Switching Considerations

This section contains information on switching concepts you may need to consider when designing your switching application.

Relay Life

All NI switches specify a conservative estimation of the expected life of the electromechanical relay components. Relay life is specified as a minimum number of cycles before the end of the relay life. One cycle is defined as the action of opening and closing the relay. The expected life is divided into two main categories: mechanical and electrical life.
Mechanical Life

The contacts of mechanical relays wear with usage, and worn contacts have a higher contact resistance. The mechanical life specification is typically the number of switch cycles before the contact resistance rises above 1 Ω. This rating assumes no electrical load across contacts during actuation.

Electrical Life

Switching active electrical signals, especially high power signals, causes arcing across the relay contacts. This arcing produces pits on the contact surface and accelerates the contact wear described above in Mechanical Life. The electrical life specification is the number of switch cycles, under load, before the contact resistance rises above 1 Ω.

Contact Resistance

Contact resistance refers to the DC resistance through one set of closed contacts in a relay.

Path Resistance

Path resistance is the resistance of a complete signal path from source to destination terminals on a switch module. The total resistance includes the resistance of PCB traces, relays, and connectors. Trace and connector resistance is generally stable, but relay contact resistance increases with use. The following figure shows a typical path resistance of a module with a mechanical life of 50 million cycles.
Thermocouple

When two dissimilar metals are joined, it creates a voltage (the Seebeck voltage) which is a function of the temperature of the junction and the composition of the two metals. This voltage is also known as thermal electromotive force (EMF). Table 1 lists the most common metal and their thermal emf.

Thermal EMF in Switches

Leads of electromechanical relays are generally made of different metal alloys than the switch module PCB which are usually copper or copper alloy. The junction between these two alloys creates a thermocouple as shown in Figure 2.

The thermocouples developed in a relay are a function of the temperature of the junctions. The temperature will vary with the ambient temperature, the type of modules in the adjacent slot, the number of relays being activated, and the air flow inside the module. A signal path can transverse a single relay or multiple relays. The sum of all the thermocouples in a signal path is expressed as the thermal EMF. This Thermal EMF can be specified as single path (single wire) or differential path thermal EMF.

Figure 3 illustrates thermal EMF measured in a single path.



Figure 4 illustrates thermal EMF measured in a differential path


When measuring voltage with a DMM and a switch, this thermal EMF needs to be accounted for in the overall system accuracy calculation.

For example if the DMM has an accuracy of 4 uV and a switch has a differential path thermal EMF of 3 uV the overall system accuracy will be:

or 0.01 % accuracy when measuring a 50 mV signal.

When measuring resistance, there are methods for compensating varying thermal EMF such as Offset Compensated Ohms (OCO) or other calibration techniques for very accurate measurements.

Thermal Offset

Solid State Relays (SSR) and FET switches have offset voltages that are larger than their thermal EMF. Once the switch modules has been powered on for 10 to 15 minutes, the thermal offset is generally constant and therefore can be calibrated and compensated out.

Settling Time / Cycle Process

Settling Time

Settling time refers to the time required for a signal to reach a steady state after sending an actuation command to the relay. Steady state is determined by the required accuracy of the measurement. Highly accurate measurements require longer settling times than less accurate measurements.

Settling time is an important consideration for solid-state relays with high path resistance and R-C time constants.

For certain situations you may need to increase the default settling time. Refer to Adding Additional Settling Time in the NI Switches Help for more information on increasing the default settling time.

Cycle Process

The following diagrams represents the process an electromechanical general-purpose relay performs during a cycle. The full diagram is located in the NI Switches Help.

For certain situations you may need to increase the default settling time. Refer to "Adding Additional Settling Time" in the NI Switches Help for more information on increasing the default settling time.

Switching Inductive Loads

Switching Inductive Loads

When inductive loads are connected to the relays, a large counter electromotive force may occur when the relay actuates because of the energy stored in the load. These flyback voltages can severely damage the relay contacts and greatly shorten the relay life.

Limit these flyback voltages at your inductive load by installing a flyback diode for DC loads or a metal oxide varistor for AC loads, as shown in the following figure:

Accounting for flyback voltages is particularly important for the NI SCXI-1160 / 1161 / 1163R modules. Refer to the switch module documentation for information on preparing the switch module to properly handle inductive loads.

Switching Capacitive Loads

Using reed relays to switch capacitive loads, especially with high voltages, requires special care. When a switch closes, a transient current flows to charge the capacitance. This inrush current may be substantially higher than the steady-state current through the system. Reed contact welding may occur because of this high inrush current, even though the voltage and steady-state currents are within the switch specifications. Inrush currents can be controlled with series impedance, such as a resistor or ferrite, between the switch and the capacitance. Any capacitance in the system can contribute to inrush currents, whether it is in a reactive device under test or from a shielded cable.

Signal levels through a switch must account for specifications on switching voltage, current, and power.


Switching Voltage
Switching voltage refers to the maximum signal voltage that the switch module can safely maintain. Switching voltage is defined from channel-to-ground and from channel-to-channel. Channel-to-ground is the voltage potential between the signal line and the grounded chassis. Channel-to-channel is the voltage potential between any pair of signal lines within the module. This voltage includes voltages across open relay contacts, as well as voltages between adjacent connection terminals.
Switching Current
Switching current is the maximum rated current that can flow through the switch as it makes or breaks a contact. Switching active currents results in arcing that can damage the contacts of electromechanical relays. A minimum current specification indicates the smallest current that can reliably flow through the switch.

Switching Power
Switching power is the limit on the combined open-contact voltage and closed-contact current of a signal in the switch.

Switching Power = Switching Voltage * Switching Current

Switching high-power signals causes high-energy arcing at the electromechanical contacts during actuation, reducing the useful life of the switch.

Selecting Switch Bandwidth

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Very informative documents
- Aug 30, 2007