Perhaps a NCVD would serve the same purpose.

A quick Google turned up this. Saw another one for little over $300 but that's a bit pricy.

That's a way. Thing is, if this is critical as the TS seems to be suggesting it is, then doing it faster and dirty as he is choosing to do, to avoid a board revision, seems odd. Since this is a commercial product, it appears, and safety related, I'm out of this one.

 

I understand the desire to invent something new. Or something purpose built that may become something others want to do as well. Whatever the TS is after, the best we can do is work with the information provided. What I gather is the TS has a 12 volt DC source that controls a relay. When the relay is activated he wants a clear indication that the relay is energized. Two ways - either detect the presence of the 12 volts or detect the presence of the high voltage being switched. I, too, think I've offered as much as I can to this thread. If the TS wishes to pursue a different route - that's up to him/her. I'm out of ideas.

 

Hi Yaakov,

You misunderstand - this is NOT critical through a safety issue. The identification of the coil operating is critical to us so we do not throw 1200 people out on the streets from a hotel, for example, at 2 in the morning when the fire alarm goes off There are no safety factors that have been overlooked on the project and if you read the posts back through, I am sure you will see what I actually wanted to achieve.

It is NOT yet a Commercial product, as is currently being run as a prototype and under development to CE conformity.



I appreciate all your input, but we never required, as per my original post, to monitor the 230V feed as this will always be live

 

So, although does not deal with the issue of monitoring the actual relay contacts working, your solution mvas works and the unit functions as expected and indicates the coils energized.

Thank you.

Even if you add an indicator to the "Contact" side of the circuit,
I do like having the additional confirmation that the Relay Coil, is or is not energized.

Also, do not use a series connected resistor to dim your Red LED ...

Add a ( 50 - 100 ohm ? ) resistor connected in parallel to your Red LED:
a) Reduces the LED brightness
b) Reduces the milliamps flowing through the LED to below 20 ma ( max ma rating ? )
c) Increases the milliamps to the Relay Coil.
It is a win-win-win.

 

Hi Yaakov,

You misunderstand - this is NOT critical through a safety issue.

OK. However adding a point of failure to the relay doesn't seem right. I think it would be better to use a high impedance connection to the low side of the relay to tell if the circuit is complete. It wouldn't have to be in the path and become a dependency.

Good luck.

 

I do like having the additional confirmation that the Relay Coil, is or is not energized.

One end of the coil is the business end, the end that throws the armature switch. The other end just sits there. An HET (Hall Effects) can sense the presence of a magnetic field and can indicate when the coil is active. Whether the armature throws or not or makes contact or not - the only way to be certain of that is to monitor the high voltage. A line is only live as far as it is connected. However, the close proximity of the switch contacts may cause false indications of the presence of high voltage. Monitoring the magnetic field will only indicate when the coil is powered or not. Whether the relay does its thing or not - that is another question.

 

The "alarm" industry uses "monitored contacts". You can detect a wiring short, a wiring open, an alarm and an "OK" signal from just two wires.

I did a simple latch/indicate generate a shutdown/enable signal for about 8 inputs. None of my inputs initiailly had monitored contacts.
I used 3 PDT relays: 1 or the alarm loop, one for the indicator loop and one for the latch.

It was a 24 VAC. Contact closures activated the "alarm". The "alarm" didn't have to make noise. It just had to indicate the cause.
A "real alarm" would come from other devices that activated the fire alarm. The fire alarm, was also a shutdown signal. RESET was from power fail and Panic buttons, one labeled OFF(mom) on the control panel itself.. Panic button(s) were not indicated. In the long run, it was probably safer. They were NC circuits.

It became necessary to "monitor the fire alarm contacts" for two reasons:
1. We could not tell when the fire alarm had cleared.
2. There was a way to "disconnect" the fire alarm signal on purpose.

The fire alarm already has SPDT contacts, so a LED could show the current state. Later, during an upgrade. a low level Hydrogen and Low Level toxic gas got the same indicators. A single wire cut could still be a problem.

It was wise to latch the alarm source. Another system was designed (not by me) using the sequential interlock model and the system was designed using two rack mounted enclosures.

They has nuscense trips of unknown origins. Most of the time it was "hood velocity". They chose no display of actual velocity, so a gust of wind could cause a shutdown if the sensor was dirty or mispositioned. Repair was impossible. Later. I, I got power to the wall box and now, at least, you could disconnect the "interlock control", which it should not have been designed as, from the building signals.

One thing I insisted on is that all "communication" between various room panels were done with contact closures and any lighting of indicators must be done with the "local power supply".

I can say, that the lab "blew up" once and the system worked because the explosion broke the hood and gas cabinet velocity sensors. The Hydrogen alarm got installed. The alarm was from a Hydrogen explosion. The investigation, which I was part of, led to a large number of changes to gas handling, gas shutdown, removal of the excess flow valves that failed and now replaced with orfices. Some Cylinders had dedicated purge tanks. A line rated for vacuum basically bursted when the excess flow valve failed open during a cylinder change. During pump an purge of the regulator cylinder pressure "could" reach the vacuum piping.

My involvement included the upgrade of the velocity alarms and changing the gas cylinder valves from 120 VAC ones in the gas cabinet to air activated pilot valves outside the cabinet. A fire within the cabinet would hopefully sever the air supply to the air operated cylinder valve and shut off the cylinder.

Part of the very original design was to only select the gases that were needed in the experiment.

The major point is the use of monitored contacts. You can create them with two equal valued resistors and a DC supply. Say 1K.
If when an external point is activated from an alarm loop that is normally closed. Place a 1K resistor in series with the wiring and a 1K resistor in parallel with the NC switch. If you drive the loop with a current source, you can differentiate open wiring, shorted wiring, alarm and OK. Open and shorted wiring is efectively "TAMPER" or "TROUBLE".