As an applications engineer, I hear about plenty of test and measurement problems: “What’s wrong with my instrument? I keep getting the same errors over and over.” Sometimes, a measurement problem isn’t really a problem with the instrument itself but with the switch hardware connected to it.
For many engineers, solving a measurement problem is a matter of going back to the basics of switching. Here are some common switching-related error sources to consider when troubleshooting measurement problems:
Contact failure can occur when excessive heat or a high current or voltage pulse causes the contacts to weld together, resulting in short circuits. When configuring a switching circuit for high current (>1A), pay particular attention to the maximum current, maximum voltage, and VA specifications of the switch card under consideration. It’s also important to choose a switch card with low contact resistance to avoid excessive heating that can cause contact failure by welding the contacts together. Contact heating is caused by I2R power dissipation.
Contact potential is an offset voltage that’s primarily a result of the thermoelectric EMFs generated by the relay contacts in the switching cards but will also include any other spurious voltages introduced by connectors and junctions in the signal path. This offset voltage adds directly to the signal being switched, so it can cause significant errors when switching very low voltages. Depending on the relays in the signal path, the error voltage could range from less than one microvolt to tens of microvolts. If that level is significant with respect to the source or measure value, it’s critical to measure the contact potential and compensate for it to improve measurement accuracy. Changes in ambient temperature as well as temperature gradients present across a switch card can affect the level of contact potential. Fortunately, it’s possible to reduce these effects by ensuring a switch card is located properly within the switch mainframe and the mainframe is located properly with the equipment rack. For example, whenever possible, install a switch card being used for low voltage measurements in the card slot furthest from the mainframe’s power supply.
Contact failures and contact potential are only two of the many potential sources of measurement error that can be traced back to switching.
About the author:
Dale Cigoy is a lead applications engineer at Keithley Instruments, Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. His responsibilities include helping customers with measurement applications that include Keithley instruments. Cigoy joined Keithley in 1976 after earning a Bachelor of Science degree in Electronic Technology from Capitol College in Laurel, Maryland.
A new 32-page booklet, "Reed RelayMate" from Pickering Electronics provides a lot of good technical non-commercial information about using reed relays in measurement equipment. To request a copy, visit: http://www.pickeringrelay.com/relaymate.html. I have no affiliation with Pickering. --Jon Titus
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