Monitoring a relay for safety matters

[mattiad]

Dear All,

I have a circuit in which I drive 2 separate relays with a micro and now I need to monitor the status of these two, in order to guarantee a particular safety.
I would like to avoid to use a "safety relay" which has a double contact that let me check the status of one contact with another one but because they are too expensive for my project.
So, my idea is to use a single contact relay and to sense the current on it (or, eventually, to measure the voltage in parallel).
My biggest problem is that I don't know which kind of output voltage is applied to the relay: the voltage could vary from 24 to 48 in DC but could also be up to 115V in AC. Furthermore I don't know the average current which flows into the relay.
Do you have any suggestion of monitoring?

Thank you in advance,


Mattia

 

[FvM]

A regular microcontroller without hardware redundancy will be hardly recognized as safety device, neither current monitoring as "poor mans" safety relay replacement.

 

[DeadBugEngineering]

You write that you want to sense the current of the single contact relay. Which current? The current in the coil or the current that flows through the contacts?
Option 1 is not safe, since the relay may fail.

What's your budget for this circuit?

 

[D.A.(Tony)Stewart]

You need to define the criteria for safety as minimum voltage or minimum current in the active state then design accordingly.

I once designed a box with 100 Power Relays 15A and 30A , each had signal contacts for confirming operation. Unfortunately my signal was TTL and the contacts were not gold plated, so it caused contact oxidation and failure. I got around this problem by wetting the contacts with a tantalum cap across the contacts and pullup resistor to 5V so the contact clousre would burn off any oxide and keep it operational. Gold plated contacts are expensive and rated for <2A but tin plated nickel-silver contacts for signal switching are cheap. The problem is the silver oxidizes.

Another solution is use an optocoupler with a logic level output , reverse protection diode on input and current limiting resistor and possibly a 12V zener so that it MUST Activate at 20V but never activate at 15V and not exceed input current at 200V. Careful selection using very low input currents and tight CTR range of 2:1. This is often harder than it sounds and could be done perhaps with a PTC.

Dual contacts are probably best.