what is the open collector output?！

    An open collector output is an NPN transistor. NPN transistor allow leakage current to the common terminal. It can be considered as a switch that allows the circuitry is connected to a common load after the end. This means that if you need to have a power output of work. After the power load must be connected to the output, otherwise the NPN transistor is to establish a common path that is dry contact. So, if you are not connected to any measure a open collector output voltage of the power supply, will not see the change in voltage. If the open collector normal operation, after the output voltage of the load should be detected.

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