Difference between Switch Resistance and Path Resistance in FET Switches Hardware: Switches
Problem: I am using an NI Switch module with FET switches (such as the PXI-2501). I notice in the module's specifications document that the total path resistance is significantly larger than the FET switch on resistance. Why is the overall path resistance so much larger than the on resistance of the FET switch? Solution: The additional path resistance is added to provide sufficient overvoltage protection. This extra resistance dissipates additional power so that the FET relay is protected at higher voltage levels. For example, the FETs on the PXI-2501 are rated for a maximum continuous current of 30 mA. The on resistance of these FETs is 50 Ω min and 100 Ω max. If there were no additional resistance added to the signal path, the switch would only be protected to 30 mA * 50 Ω = 1.5 V. A voltage greater than 1.5V would apply more than 30mA across the FET and would increase the likelihood of damaging the relay. For this reason, additional resistance is inserted in series with the FETs on this module, so that it is protected to 25 V. Related Links: Attachments:
Report Date: 08/12/2007 Last Updated: 08/24/2007 Document ID: 4CBKJQ9S |
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