(Dry contact) When the DI channel is open, voltage is detected between the two terminals; when it is closed, no voltage is detected. What is the principle behind this?
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(Dry contact) When the DI channel is open, voltage is detected between the two terminals; when it is closed, no voltage is detected. What is the principle behind this?
You are asking about checking the voltage, right? When the DCS detects a dry contact signal, it outputs a voltage from the positive terminal, such as 48VDC. If the dry contact on the ground side is closed, this voltage returns to the DCS via the negative terminal, allowing the DCS to determine that the contact is in a closed state. This 48VDC is the query voltage, responsible for checking the on/off status of the on-site switch.
One end of the dry contact is connected to the positive power supply, and the other end is connected to the negative power supply. When the dry contact is open, it functions as an open circuit, and in this case what is measured is the open-circuit voltage of the power supply ； When a dry contact is closed, it acts like a section of wire with no resistance, and there is no voltage across the two ends of the closed dry contact.
Open circuit: resistance is infinite, the voltage drop is maximum; it’s equivalent to a power source； And the series resistance is minimal, so its voltage drop can be ignored; therefore, the voltage is zero.
Is the voltage drop you mentioned the same as the voltage drop under load? For example, if the voltage measured under load with a 24V power supply is 16V. Is this the same concept?