Electronic fuses for the power supply. Electronic fuse for alternating current. Current protection comparator INA300
This device is designed to protect DC circuits from overcurrent and load circuit short circuits. It is connected between the power source and the load.
The fuse is made in the form of a two-terminal network and can work in conjunction with a power supply with an adjustable output voltage within 3...35 V. The maximum total voltage drop across the fuse does not exceed 1.9 V at maximum load current. The tripping current of the protective device can be continuously adjusted from 0.1 to 1.5 A, regardless of the load voltage. The electronic fuse has good thermal stability and speed (3...5 μs), and is reliable in operation.
The electrical circuit diagram of the electronic fuse is shown in Fig. 1. In operating mode, thyristor VS1 is closed, and the electronic switch on transistors VT1, VT2 is open by the current flowing through resistor R1 to the base of transistor VT1. In this case, the load current flows through the electronic key, a set of resistors R3-R6, variable resistor R8 and the contacts of the SB1 button.
During an overload, the voltage drop across the circuit of resistors R3-R6, R8 reaches a value sufficient to open the SCR VS1 along the control electrode circuit. The opened SCR closes the base circuit of transistor VT1, which leads to the closing of the electronic key. The current in the load circuit decreases sharply; an insignificant residual current remains, equal to Iost=Upit/R1. At Upit=9 V Iost=12 mA, and at 35 V - 47 mA.
In order to restore the operating mode after eliminating the cause of the overload, you need to briefly press the SB1 button and release. In this case, the SCR will close, and transistors VT1 and VT2 will open again.
The residual current can be reduced by increasing the resistance of resistor R1 by 1.5...2.5 times and using transistors VT1 and VT2 with a large static current transfer coefficient. However, an excessive increase in the resistance of resistor R1 leads to an increase in the voltage drop across transistor VT2, i.e., an increase in the voltage drop across the fuse in operating mode.
The residual current can be significantly reduced (up to 2...4 mA) at any supply voltage by using a current source on a field-effect transistor KP303A or KP303B with an initial drain current of 1...2.5 mA to bias transistor VT1. In this case, resistor R1 is excluded. The gate and source of the field-effect transistor must be connected together and connected to the base of transistor VT1, and the drain to its collector. It should be borne in mind that in this case the device is operational in circuits with a voltage of no more than 25 V.
Figure 2 shows the dependence of the fuse operation current on the resistance of resistor R8. The type of this characteristic strongly depends on the opening voltage of the thyristor.
It should be borne in mind that with a supply voltage that has significant ripple, the electronic fuse trips at voltage peaks, so the average current through the load will be slightly lower than when using a well-smoothed voltage.
The fuse operation current can be determined from the expression: I open =U openVS1 /(R eq +R8), where U openVS1 is the opening voltage of the trinistor, and R eq is the equivalent resistance of the circuit of resistors R3-R6. As the graph in Fig. 2 shows, regulation of the operating current by resistor R8 in the limit value zone is quite rough, so it is advisable to either reduce the control limits by reducing the resistance of resistor R8 by 1.5...2 times, or introduce multi-stage regulation with a switch with a set of precisely selected resistors .
The fuse is mounted on a printed circuit board made of fiberglass 1.5 mm thick (Fig. 3). The board contains all the parts except transistor VT2, resistor R8 and button SB1. Transistor VT2 must be installed on a small heat sink, for example, on a duralumin plate measuring 90x35x2 mm with bent edges.
The device can also use transistors in a metal case; you just need to change the design and dimensions of the heat sink. The KT817B transistor can be replaced with KT815B-KT815G, KT817V, KT817G, KT801A, KT801B, and KT805AM with KT802A, KT805A, KT805B, KT808A, KT819B-KT819G. The static current transfer coefficient of transistors must be at least 45. Fixed resistors - MLT, MT and MON; variable resistor - any wire; button SB1 - P2K without lock.
It is better to use KU103A thyristors in the fuse with an opening voltage of 0.4...0.6 V.
As a rule, the assembled fuse does not require adjustment. In some cases, it is necessary to select the resistance Req by adding another resistor to set the maximum operating current. The board provides space for four resistors R3-R6.
Rice. 2
Rice. 3
Radio No. 5, 1988, p. 31
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| VT1 | Bipolar transistor | KT817B | 1 | To notepad | ||
| VT2 | Bipolar transistor | KT805AM | 1 | To notepad | ||
| VS1 | Thyristor & Triac | KU103A.B | 1 | To notepad | ||
| R1 | Resistor | 750 Ohm | 1 | 2 W | To notepad | |
| R2 | Resistor | 2.4 kOhm | 1 | To notepad | ||
| R3-R6 | Resistor |
Household automation
This device (Fig. 7.21) acts as an electronic fuse; it turns off the load if the current flowing through it exceeds the permissible limit. The current flowing through the load connected to connector XI creates a voltage drop across resistor R3. Part of this voltage, removed from the variable resistor R2, is supplied to the base circuit of transistor V3. An electromagnetic relay K1 is connected in the collector circuit of this transistor. If the load current exceeds the specified value, then relay K1 will operate and its contacts Kl.l, K1.2 will disconnect the load from the network and be blocked. The device remains in this state until the S1 “Reset” button is pressed.
Device diagram
Resistor R1, diode V2, zener diode VI and capacitor C1 form a stabilized power source. Diode V4 protects the emitter junction of transistor V3 from being exposed to reverse polarity voltage. The limiting current is set by variable resistor R2. The minimum limiting current is determined by the resistance of resistor R3.
With the rating indicated in the diagram, it is 0.2...0.3 A. To protect the network from short circuits in the load, fuse F1 is used. Contacts Kl.l, K1.2 of the relay are connected in parallel to increase the possible maximum load current. Transistor V3 can be from the MP25, MP26 series with any letter index, diode V4 - from the D7, D9, D311 series. The D816G zener diode can be replaced with three D814D zener diodes connected in series. Relay K1 - RES9 (passport RS4.524.205). Button S1 -MT1-1 or P2K. The maximum load current limited by the device should not exceed 1.5 A - otherwise the contacts of relay K1 may burn out.
O. SIDOROVYCH, Lvov, Ukraine
In the article, the author proposes a number of original electronic fuses for low-voltage circuits, made using relays or relays and thyristors. The fuses are reset to their original state using a button.
As you know, a reed switch (sealed contact) is a glass cylinder into which contacts made of an alloy with high magnetic permeability are soldered. If the reed switch is placed in a magnetic field, then the magnetic force arising in the gap attracts the contacts, which close after this force exceeds the mechanical elastic forces of the contacts. If a coil wound on the reed switch body is connected to an open circuit through which the current needs to be controlled, then the reed switch can be used as an element of an electronic fuse that combines a current sensor (coil) and a circuit disconnect device (contacts). Let's consider electronic fuses based on the KEM-3 reed switch, which has the following parameters: response time - 1.5 ms; release time - 2 ms; maximum switched direct current - 1 A; maximum contact resistance - 0.15 Ohm; MTBF - 10 to 6 cycles.
From this it can be seen that the speed of the reed switch is higher than that of a conventional relay, and even more so than that of fuse-links. For fuse link VP1-1, for example, according to technical conditions it is equal to 0.1 s at four times overload. The electronic fuses described below require a reed relay, which is easy to make yourself.
In Fig. Figure 1 shows the design of a homemade reed relay.
The glass body of reed switch 1 serves as a frame for winding 2 of the relay coil. Coil cheeks 3, which are textolite washers with cutouts for leads, are glued along the edges of the KEM-3 reed switch with epoxy glue 4. The drawing of the cheeks is given in Fig. 2.
The coil winding contains 60 turns of PEV wire with a diameter of 0.3 mm (for an operating current of 1 A). The winding resistance is so small that it can be neglected.
In Fig. Figure 3 shows a diagram of a simple electronic fuse made on such a relay (K2).
In addition, it includes a factory-made reed relay RES55A (K1). In normal mode, the load current passes through the circuit: input terminal ("+" of the power supply), closed contacts of the SB1 button, relay winding K2, normally closed contacts K1.1 of relay K1, normally closed contacts K2.1 of relay K2. When a current overload occurs, the current through the winding of relay K2 increases sharply, which causes its contacts K2.1 to operate, which open the current circuit. Almost all the supply voltage is supplied to relay K1, the relay is activated and opens the circuit of the winding of relay K2 with contacts K1.1. Thus, the overload current circuit is broken, and a current limited by the parallel connection of the resistances of the relay winding K1 and the indication circuit consisting of the LED HL1 and resistor R1 flows through the emergency load. The glow of the HL1 LED indicates that the fuse has been turned off. To activate the fuse, you must briefly press the SB1 button.
The fuse operation current is selected to be no more than 1 A based on the maximum permissible current for KEM-3 reed switches. A drawing of the fuse circuit board is shown in Fig. 4.
In Fig. Figure 5 shows a diagram of another version of the electronic fuse.
It contains, in addition to the reed relay K1, made in accordance with Fig. 1, includes thyristor VS1. The device is started by briefly pressing the SB1 button. In this case, the thyristor VS1 opens and along the circuit: plus the power supply, thyristor VS1, relay winding K1, normally closed contacts K1.1, load - current flows. When the load resistance decreases, i.e. when a current overload or short circuit occurs, the current through the winding of relay K1 increases, the contacts K1.1 of which open, opening the circuit of the thyristor VS1. SCR VS1 closes, thereby disconnecting the power source from the load. At the same time, the HL1 LED lights up, indicating that the fuse has been turned off. To restart it, you must briefly press the SB1 button. The voltage drop across the fuse is determined mainly by the voltage drop across the SCR VS1 (about 1.5 V at a current of 1 A). A drawing of the fuse circuit board is shown in Fig. 6.
The table shows the number of turns of the winding of a homemade reed relay for different tripping currents of fuses made according to the diagrams in Fig. 3 and 5.
The winding wire in all cases was chosen with a diameter of 0.3 mm.
In Fig. Figure 7 shows a diagram of the third version of the electronic fuse, containing a thyristor VS1 and two reed relays K1, K2 type RES55A.
One of the relays, K2 (passport RS4.569.610P2), is used as a threshold element. It has a response voltage of 1.46 V and is connected by its winding in parallel to the series-connected thyristor VS1 and resistor R3, the voltage drop across which is the measured value. For a load current of 1 A (fuse current), the resistance of resistor R3 is 0.2 Ohm. By increasing the resistance of resistor R3, you can change (downward) the fuse operating current. The response voltage of relay K1 (RES55A passport RS4.569.602P2) is 7.3 V.
To bring the fuse into working condition, you must briefly press the double button SB1. In this case, the thyristor VS1 turns on and relays K1 and K2 are de-energized. The current from the positive of the power source passes through the circuit: thyristor VS1, resistor R3, normally closed contacts K2.1, load. This current increases during overload or short circuit. Accordingly, the voltage drop across the fuse increases. When it reaches the threshold value, relay K2 is activated, the contacts K2.1 of which open, disconnecting the load from the power source. In this case, a voltage almost equal to the voltage of the power source is applied to the fuse. Relay K1 is triggered, its contacts K1.1 are opened, relay K2 is de-energized, its contacts K2.1 are closed, but no current passes through them, since due to their previous opening the thyristor VS1 is closed. The HL1 LED lights up. Relay K1 is necessary in order to turn off relay K2, to which, when its contacts K2.1 are opened, a voltage is applied that significantly exceeds the rated voltage of this relay. Due to the presence of relay K1, the time of application of this voltage to the winding of relay K2 is equal to the turn-on time of relay K1 - approximately 1 ms. After the fuse trips, a small current will flow from the source to the load through the resistance of the parallel-connected relay windings K1 and the circuit: resistor R1, LED HL1. After eliminating the overload, you must briefly press the SB1 button to bring the fuse into working condition.
The printed circuit board drawing of this device is shown in Fig. 8.
In the last two devices (see Fig. 5 and 7), the thyristor is mounted on a bracket, the drawing of which is shown in Fig. 9.
All electronic fuses described are tested with a power supply voltage of 12 V. This, however, does not exclude the possibility of their use with other voltages.
LITERATURE
1. Switching devices of radio-electronic equipment. Edited by Rybin G. Ya. - M.: Radio and Communications, 1985.
2. Tereshchuk R. M. et al. Amateur Radio Handbook. - Kyiv: Naukova Dumka, 1982. Radio No. 12 2005
Are you tired of changing fuses every time they blow? Use an electronic DC fuse that will protect your devices connected to the power supply. This "fuse" can be reset by simply turning it off and on again. This fuse uses an N-channel FET field effect transistor as a current sensor. The transistor also turns off the load line by ground when the current exceeds the maximum permissible value.
Fuse diagram
Printed circuit board
The cutoff (trigger) current can be adjusted by variable resistor P1 from 0 to 5 A. This circuit can work correctly with a maximum load current of up to 5 amperes. Don't overload it if you don't want to burn the parts. At prolonged high current the transistor can become hot, so a small heatsink is needed.
Now about the capacitors in the base circuit - C1 and C2 of transistor T2. Depending on their capacity, the response speed changes. For example, C1 will turn off slowly (skipping short-term load peaks), and C2 instantly. When tuning, adjust resistor P1 until the fuse blows. Resetting a fuse is simple: turn off the power to it and when power is reapplied, the circuit is ready to protect your appliances again. The device is suitable as an attachment for any DC power source (with AC the circuit will not work) for an output voltage of up to 25 V. At higher voltages, you will need to change the values of some resistors and install more powerful transistors.
Electronic fuse are an effective way to protect all kinds of electronic devices from current overloads.
Basically, electronic fuses must meet the following requirements: they must be economical, simple and at the same time reliable and small in size. By the way, high-power field-effect transistors are ideally suited to implement all of the above requirements.
A schematic diagram of one of the options for such an electronic fuse is given in this article.
Description of the operation of the electronic fuse
This electronic fuse is connected to the open circuit between the power source and the protected load. The circuit provides protection at a voltage of 5...20 volts with a load reaching up to 40 amperes.
(DA1) is built, input 3 of which is supplied with a reference voltage from (DA2). Field-effect transistor VT1 embodies two functions at once: a current sensor and a powerful electronic switch. As noted above, the specificity of the electronic fuse lies in the use of the resistance of the field-effect transistor channel as a current sensor.
Key characteristics of the field effect transistor used
- maximum dissipation power - 110 W.
- channel resistance - 0.027 Ohm.
- Maximum drain-source voltage is 55 V.
- maximum drain current - 41 A.
To activate the fuse, use the SA1 button (without locking). When you press it briefly, voltage is supplied to the gate of the field-effect transistor through resistance R4 and diode VD2. As a result, the transistor connects power to the load.
The output state of the LM358 operational amplifier is related to the voltage level at its input 2. If the current drawn by the load is less than the set electronic fuse threshold, then the voltage at input 2 of the comparator will be lower than the reference voltage at pin 3. As a result, output 1 will be high voltage that keeps the transistor open.
Simultaneously with the increase in current consumption, the voltage on the field-effect transistor VT1 will also increase. When this voltage exceeds the voltage across resistance R1, the voltage at the output of the comparator will begin to decrease, transistor VT1 will begin to close while the voltage across it increases.
In this regard, the voltage at the output of the comparator decreases even more, which ultimately leads to the instantaneous closing of the transistor and de-energizing the load. To reactivate the electronic fuse, press the SA1 button again.
The required value of the fuse operation current is selected by trimming resistance R1. If the controlled power supply is stable, then the DA2 stabilizer and resistance R3 can be removed from the circuit by installing a jumper in place of R3. To reliably disconnect the controlled load at a small operating current (no more than 1...1.5 amperes), it is necessary to increase the resistance of the current sensor by connecting a resistor of about 0.1 Ohm to the electrical drain circuit of transistor VT1 (point “A” in the diagram).
It is possible to use an arbitrary op-amp (DA1) in the circuit, which can operate at zero voltage on both inputs in single-pole power supply mode, namely K1464UD1R, KR1040UD1A, K1464UD1T. DA2 can be replaced by the domestic KR142EN19. Trimmer resistor brand SPZ-28, SPZ-19a. All fixed resistors C2-33, MLT. Non-oxide capacitor C1 type K10-17V