Simple and convenient inverter overcurrent protector

The power of a household power inverter is only about 300 watts, which can easily cause damage to the machine due to overload. Overcurrent protection usually uses fuses, which have poor effects and are troublesome to replace frequently. This overcurrent protector can effectively limit the current and avoid the trouble of frequently changing fuses in the dark.

Working principle: The figure shows the simplified main circuit of a general inverter, and the dotted box is an additional overcurrent protection circuit. IC is a 555 time base circuit, which forms a delay circuit with RP, C, etc. After the power switch K is turned on, the timing circuit starts working first. Since the voltage on C1 cannot change suddenly, initially the ②⑥ pins of IC are at high potential, the ③ pin is at low level, and the relay J is in a released state. At this time, the reed relay JAG cannot be attracted because the inverter has not started working. Then, the power supply charges C through RP. After a delay, the potential of the ②⑥ pins of IC gradually drops to 1/3VCC, the ③ pin outputs a high level, J is attracted, the normally open contact is closed, the inverter push block is powered, the power tube works, and the secondary of transformer B outputs 220V AC. If there is no overcurrent, the main DC current flowing through JAG will not cause JAG to attract, and the inverter works stably.

When the battery output current is too large for some reason, JAG is energized, the contact is closed to short-circuit C, so that the ② and ⑥ pins of lC are suddenly high level, and ③ is flipped to low level, and J is released immediately, cutting off the power supply of the inverter drive block, and the inverter stops working, achieving the purpose of protection. JAG immediately discharges the charge on C, so when the main circuit current disappears and JAG is released, C starts a new round of charging process, so J remains in the released state at this time. At this time, the excessive load must be removed quickly. When the lC delay time is up, J automatically attracts and tries to send power. If the current returns to normal at this time, the inverter will also work normally. On the contrary, J is released immediately.

In the figure, AN is the quick start button. Press AN once and the ②⑥ pins of IC will immediately change to low level, and J will immediately close to test power. It can be used for testing or to make sure that the overcurrent has disappeared.

Component selection and production: C should use electrolytic capacitors with small leakage current as much as possible, IC can use various 555 time base circuits, and J can use 12V small relays such as 4088, 4098, etc. The dry reed relay JAG needs to be made by yourself. The method is to use a normally open reed switch and wind the inverter DC power line around it for several turns to test. The number of turns required is determined according to the rated power of the machine. Generally, less than 5 turns can meet the requirements. All components can be welded by foot or a small circuit board can be welded and placed in an empty space inside the machine. AN is installed on the panel.

By adjusting RP, different delay test power-on times can be set, and no other adjustments are required.