AC Electronic Fuse Manufacturing

AC Electronic Fuse Manufacturing

How it works

The circuit of the AC electronic fuse is shown in the figure below. It is actually an overcurrent protection switch. After the 220V AC is stepped down by the power transformer T, half-wave rectified by the crystal diode VD2, and filtered by the capacitor C2, it outputs about 12V DC to supply power to the control circuit.

　　R1 is a detection resistor connected in series in the user phase line L loop, and its resistance is only 0.2Ω (variable); it will not have any effect on the main circuit. The current passing through it is the sum of the currents of the user's electrical appliances. When the current passing through R1 reaches 3A (effective value), the voltage U1 (effective value) on R1 = 0.6V, Um (peak voltage) = 2 to the power of -2 × 0.6V ≈ 0.8V. After half-wave rectification by crystal diode VD1 (the forward tube voltage drop is 0.5V when the actual current is small) and filtering by capacitor C1, the output is about 0.3V DC voltage, which is not enough to trigger the unidirectional thyristor 'VS to turn on (VS trigger threshold voltage is 0.6V). If the power supply is abnormal and the current continues to increase, for example, it reaches 4A or above, then Um at both ends of R1 ≥ 2 to the power of -2 × 4A × 0.2 Ω ≈ 1.1 V, the output voltage of C1 reaches above 0.6V, VS obtains the appropriate trigger voltage and turns on, the electromagnetic relay K is attracted, and its normally open contact KH connects the power supply of the AC contactor KM, and the large-capacity normally closed contact of the AC contactor automatically cuts off the AC power supply circuit, realizing the "fuse" function.

In the circuit, R2 is a current limiting resistor to protect the VS control electrode of the unidirectional thyristor from being damaged by overcurrent. When the load is short-circuited, the 220V AC voltage is almost completely applied to both ends of R1 instantly. Due to the existence of R2, the average current flowing through the VS control electrode will not exceed 9.9mA.

Without R2, VS will be damaged by overcurrent at this moment. R2, R3 and C1 also form an anti-false operation circuit, which can effectively eliminate the false operation of the electronic "fuse box" caused by various instantaneous strong currents in the power grid (such as the startup current of color TVs, refrigerators, etc.). The capacity of C1 directly affects the sensitivity of the electronic "fuse box" action. R4 and light-emitting diode VD3 form the electronic "fuse box" action indication circuit. SB is a normally closed self-reset push button switch. After the power supply load fault or line short circuit fault is eliminated, just press this button switch to restore normal power supply.

2. Component Selection

VS uses BT169D or CR1AM-6, MCR100-1 type ordinary small plastic-sealed unidirectional thyristor. VD1 and VD2 both use IN4004 or IN4007 type silicon rectifier diodes, and VD3 uses φ5mm ordinary red light-emitting diodes.

The detection resistor R1 can be replaced by a section of electric furnace wire. Different resistance values ​​will result in different operating currents for the electronic "fuse box". According to Figure 1, 0.2Ω is selected. As long as the current in the power supply circuit reaches 4A, the "fuse box" will operate. If other operating currents are required, the resistance value of R1 can be estimated by the formula R1=0.8/I1, and I1 is the required operating current of the electronic "fuse box". However, the maximum value of I1 should not exceed the maximum allowable current value of the AC contactor KM contact. R2~R4 are all RTX-1/4W carbon film resistors. C1 and C2 are all CD11-25V electrolytic capacitors.

T uses a 220V/12V, 3W small high-quality power transformer, which requires long-term continuous no-load operation without heating. K uses a JZC-22FA/O12-1Z ultra-small medium-power electromagnetic relay suitable for direct welding on a printed circuit board. Its contact load is 220V×3A (AC), and its volume is only 22.5mm×16.5mm×16.5mm. It fully meets the use requirements. KM uses a single-phase AC contactor, and the contact capacity depends on the actual power consumption. Generally, 5A is sufficient. SB uses a small normally closed self-reset push button switch, which can also be replaced by a KWX-2 micro switch (only a normally closed group contact).

3. Production and Use

Figure 2 shows the wiring diagram of the printed circuit board of the AC electronic "fuse box" (the dotted box represents the shell). The printed circuit board is preferably made of epoxy-based single-sided copper foil, with an actual size of about 50mm×35mm.

The soldered circuit board can be placed in a bakelite or plastic box of suitable size. The box panel has holes to fix the light-emitting diode VD3 and the self-reset button switch SB. It is best to connect the input and output wires on the circuit board to the four-hole bakelite terminal blocks commonly used by electricians. This makes it easier for users to connect external lines.

After the installation and wiring are correct, the only thing to do is to check whether the "fuse box" plays a role of insurance. The specific method (R1=0.2Ω) is: connect 220V AC mains to the incoming line, and connect 800W load (800W electric stove can be used) to the outgoing line. At this time, the "fuse box" should not work; if it works, the resistance of R1 should be appropriately reduced. Then, increase the load to 900W (a 100W light bulb can be connected in parallel), and the circuit should be quickly cut off. That is, the electromagnetic relay K and the AC contactor KM are successively attracted, and the light-emitting diode VD3 lights up. Otherwise, the wiring and component quality should be carefully checked.

The inspected "fuse box" can be installed on the user's distribution board (panel). Its two incoming and outgoing terminals should be correctly connected in series to the power supply circuit as shown in the figure, and then it can be put into formal use.

In addition to being used to directly replace the ordinary main fuse box of the household lighting circuit, this AC electronic "fuse box" can also be directly installed (without the shell) into multi-purpose sockets and various electrical equipment to replace the original ordinary fuse. When installing, please note that the input and output lines cannot be mistaken, and the installation environment must be dry, without strong magnetic interference and vibration.