2023-10-17
664
Small relays are commonly used in electronic systems to control circuit switching, and they have widespread applications in industrial equipment, instruments, and household appliances. The heart of modern control circuits is usually computer chips such as MCUs, DSPs, and FPGAs. These core chips do not have enough output power to directly control small relays and can be driven by SLKOR's small transistors BC817-25, BC848B, BC847B, and small MOSFETs 2N7002E, SL2302S, SL2300, SL3400, SL3402, and Darlington arrays ULN2001D, ULN2003AU, ULN2803AS, etc.
In the following example, we will use the driving of a small high-power relay JQC-3FF/12VDC-1ZS (551) from HF as an example. The coil of the relay has a rated voltage of 12V, an action voltage of 9V, a maximum voltage of 15.6V, and a coil resistance of 400Ω. When powered by 12V, the coil current is 30mA. Here we will introduce the design of driving the small relay using BC848B, SL2302, and ULN2803AS, respectively. Taking the STM32F030C8T6 MCU as an example, any IO pin can provide a maximum limit of 25mA of pull-up or pull-down current, and the total limit of pull-up or pull-down current for all IO pins is 80mA. Therefore, when applying the IO pins, there should be enough margin. The typical value of the weak pull-down resistor for the IO pin is 40KΩ. When powered by 3.3V, if the IO pin outputs 8mA of current, the minimum output high level is 2.9V, and if the IO pin pulls 8mA of current, the maximum output low level is 0.4V.
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