Three-channel high temperature alarm. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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It is no secret that the cause of a significant part of the malfunctions of consumer electronic equipment is the non-optimal thermal mode of operation of its components, leading to their accelerated degradation and subsequent failure. The proposed device allows you to control the temperature simultaneously at three points: at two - by exceeding the pre-set fixed values, and at the third - by the value set in advance or during the test. The device may be useful in the development or repair of devices such as switching power supplies, voltage stabilizers, power amplifiers, etc.

The device, which will be discussed, is designed to control the operating temperature of the components of the devices being adjusted or repaired during the period of their testing, but it can also be built into any device on a permanent basis. It differs from the design [1] by the presence of three temperature control channels instead of one. Two of them include an alarm when the temperature exceeds the pre-set fixed values, the third channel is adjustable, it can be quickly set to any temperature in the range of 5 ... 100 ^оC.

Rice. 1 (click to enlarge)

The scheme of the three-channel light-sound signaling device of elevated temperature offered to the attention of readers is shown in fig. 1. The device is based on the popular LM339N microcircuit, which consists of four independent comparators with an open collector output, capable of operating at a unipolar supply voltage from 2 to 36 V. As you can see, an exemplary voltage with divider R2.1R2.3, and non-inverting - voltage from dividers, one shoulder of which is formed by a thermistor (RK5-RK2), and the other by a trimmer (variable) resistor (R1, R3, R4) and a constant connected in series with it (R8, R11, R3) . While the temperature of the thermistor, for example, RK7, is less than the specified maximum, its resistance is relatively large, the voltage at the non-inverting input (pin 10) of the DA1 comparator is greater than at the inverting one (pin 7), its output transistor is closed and the output voltage (pin 2.1) has a high level, so the HL6 LED does not shine. As the temperature rises, the resistance of the thermistor decreases. As a result, the voltage at pin 1 of DA2 decreases, and as soon as it becomes less than the voltage at pin 7, the comparator switches (the high voltage level at pin 2.1 changes to low) and the HL6 LED starts to shine. Signaling channels on comparators DA1 and DA2 work similarly. Capacitors C2.2-C2.3 reduce the sensitivity of the device to interference and interference.

An audio frequency signal generator is assembled on the DA2.4 comparator, which turns on when any of the DA2.1-DA2.3 comparators is triggered (when the voltage level at its output becomes low). While none of them has worked, the transistor VT1 is open and blocks the operation of the generator, at its output at this time there is a high level voltage. When any of these comparators is triggered, the transistor VT1 closes and the generator on the comparator DA2.4 starts to work. The frequency of its oscillations depends mainly on the capacitance of the capacitor C11 and the resistance of the resistor R19. Resistor R1, connected in series with the sound emitter HA20, reduces the sound volume. Resistors R1, R6, R9, R12 limit the current through the LEDs.

The DA2 chip is powered by a stabilized voltage of 5 V from the stabilizer on the DA1 chip. The Schottky diode VD1 protects the DA1 chip when the supply voltage is reversed, and also allows you to power the device from an AC voltage source of 7 ... 15 V. The HL1 LED lights up when there is voltage at the output of the stabilizer. In standby mode, the device consumes a current of about 8 mA from the power supply, and about 25 mA when the light and sound alarm is turned on.

Fig. 2

Most parts of the signaling device are installed on a mounting plate with dimensions of 65x40 mm (Fig. 2), the mounting is hinged, the connections are made with thin multi-colored wires in PVC insulation. To prevent accidental short circuits and increase mechanical strength, the mounting on the connection side is covered with zaponlak.

Fixed resistors MLT, S2-33, tuning resistors R4, R8 and variable R11 are imported small-sized ones. To facilitate the precise setting of the alarm thresholds, you can use the so-called multi-turn tuning resistors, for example, SP3-39, SP5-2, SP5-14.

Thermistors RK1-RK3 are small-sized with negative TCR and resistance at room temperature of 10.100 kOhm. Thermistors of the right size and size are often found in dot matrix printer printheads and small stepper motors. To connect the thermistors to the signaling device board, thin shielded wires about 1000 mm long were used, the shielding braids are connected to a common wire. The last 50 mm on the side of the thermistors are made with a thin MGTF wire.

When using thermistors much larger than indicated in the diagram, trimmers and variable resistors should be used in proportion to the greater resistance. In the absence of thermistors, low-power small-sized germanium diodes or germanium transistors can be used as temperature sensors [2].

Capacitors C1, C3, C4, C7-C11 - small-sized ceramic, for example, K10-17, K10-50, the rest - oxide K50-68, K53-19, K53-30 or analogues. Schottky diode MBR0540T1 can be replaced by any of 1N5819, SB140, SB150, MBRS140T3, and diodes 1N4148 - by any of KD510A, KD521A-KD521D, KD522A, KD522B, 1N914, 1SS244.

Instead of the 2SC3199 transistor, you can use any of the 2SC815, 2SC1815, 2SC1845, SS9014, as well as the KT645, KT3102 series. Possible replacement for the LM339N chip - LM139, LM239, LM339, LM2901, MC3302, KIA339, BA10339 (for ease of installation, it is preferable to use a chip in a DIP14 package). The integrated voltage regulator KA78L05AZ can be replaced by any of the 78L05 series in the TO-92 package. With a supply voltage of more than 15 V, it is advisable to include an additional resistor with a power dissipation of 1 W in series with the VD0,5 diode, the resistance of which should be selected so that when the alarm is running, the voltage at the DA1 input does not go beyond 10.13 V.

LEDs RL30N-YG414S (green glow), RL30N-HY214S (yellow) and RL30N-DR314S (red) can be replaced by any similar ones without built-in resistors. It is possible to use flashing LEDs as HL2-HL4, for example, DFB3b-145, L-36BSRD/B, L-36BYD. Possible replacement of the electromagnetic sound emitter DBX-12PN (winding resistance - about 133 Ohm) - dynamic SD-150 (120 Ohm). In order not to overload the output stage of the comparator, the total resistance of the sound emitter and resistor R20 must be at least 150 ohms. A sound emitter with a winding of much lower resistance or a small-sized dynamic head is connected either through an output transformer from a pocket radio receiver, or by changing the circuit of the device, as shown in Fig. 3.

Fig. 3

All parts of the signaling device are placed in a plastic case measuring 92x48x17 mm from a pencil sharpener (Fig. 4). For the convenience of using the adjustable channel, a control knob with a dial is fixed on the roller of the variable resistor R11, on which a scale with marks from 0 to 100 ° C is applied. It is convenient to use a digital multimeter with an external thermocouple to set the device response thresholds. She and the temperature sensors of the device are tied together with a thin copper wire, placed in a waterproof plastic bag and lowered into some compact closed container filled with water. Heating it to the desired (according to the readings of the multimeter) temperature, using trimmer resistors R4, R8 or variable R11 (depending on the channel being calibrated), they ensure that at this temperature the sound signal turns on and the corresponding LED starts to shine.

Fig. 4

In the author's version of the device, unregulated channels are set to a threshold of 65 with the help of tuning resistors. ^оC. This temperature is generally considered optimal when controlling the heating of power transformers, power transistors, and microcircuits mounted on heat sinks. The adjustable channel can be used, for example, to control the temperature in the device case.

Literature

1. Butov A. High temperature indicator on KIA6966S. - Radio, 2010, No. 6, p. 27, 28.
2. Butov A. Indicator of heat sink overheating. - Radio, 2002, No. 5, p. 53.

Author: A. Butov

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