Go 'back to the basics' to solve switching and measurement problems

6/15/2011 12:46 PM EDT

• 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.