of 220 - 470 uF in parallel with the relay coil. 16 volts.

Comments:

# 17 wrote: | [quote]

The control circuit works, but there is one problem in the off state after 25-30 seconds, the triac starts to pass a voltage of 127 V. Does R3 open the triac? Why is a voltage of 127 V passed?
In the on state, everything is as it should i.e. 220 V.

Comments:

# 18 wrote: | [quote]

What is the temperature range of the regulator? Need up to 220 degrees. If the thermistor is 1kom, then what is the nominal value of R1 and R2 to reach 220 degrees? Perhaps there is a calculation formula? The power of the stove is 380 watts.

Comments:

# 19 wrote: Boris Aladyshkin | [quote]

AndrewPerhaps the whole problem is in the KU208G triac. 127V is obtained from the fact that the triac passes one of the half-cycles of the mains voltage. Try replacing it with an imported BTA16-600 (16A, 600V), they work more steadily. Buying a BTA16-600 now is not a problem, and it is not expensive.

sta9111, to answer this question you will have to remember how our thermostat works. Here, the paragraph from the article: “The voltage at the control electrode 1 is set using the divider R1, R2 and R4. As R4, a thermistor with a negative TCR is used, therefore, when heated, its resistance decreases. When the voltage higher than 2.5V at pin 1 is open, the relay is on. ”

In other words, at the desired temperature, in your case 220 degrees, on the R4 thermistor should be voltage drop 2.5V, we denote it as U_2.5V. The nominal value of your thermistor is 1Kohm - this is at a temperature of 25 degrees. This temperature is indicated in the directories.

Thermistor Reference msevm.com/data/trez/index.htm

Here you can see the operating temperature range and TKS: little is suitable for a temperature of 220 degrees.

The characteristic of semiconductor thermistors is non-linear, as shown in the figure.

Picture. The current-voltage characteristic of the thermistor is e.imadeself.com/vat.jpg

Unfortunately, the type of your thermistor is unknown, so we will assume that you have a MMT-4 thermistor.

According to the graph, it turns out that at 25 degrees the resistance of the thermistor is just 1KΩ. At a temperature of 150 degrees, the resistance drops to about 300 Ohms, more precisely, it is simply impossible to determine from this graph. We denote this resistance as R4_150.

Thus, it turns out that the current through the thermistor will be (Ohm's law) I = U_2.5V / R4_150 = 2.5 / 300 = 0.0083A = 8.3mA. It is at a temperature of 150 degrees, it seems, so far everything is clear, and there are no errors in the arguments, as if. Let's continue further.

With a 12V supply voltage, it turns out that the resistance of the circuit R1, R2 and R4 will be 12V / 8.3mA = 1.445KΩ or 1445Ω. Subtracting R4_150, it turns out that the sum of the resistances of the resistors R1 + R2 is 1445-300 = 1145Ohm, or 1.145KOhm. Thus, it is possible to apply a tuning resistor R1 1Kohm, and a limiting resistor R2 470ohm. Here is a calculation.

All this is good, only a few thermistors are designed to operate at temperatures up to 300 degrees. Most of all, thermistors CT1-18 and CT1-19 are suitable for this range. See the msevm.com/data/trez/index.htm manual

Thus, it turns out that this thermostat will not provide stabilization of temperature of 220 and above degrees, since it is designed for the use of semiconductor thermistors. You will have to look for a circuit with metal thermistors TCM or TSP.

Comments:

# 20 wrote: Sergei | [quote]

At 18 degrees, will this device turn on, or what needs to be changed so that it works from 18-26 degrees?

Comments:

# 21 wrote: | [quote]

Good evening. Assembled the circuit and the reference voltage of the stabilizer 1.9 in. Why could this be ??

Comments:

# 22 wrote: | [quote]

Vyacheslav,
check the integrity of the diode.

Comments:

# 23 wrote: | [quote]

Boris Aladyshkin,
So that the thyristor plowed in full force, i.e. on both half-cycles, it is necessary to turn on the diode in parallel in the thyristor circuit in the opposite direction, the current calculated for the load and thereby you compensate for the second half of the life losses and to turn this work on both half-periods to sleep, you can plug the diode in series ..... .......................

Andrew,

The network has two half-periods, respectively, one of them opens, and the second closes, the question - WHAT TO DO ...... ANSWER - again, the diode will save our lives, regarding the Anode and Control. put the diode in that direction so that the locking half-cycle works for you and not against you :)

Comments:

# 24 wrote: | [quote]

I assembled this scheme. R1 - 68k? R2 - 100 Ohms. Power contacts K1 shunted 1uF so that it sparkles less. Power supply through 12-volt krenk. Works fine. Hysteresis is provided by the properties of the relay itself. I don’t understand what problems some comrades are talking about here. As they said in our training: TEACH THE MATCH!