The open-collector output of a power supply is a method of control circuitry used to output the state (status) and other information of the power supply. Alternatively, an open collector may be used as an example of a circuit method to externally control the power supply, such as output ON/OFF.
The open collector output connects an external circuit to the collector and emitter terminals of the transistor inside the power supply. There are two types of transistors, NPN and PNP type, and their usage differs. It is important to note which transistor type is used in the internal circuit.

Difference between NPN and PNP transistors

Most of Matsusada Precision's power supplies use an NPN open-collector output for the status output of the analog remote control terminal.
For example,
If the status is High, the transistor is in the ON state, and the output pin is in a low impedance (current flowing) state.
If the status is Low, the transistor is in the OFF state, and the output pin is in a high impedance (no current flow) state.
Generally, it is used by connecting a power supply and a pull-up resistor to an external circuit and converting it to a signal of the required voltage.

connection diagram of NPN open-collector output

For reference, a connection diagram of PNP open-collector output is shown below.

PNP open-collector output

The transistors used in open-collector input/output circuits are often optocouplers (also called optoisolators, photocouplers) to prevent equipment malfunction and reduce the risk of failure.
The NPN type is widely used for sensors and PLC outputs in the U.S. and Japan. In Europe, on the other hand, the PNP type is often used to prevent malfunction in case of wire breakage or short circuit to the enclosure.
The input/output logic circuit method may be transistor-transistor logic (TTL) or CMOS, in addition to the open-collector type.

Power supply analog remote control internal circuit example