Hello peeps,
Problem I have is for Emergency Lighting.
I need a circuit that turns on one supply when another is turned off -
When mains supply fails, circuit sends 12vdc from battery to emergency lights (turns off when supply resumes)

I know how to do this with a simple relay, but the relay would be energised for months at a time.
I'm worried about coil and contact reliability, the coil overheating and generally drawing current when not in use.

I'm looking for an electronic relay (thyristor?) which switches on when the supply is off.
12vdc is always available for control circuitry. Reliability, low temp and low current draw will be important.

I have to pass this idea by my boss so must be cheap and not likely to short or overheat.
Cheers guys and gals

 

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.

 

Reduce the power to the relay after it has switched on. A relay will generally hold in at 30% of its rated voltage, and that is 9% of the power.

Or you could use a MOSFET with its gate shorted to its source by a transistor to keep it off, and a pull-up resistor to switch it on.
If you use a thyristor to switch a battery, how are you going to switch the thyristor off? If you leave it on until the battery is depleted you will ruin the battery.

 

Cheers nsaspook, so you're saying use a relay and get on with life? Cool! Care to recommend such a relay (UK so 230VAC - I'd rather not use the 12v for control to keep it simple).

Great idea Ian0, a double throw relay can easily cut a resistor into the coil feed. I was never convinced by my thyristor idea anyway, but the phrase " MOSFET with its gate shorted to its source by a transistor to keep it off, and a pull-up resistor to switch it on " makes no more sense to me than greek. Is there a readily available (and cheap) circuit I can buy?

Thanks again peeps for your assistance.

 

That's exactly how emergency lights work. When mains power is present a relay holds a NC contact (Normally Closed) open. When power fails the NC contacts close and turn the lamps on. Also on board is a charger to keep the battery at float voltage. The ones I'm familiar with use SLA's (Sealed Lead Acid). Some big systems I've seen use Ni-Cad's, huge banks of them anywhere from 36 to 72 VDC, depending on the system. A simple relay with C (Common), NC and NO (Normally Open). Since the NC is closed when the relay is NOT energized (hence power failure) whatever is connected through C & NC will come on.

Here's a crude drawing of how they work:

 

Hello peeps,
Problem I have is for Emergency Lighting.
I need a circuit that turns on one supply when another is turned off -
When mains supply fails, circuit sends 12vdc from battery to emergency lights (turns off when supply resumes)

I know how to do this with a simple relay, but the relay would be energised for months at a time.
I'm worried about coil and contact reliability, the coil overheating and generally drawing current when not in use.

I'm looking for an electronic relay (thyristor?) which switches on when the supply is off.
12vdc is always available for control circuitry. Reliability, low temp and low current draw will be important.

I have to pass this idea by my boss so must be cheap and not likely to short or overheat.
Cheers guys and gals

SCR/Thyristor and a NOT gate on the trigger. Is it 12V everywhere, or is the voltage higher from regular source than battery? If so, a diode might do what you need, depending on current needed.

 

Only as an example of a 230VAC relay in a socket: Something like this should work in a general industrial application where size and bulk is not a problem.
https://www.phoenixcontact.com/en-us/products/single-relay-rel-or3l-230ac3x21-2903696?type=pdf
https://www.phoenixcontact.com/en-us/products/relay-socket-rif-3-bsc3x21-2900936

 

 

Does this look good?

 

Does this look good?

Sort of.
Compliance with BS5266-1 requires a more accurate sensing of the mains voltage than simply when a relay drops out.
Wherever you are there will probably be an equivalent standard.

 

Good point, Ian0. If the supply drops below the level required to run fluorescent tubes, but high enough to keep the coil energised, the system will not work. Somehow I'm OK with that (seems an improbable circumstance).
I'm in the UK and I'm referring to the IET electrician's guide to emergency lighting, bearing in mind some of my workplace has no emergency lighting at all, I consider anything an improvement!
As long as it looks good to the boss.

 

Does anyone see a problem with running the relay coil for months at a time?

 

Good point, Ian0. If the supply drops below the level required to run fluorescent tubes, but high enough to keep the coil energised, the system will not work. Somehow I'm OK with that (seems an improbable circumstance).
I'm in the UK and I'm referring to the IET electrician's guide to emergency lighting, bearing in mind some of my workplace has no emergency lighting at all, I consider anything an improvement!
As long as it looks good to the boss.

Might not look so good to the HSE, and they could shut your business down until you got a complaint system.

 

Does anyone see a problem with running the relay coil for months at a time?

No - that’s what they are designed for.
AC coil relays have a neat trick. The current is inversely proportional to the inductance. When the contacts close, the gap in the magnetic circuit also closes, increasing the inductance, and reducing the current.

 

Ooh Ian0! Seems a bit like the Nuclear Option! Well, we haven't had emergency lighting in some areas since the factory went up in the days of Queen Victoria, and the HSE haven't closed us down yet!

Thanks for putting my mind at rest re. the constantly energised relay. Are AC coils rectified (I've assumed a diode and a smoothing capacitor)?

 

While the relay circuit will work, the selection of the relay will indeed require one with a dropout voltage high enough to cause it to release in a brown-out, MAYBE!
But I have not seen a mechanical relay in an emergency lighting system since 1980. Presently all of the battery backed systems use a transistor of some variety, either FET or Bipolar, that is biased off while the mains power is active, and biased into conduction when the mains power fails. The transistor switching is conveniently added to the charging assembly needed to keep the standby battery charged, so the added complexity is not much.
The transistor switch uses much less standby power, and it is less expensive both initially and in operation.

 

MisterBill2, I agree, I don't like my relay idea very much. But I don't know what I need to know to make it transistor controlled.
What am I missing?

 

the circuit to have the transistor switch on is not complex, until you try to add charging the battery and keeping the cost very low. If there is already a battery charging circuit then it can be simpler. I don't have that circuit handy, but somebody who reads this may. What makes it a challenge is how to use the same supply that charges the battery to bias the transistor off while the power is present, without a lot of parts.
The challenge with charging the battery is that to have the battery last many years the trickle charge must be quite low, just enough to compensate for the internal battery leakage. But then some agencies demand that if the thing is useda bit that the battery must be recharged in just a couple of hours, which is a totally different charge rate, much greater. That high a charge rate will damage a battery in a day.

 

I have bought - apparently - a smart charger that changes charge rate and can be left connected.
I have no idea about using a tranny as a switch - I could just about make an amp, but I'm baffled - would the whole of the LED current flow through the tranny?

 

Is this an improvement? R1 keeps the coil voltage and current down, and makes the coil more sensitive to a brown-out voltage.
I can adjust it until the LEDs come on just before most of the fluorescents die.
SW1 is a reset. As the voltage required to trigger the relay is higher than that needed to hold it (cheers for the tip, Ian0), the switch can turn the LEDs off when supply returns (if supply isn't sufficient to trigger it).