Typical problems relating to electromechanical relays and optocouplers

Typical problems with signal transferring

Capacitive noise
When an AC cable is lying close to the signal cable, there is a capacitive link formed between the two cables. This link, sometimes called crosstalk or pickup, enables a small current to flow from the AC cable to the signal cable. When the signal cable is in the off state, this small current may affect the I/O relay concerned.

For instance, this may cause the electromechanical relay or optocouler to "latch on", LED can glow when the signal is off or in worst case it can switch the relay on even when the signal is off.

Long AC signal cables
Lond AC signal cables cause similar kind of problems to interface relays as mentioned above. This happens because AC cable has it's own inner capacitance. The capacitance of about 100 m AC cabel is enough to keep relay latching on and the capacitance of about 200 m cable could be long enough to switch the relay on. 

Delcon's solution
Delcon's interface relays contain a diode circuit which prevents this small AC current causing a false operation. The result is that when using Delcon's interface relays, the signal cable does not need to be physically separated from AC cables and that when usin AC for the signal, unshielded multicore cable can be used even on long runs.

Leakage currents
AC-proximity switches causes also similar kind of problems as mentioned above. These switches are protected my means of snubber and it leaks. Delcon has designed special relays which can stand both relatively high capacitive noise and the leakage current from the promixity switches. These Delcon relays are SLI 120CRP, SLI 230CRP, SLO P120TR and SLO P230TR.

Transient and high frequency suppression
The effect of magnetically coupled interference, typically RFI or transients, are eliminated in Delcon relays by the use of an RC circuit. This circuit absorbs interference up to the energy levels specified.

Switching points and hysteresis
The switching points of the relay should be clearly separated. In electromechanical relays this hysteresis is normally strong but in some cases the switch-off point might be too low and in some circumstances the relay might latch on.

In optocouplers this hysteresis is really weak. Switching-on and -off points are really close to each others and the voltage level is extreamely low comparing to the nominal.

The operation of the Delcon relay at its switching points is determined by the hysteresis circuits. The relays have switch-on and switch-off points which are clearly separated. The switch-on point is set at around 2/3 of nominal and the switch-off point at around 1/2 of nominal. This secures that Delcon relay does not latch on at any circumstances.

Status indication
Normal way to indicate status of the relay is to use LED. Common problem with status indication is that LED glows and gives wrong indication of the relay status.

Only 100 % status indication is in Delcon relays. Delcon relays have also build in current hysteresis which means that the relays impedance changes at the switching points. At switch-on the step increase in impedance tends to force the relay further on and at switch-off the reverse. Thus it is impossible for the Delcon relays to chatter or be in a middle state and switching is alway clean - even in noisy environments.

Short lifetime of the relay
Switching frequency and the size and type (resistive or inductive) of the load determines the life time of the relay.

With electromechanical relays the life time must be calculated with real load. Thumb of rule could be that if swithcing frequency is higher than once a minute then electromechanical relays must be changed during the life time of the automation system. Other option is to choose solid state relay (optocoupler or Delcon type of relay) where switching frequency does not effect to the lifetime.

Inductive AC and DC loads are hard for electomechanical relays where welding contacts cause problems. Normal way to solve this problem is to use clamp diodes or zener diodes in the poles of the load. However this lenghtens the off delay and it does not totally eliminate the sparking in relay contacts.

Switching the high DC voltages is also difficult for the electromechanical relays. For example the effective current of 12 A electromechanical relay in 250 VDC load is only about 0,3 A.

In Delcon relays these problems with inductive loads and high DC voltges are solved by using varistor as a protection element (Note! check correct relay type). With Delcon relays diodes or other extra components which makes process slower are not needed. Delcon relay features are effective and there is no need to overdimensioning of the relay.