Recommendations for use

Comparison of properties depending on switching technology

Technology

Reed

FET/Solid State

EMR, Electromagnetic Relay

Examples

ELM264x, EL264x

ELM274x, EL27xx

EL26xx

Switching characteristics

"real" switch, completely disconnects

semiconductor switches, always low leakage current

"real" switch, completely disconnects

Transition resistance

Higher

Low

Low

Repeat stability Ron

Good

Very well

Very well

Overload behavior

Sensitive

Less sensitive

Sensitive

Switching speed

Fast

Very fast

Medium

Dependence of properties on the ambient temperature

Low

Higher

Low

Leakage current when open

Very low

Higher, increasing with operating temperature, several nA

Very low

Resulting offset/thermovoltage

Yes, several ±10 µV

Low, a few ±1 µV

Yes, higher than reed, also due to self-heating

Wear

Yes, gradual aging with normal use

Typ. failure: non-conductive

Low, if at all, then sudden failure

Typ. failure: conductive

Yes, gradual aging with normal use

Typ. failure: non-conductive

AC behavior (alternating signals)

Good

Questionable

Good

Recommended use

Use at fluctuating ambient temperature

4-wire resistance measurements

Use at room temperature

2-wire resistance measurements

Thermocouples (as long as the ambient temperature does not change significantly)

Current peaks

Frequent/fast switching

 

*) it is not so much the Ton/Toff time of the switching element that is decisive with regard to the switching speed, but rather the time it takes for the device's internal firmware to control the switches.

If a switching element such as the EL2xxx/ELM2xxx is used for the manipulation of demanding analog signals, it should always be borne in mind that temperature changes affect properties of all kinds and can change them adversely. This must be taken into account when designing the system.

In addition, the switching elements always bring ohmic/inductive/capacitive influences into the system. Offset matching is recommended for voltage or resistance measurement.

In general, "complete" all-pole switching of the signals is recommended, i.e. all six lines in the case of a 6-wire DMS connection. If this is deviated from and, for example, only the signal line (+) is switched in a 2-pin IEPE connection

  • earth loop(s) may be created, depending on the design of the sensor and wiring,
  • interference inductions are facilitated by an N-fold enlarged line network,
  • the switched signal is to be given a sufficient settling time, e.g. a constant IEPE current signal must first stabilize for a few 100 ms,
  • "bouncing" can occur when mechanical contacts close,
  • the switched devices must be suitable for the on/off switching operation,
  • in the case of resistance simulation, the parallel circuit
    Recommendations for use 1:
    can be used.
    To get nearly equal resistance gradations with as many switching combinations as possible, the serial circuit with e.g. binary stepped values is recommended:
    Recommendations for use 2:.
    It requires switches that are accessible on both sides, such as provided by the EL2624.
  • To create a resistance simulation based on this serial circuit with the multiplex terminals is only possible with two switches of a group of 4 switches:
    Recommendations for use 3:
    A combination of both circuit types leads to further possible resistance values and partially redundant switching combinations.

Like any technical device, a switching terminal can fail: on the one hand by non-switching although it is supposed to switch, e.g. "contact destroyed" and, on the other hand, by permanent connection although it is supposed to disconnect, e.g. "contact sticks". Sufficient plausibility queries should be provided for this purpose on the receiving side.

Additional notes:

  • An external protective circuit is necessary with inductive/capacitive loads, e.g. a short-circuit diode.
  • External strong magnetic fields or vibrations/shocks can affect the function, for example, of reed relays
  • The devices discussed here have switching cycle counters in the CoE and it is recommended to observe them.
  • With long/frequent use, aging switching contacts exhibit a slowly increasing resistance before complete failure. Occasional checking of the switching function can be useful.
  • The repair of individual switches in the Beckhoff Service Dept. is possible.