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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).  The module's specification document lists the total path resistance as 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:
KnowledgeBase 4J4DMPLE: NI PXI-2535 or PXI-2536 FET Switch Doesn't Close
Developer Zone Tutorial: How to Choose the Right Relay

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Report Date: 08/12/2007
Last Updated: 03/17/2009
Document ID: 4CBKJQ9S

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