Mixed and un-matched electronics

[bernardgreen]

How often does one hear this about the switching outputs of a central heating controller.

""It is only a switch, it is either ON or OFF ""

The simple switch is most probably a relay or it might be a solid state switch.

Which often invokes a comment

"" A relay is only a switch that switches ON when current flows through the coil ""

It is far more complicated than that. https://www.machinedesign.com/archive/article/21814917/tips-for-selecting-relays
as a strating point for learning the reality of relays.

Some controllers with simple ON - OFF control are said to be capable of switching either mains 230 V AC or Extra Low Voltage ( typically 12 V or 24 V AC or DC )

The public demand for "fashion" controllers and mix and match installing by DIYers can end up with a relay being abused.

A relay designed to switch 230 Volt AC but used to switch 24 V DC may have a short life due to prolonged arcing when the contacts open. ( when switching an AC voltage the arc will extinguish within 20 mSec ( at the zero crossing point of the AC waveform ).

Controllers with failed relays ( burnt out contacts ) seem to be too common in heating systems.

 

[Harry Bloomfield]

The simple switch is most probably a relay or it might be a solid state switch.

Which often invokes a comment

"" A relay is only a switch that switches ON when current flows through the coil ""

Just a comment on that - a relay can be either NO or NC.

 

[bernardgreen]

Just a comment on that - a relay can be either NO or NC.

or even CO ( change over )

and to compound the issue a relay can have QO and QC (Q= quiescent ) when the normal operating state is for the coil to be continuously energised and only de-energised when something happens.

 

[DP]

Bernard, are you implying the relay that will get the boiler to start up or stop is going to arc and fail?
I have in 30 years not had one such problem ie where relay ( there are vast shapes, sizes etc type of relay here) contacts have eroded due to boiler starting and stopping.

 

[DP]

and to compound the issue a relay can have QO and QC (Q= quiescent ) when the normal operating state is for the coil to be continuously energised and only de-energised when something happens.

Relay operation will be to perform a function. Why energised a single relay so that a contact can be on or off when same can be done with relay unenergised. Obviously when several relays perform different functions, then a relay may need to be energised, short circuit the coil ( yes I did say short circuit) or whatever to carry out a certain function.

 

[bernardgreen]

Why energised a single relay so that a contact can be on or off when same can be done with relay unenergised.

Alarm circuits feed the coil of a relay via a loop of normally closed contacts. Open a contact or cut the cable and the relay drops out and it NC ( or QO ) contact switches the alarm sounder ON

short circuit the coil

Used to slow the drop out time of the relay, requires a current limiting resistor to prevent the short drawing excessive power for the supply.