The output of DC power supplies can be externally controlled using external analog signals or external digital signals (transmitted via a communication interface such as a LAN and RS232C). In this white paper, we’ll mainly focus on explaining how to control DC power supplies with external voltage using external analog signals and briefly explain how to successfully utilize the digital controls in the last section.
For the external analog signal control, you need to set the common of the external voltage control at either a positive (+S) or negative sensing (-S) terminal. Let’s first examine how to connect the external power supply for both settings.

1. External analog-signal control – When the common of the external voltage control is at the +S terminal

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This section describes how to wire the external power supply in the system if the common of the external voltage control is at the +S terminal of the DC power supply. Keep in mind that improper wiring may result in a product malfunction or electrical accident. When you look at the circuits and wiring examples described below, make sure that you know where the +S terminal is located.

1-1 Controlling the output voltage with the external voltage

For the system in Figure 1, the DC power supply is the isolated AC/DC converter, which electrically isolates the AC input line from the output. The common of the external voltage control is at the +S terminal. The output voltage is calculated as follows: output voltage = external voltage x R2 / R1. Therefore, the output voltage is proportional to the external voltage.

Figure 2 shows the block-wiring diagram of the above system with the ground (GND). The external DC power supply and the load are connected to the DC power supply (AC/DC converter). The external DC power supply isolates the AC input line from the output. For safety, the GND terminals of the DC power supply and the external power supply (also connected to the chassis) are connected to the GND with a three-wire power cord. Neither the positive nor the negative terminal of the load is grounded.