(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?
This post was last edited by hanhaigang128 on 2024-1-28 at 11:19. When the DI switch contacts of a typical DCS or PLC are open, the voltage drop across those two contacts is 24VDC. When the normally open contacts close, the voltage across the two contacts is the same as 0VDC; therefore, the voltage drop between the two contacts is 0. In some PLCs, the DI inputs are 220VAC; similarly, when the normally open contacts close, the AC voltage across those two contacts with respect to ground is the same, so the voltage drop between the contacts after they close is 0.