Brissieboy
13th August 2018, 02:29 PM
A lot of installations using Carling type switches require/use a relay instead of directly switching the load.
The way most of these switches are configured almost always puts the internal on/off indicator LED directly across the load - like this:
76076
This is fine for non-inductive loads, but when switching a relay there is a significant back EMF produced when the relay is de-energised.
This will put a large reverse voltage across the LED.
A LED's maximum reverse voltage is very difficult to obtain with any degree of certainty, but with the older types it is considerably greater than the more modern 'super-bright' ones which are generally speced very low (as low as 5V according to a lot of information). But they will all be far less than the peak back EMF which could be in the vicinity of 1,000 - 1,500V.
Does anyone know if any protection is provided for the LEDs within the switch (apart from the current limiting resistor)? I have not been able find any relevant information on the web.
Has anyone had LEDs fail when switching relays? Have you ever dismantled one of these switches?
It is a relatively simple matter to resolve the issue with a suitable diode across the relay like this below, but I'd like to know if it is really necessary.
76074
The way most of these switches are configured almost always puts the internal on/off indicator LED directly across the load - like this:
76076
This is fine for non-inductive loads, but when switching a relay there is a significant back EMF produced when the relay is de-energised.
This will put a large reverse voltage across the LED.
A LED's maximum reverse voltage is very difficult to obtain with any degree of certainty, but with the older types it is considerably greater than the more modern 'super-bright' ones which are generally speced very low (as low as 5V according to a lot of information). But they will all be far less than the peak back EMF which could be in the vicinity of 1,000 - 1,500V.
Does anyone know if any protection is provided for the LEDs within the switch (apart from the current limiting resistor)? I have not been able find any relevant information on the web.
Has anyone had LEDs fail when switching relays? Have you ever dismantled one of these switches?
It is a relatively simple matter to resolve the issue with a suitable diode across the relay like this below, but I'd like to know if it is really necessary.
76074