Why is the Thermal EMF Specification for the General-Purpose Relays So Large?Hardware: Switches>>General-Purpose
Problem: I am interested in purchasing a General-Purpose Relay module, perhaps the PXI-2568, PXI-2569, or PXI-2570. In the specifications page for these boards, it says they have a typical Thermal EMF specification of < 12 uV. This seems high and I would like to know why. Solution: With no or minimal load, the Thermal EMF performance of these SPST and SPDT switch is actually around 1 to 2 uV. The published Thermal EMF specification of < 12 uV was a worst case scenario intended to include application use-cases where 1 to 2 A is connected across the relay. The more current passing through a relay, the more heat generated and the greater the amount of thermal noise a relay can expect to see. Verification and Validation (V&V) Testing done for these modules showed that, while typically lower, it was possible to get Thermal EMF readings as high as 12uV if the right heating conditions were met which is why the <12 uV specification was selected. When comparing switch specifications, it is always best to check with the manufacturer to see under what conditions the specification measurements were made.Related Links: Developer Zone Tutorial: Switching Considerations Developer Zone Tutorial: Switching Low-Level Signals to a DMM Product Page: NI PXI-2568 General-Purpose SPST Relays Product Page: NI PXI-2569 General-Purpose SPST Relays Product Page: NI PXI-2570 General-Purpose SPDT Relays Attachments:
Report Date: 05/14/2009 Last Updated: 05/14/2009 Document ID: 4XDB8H9G |
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