All of that extra circuitry increases cost and decreases overall reliability over a simple passive device designed to run for decades energized.

 

I can tell you that ALL of the current exit lights and small emergency light packages have been using transistor switching for many years. At least 20 years.

 

The relay being energised for years at a time shouldn't be a problem with a good sealed relay. We have industrial relays (in life-safety systems, EMO buttons in series with relay coils) that have been energized for decades.

Way back in the beginning I liked this simple solution and have to agree with it. I had common 120 and 240 volt coil relays in service for decades. This is what they were designed for and what they did. They were off the shelf good quality relays. They were also inexpensive.

Next when we start wanting to consider things like brownouts we start adding parts and complexity. So ask yourself just how important that is. All you really care about is emergency lighting. Mains power fails you want lights so people don't get hurt. Your options are pretty simple. You roll your own or you invest in a off the shelf system. Using a 12 volt battery you just look at your load and decide how long you want your batteries to last. I would add a battery maintainer / tender just to make sure your batteries or battery are always topped off.

Anyway as to roll your own personally I would just run with a simple relay capable of handling your DC load. They work fine and last a long time. Today's commercial units also use low current LED lightswhich work just fine and yes. they use electronic switching.

Ron

 

Wow guys and gals,
what a complete answer you have given me!
On the Solid State Vs. Electromechanical switching argument (which will continue for decades in some applications), I'll make a mock-up of this circuit and test it. If I remember I'll upload a photo and circuit drawing for y'all to browse over. I'll let you know how it functions, and whether my boss will let me install it.

Cheers Nsaspook for supporting my initial idea of relay control, and Ian0 for your advice about dropping the coil voltage once energised.
Thanks everyone else for your tuppences, all helped me figure this all out.

 

Just remember, with Electromechanical switching that emergency light will likely still work after the Nuclear EMP that took out the entire power grid.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021EA001792

I still use mechanical relays as isolation elements in high voltage/high energy environments for status signals after having several very expensive vacuum system drivers die when beam transients smoked the open collector opto-isolator chips inside the modules.

 

Sounds like a painful problem, Nsaspook. Still, advocating for EM over tranny supports my cause, so thanks for that (I've no idea what a beam transient is, if you hadn't noticed).

But, you mention a Nuclear EMP.
Would my system have any defence against something like the the 1859 Carrington Event?
Say I have too much faith in human nature, but I'm more worried about a Carrington mkII than a nuclear strike.
Although I'm more worried about my company being taken over by the Chinese than either of these things!

 

Sounds like a painful problem, Nsaspook. Still, advocating for EM over tranny supports my cause, so thanks for that (I've no idea what a beam transient is, if you hadn't noticed).

But, you mention a Nuclear EMP.
Would my system have any defence against something like the the 1859 Carrington Event?
Say I have too much faith in human nature, but I'm more worried about a Carrington mkII than a nuclear strike.
Although I'm more worried about my company being taken over by the Chinese than either of these things!

I'm joking obviously but there has been work at the industrial scale to harden facilities.

 

I wasn't being entirely sensible myself!
But on the subject:
Would an earthed faraday cage protect against Armageddon or Carrington?
Cool graffix BTW!

 

I wasn't being entirely sensible myself!
But on the subject:
Would an earthed faraday cage protect against Armageddon or Carrington?
Cool graffix BTW!

The CME EM effects are very low frequency/DC magnetic fields that can saturate and overheat large utility scale transformers and magnetic devices. Conventional faraday cage shields (earthed is not a requirement for a effective shield) are useless unless they also have very high permeability magnetic flux shunting plates.
https://www.kjmagnetics.com/blog.asp?p=shielding-materials

First, one important point must be clear: Magnetic shielding does not block a magnetic field. No material can stop the lines of flux from traveling from a magnet's north pole to it's south pole. The field can, however, be redirected.

https://fluxtrol.com/composite-materials-for-magnetic-field-control-in-EPM

 

I wasn't being entirely sensible myself!
But on the subject:
Would an earthed faraday cage protect against Armageddon or Carrington?
Cool graffix BTW!

Well I figure it this way, when the party is over it's over. Matters not.

Ron

 

Well I figure it this way, when the party is over it's over. Matters not.

Ron

I'll be a ground-zero with a stiff drink and a good cigar.

 

View attachment 271464
I'll be a ground-zero with a stiff drink and a good cigar.

Funny you should post that. Planning on several days away with family and among things I bring are several Cohiba Cigars and some fine Bourbon and Scotch. My son brings same and evenings along with son in law we sit out and enjoy.

Ron

 

The world as we knew it survived that last So suddenly things will be rather different, massive CME a hundred years ago, did not end the world. The main part the next CME would damage will be everything electrical. And electronic. Other stuff should survive. So suddenly life will be different. No more cell phones and no more internet and no computer driven cars.

 

No ••• more internet and no computer driven cars.

No more internet? No more computer driven cars? How am I to go to sleep and where?

 

No more internet? No more computer driven cars? How am I to go to sleep and where?

Much of the ip backbone would survive a CME or EMP due to fiber and DOD rated countermeasures for critical infrastructure. The old hydro power grid backup systems would still be capable once the distro system stopped burning. Local power from diesel gensets and engines should be easy to recover.

 

Local power from diesel gensets and engines should be easy to recover.

Oh good. I can still cut my grass. Whew!

OK, we've slipped off topic a bit. Sorry Mod's; I'll stop now.

I can't believe we're still belaboring the argument. An AC relay designed for mains voltage will do just fine. Wired as shown previously, the battery will remain charged if held at a float voltage. 12V SLA float voltage is 13.6 minimum, 13.8 max for a float charge. After a discharge they can withstand short term charging at 14.5 volts no problem. They will take whatever current they need or whatever current the charger can provide. There are smart chargers and not so smart chargers. The not so smart ones consist of a transformer and a diode. Probably a current meter and maybe even a volt meter. Can't get more simple than that.

 

If there is a tendency for the relay to buzz the simple fix is another diode across the coil in the opposite direction from the external diode. That works better than a capacitor at stopping the buzz, because during the half cycle when the supply is cut off it allows current to keep flowing, keeping up the magnetism. It is an old trick of mine, it works well, but many people do not believe it.

Relay PREL-SL-1-UKE-M1-230W-12-5,0 :



________

 

Like I said, folks. A bit less power and not quite as warm. Simulation software was not available to most folks at the time that I first used this technique. Note hat the current does not quite reach zero.

 

Happy Monday, people!How does this compare?
Would a 12v relay, protected from battery current by a diode, work better (safer, cooler, lower wattage, greater reliability etc.) than mains?
As soon as I thought of this, I wondered why I even considered a mains relay!
Solves all of my confusion as to how an AC relay works (but thanks again peeps for your explanations).
Does introduce a new problem though - would the voltage drop over the diode confuse the "smart" charger?

I've added a regulator (for a reverse camera in cars) to hold the LED voltage steady as opposed to a "proper" LED driver.
(I know, LEDs are current, not voltage driven, but I'm not sure what that means for me or my circuit).

 

It can work, for LED emergency lighting although it will be better to use just a resistor to set LED voltage so that the LED current will stay reasonable. An added advantage of operating the relay from the supply is that if the supply has a few swcods of hold up time, the LEDs will not flash on with every mains power momentary blink. The"blinks" that set digital clocks flashing.
But if it is for LED general lighting, then you also are missing the connection from the relay N.O. contact to the 12 volt supply.