Changing the Input Threshold Voltage
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This document is part of a comprehensive tutorial on industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.
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Changing the Input Threshold Voltage
When you want a threshold higher than the default (4V max for High), you can insert a zener diode in series with an input to shift its threshold by the zener breakdown voltage. You can install the zener diode in the connector block. The following figure shows the polarity of the zener diode with respect to the NI 6624 input.
After inserting the zener diode, the new threshold becomes 4 + VZ. For example, if you choose the zener diode such that VZ is approximately equal to 7.5 V when its current, IZ, varies from 1-10 mA, the threshold voltage between IN+ shifted and IN- shifted is approximately 4 + 7.5, 11.5 V.
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