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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Universal LED indicators of current overload for power supplies. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network Exceeding the output current in power supplies indicates an increase in power consumption in the load device. Sometimes the consumed current in the load (due to a malfunction of the connections or the load device itself) can increase up to the value of the short circuit current (short circuit), which will inevitably lead to an accident (if the power source is not equipped with an overload protection unit). The consequences of an overload can be more significant and irreparable if you use a power source without a protection unit (as radio amateurs often do today by making simple sources and buying inexpensive adapters) - power consumption will increase, the mains transformer will fail, individual elements may ignite and an unpleasant smell. In order to notice in time the output of the power source in the "non-standard" mode, simple overload indicators are installed. Simple - because, as a rule, they contain only a few elements, inexpensive and available, and these indicators can be installed universally in almost any home-made or industrial power source. The simplest electronic circuit of the current overload indicator is shown in fig. 3.4. The operation of its elements is based on the fact that a low-resistance limiting resistor (R3 in the diagram) is connected in series with the load in the output circuit of the power source.
This unit can be used universally in power supplies and stabilizers with different output voltages (tested under conditions of output voltage of 5-20 V). However, the values and denominations of the elements indicated in the diagram in Fig. 3.4 are selected for a power supply with an output voltage of 12 V. Accordingly, in order to expand the range of power supplies for this design, in the output stage of which the proposed display unit will work effectively, it will be necessary to change the parameters of the elements R1-R3, VD1, VD2. As long as there is no overload, the power supply and the load node operate normally, the allowable current flows through R3 and the voltage drop across the resistor is small (less than 1 V). Also small in this case is the voltage drop across the diodes VD1, VD2, while the HL1 LED barely glows. With an increase in current consumption in the load device or a short circuit between points A and B, the current in the circuit increases, the voltage drop across the resistor R3 can reach its maximum value (the output voltage of the power supply), as a result of which the HL1 LED will light up (blink) at full strength. For a visual effect, a blinking L36B LED is used in the circuit. Instead of the indicated LED, devices similar in electrical characteristics can be used, for example, L56B, L456B (increased brightness), L8I6BRC-B, L769BGR, TLBR5410 or the like. The power dissipated by the resistor R3 (at a short circuit current) is more than 5 W, therefore this resistor is made independently from copper wire of the PEL-1 (PEL-2) type with a diameter of 0,8 mm. It is taken from an unnecessary transformer. 8 turns of this wire are wound on the frame of a stationery pencil, its ends are tinned, then the frame is removed. Wirewound resistor R3 is ready. About the details. All fixed resistors type MLT-0,25 or similar. Instead of diodes VD1, VD2, you can install KD503, KD509, KD521 with any letter index. These diodes protect the LED in overload mode (extinguish excessive voltage). Unfortunately, in practice there is no way to constantly visually monitor the status of the indicator LED in the power source, so it is reasonable to supplement the circuit with an electronic sound accompaniment unit. Such a scheme is shown in Fig. 3.5. As can be seen from the diagram, it works on the same principle, but unlike the previous one, this device is more sensitive and the nature of its operation is due to the opening of the transistor VT1, when a potential of more than 0,3 V is established in its base. A current amplifier is implemented on the transistor VT1. The transistor is selected germanium. From an old ham radio stock. It can be replaced by devices similar in electrical characteristics: MP16, MP39-MP42 with any letter index. As a last resort. you can install a silicon transistor KT361 or KT3107 with any letter index, but then the threshold for switching on the indication will be different.
The turn-on threshold of the transistor VT1 depends on the resistance of the resistors R1 and R2, and in this circuit, at a power supply voltage of 12,5 V, the indication will turn on when the load current exceeds 400 mA. A flashing LED and a capsule with a built-in AF generator HA1 are included in the collector circuit of the transistor. When the voltage drop across the resistor R1 reaches 0,5 ... 0,6 V, the transistor VT1 opens, the supply voltage is supplied to the HL1 LED and the HA1 capsule. Since the LED capsule is an active current limiting element, the LED behavior is normal. Due to the use of a flashing LED, the capsule will also sound intermittently - the sound will be heard during the pause between LED flashes. In this circuit, an even more interesting sound effect can be achieved if, instead of the HA1 capsule, the KPI-4332-12 device, which has a built-in interrupted oscillator, is turned on. Thus, the sound in the event of an overload will resemble a siren (this is facilitated by the combination of LED flash interrupts and internal interrupts of the HA1 capsule). Such a sound is quite loud (you can hear it in the next room at an average noise level), it will definitely attract the attention of people. Another diagram of overload indicators is shown in Fig. 3.6. In those designs where a fusible (or other, for example, self-resetting) fuse is installed, it is often necessary to visually monitor their operation. A simple development, the scheme of which is shown in Fig. 3.6 allows you to do this. A two-color LED with a common cathode and, accordingly, three leads is used here. Those who have tested these diodes with one common terminal in practice know that they function a little differently than expected.
The thinking pattern is that it would seem that green and red colors will appear at the LED in the common package, respectively, when voltage is applied (in the correct polarity) to the corresponding terminals R or G. However, this is not entirely true. While the fuse FU1 is working, voltage is applied to both anodes of the HL1 LED. The glow threshold is adjusted by the resistance of the resistor R1. If the fuse breaks the load power supply circuit, then the green LED goes out, and the red one remains on (if the supply voltage has not disappeared at all). Since the allowable reverse voltage for LEDs is small and limited, for the indicated design, they are introduced into the circuit. Diodes with different electrical characteristics VD1-VD4. The fact that only one diode is connected in series to the green LED, and three to the red one, is explained by the peculiarities of the LED. ALS331A seen in practice. During the experiments, it turned out that the voltage threshold for turning on the red LED is less than that of the green one. To balance this difference (noticeable only in practice), the number of diodes is not the same. When the fuse blows, voltage is applied to the green LED (G) in reverse polarity. The values of the elements in the circuit are given to control the voltage in the 12 V circuit. Instead of the ALS331A LED, it is permissible to use other similar devices, for example, KIPD18V-M, L239EGW. Author: Kashkarov A.P.
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