Emergency light to LED?

[tawa]

Ok. I got this idea of having some battery power source, like 12v, and having it hooked to LED lights, and having it plug into 110v for recharging. So in an emergency, I would have the battery charged, and have the LED lights hooked up already.

I want to know the easiest way to do this. What about converting one of those business emergency lights you see in the US? Usually they have a box with a battery, then above the box they have 2 floodlights pointing in different directions. One of those should contain all the circuitry to recharge a battery, and power detection circuits to turn on the lights when the 110v goes out.

Do Exit signs in the US work this way? Surely the construction code requires a battery backup for exit signs in the US.

Anyone have any other ideas? Are the emergency floodlight type things expensive? Like more than $50US?

[drdongle]

Some models of Exit signs do have battery back up, wither they get installed is a matter of local code. Newer ones use LED's and LED retrofit kits are available for older incandescent lamp units.

Carpe Vigor

Dr.D

[srnoth]

Ever thought of using a relay to activate the lights? This is what I did for a small emergency lights setup. Just use any old power adapter between 6 and 12 volts to power the relay. Wire it so that when the power adapter is plugged in, the relay disconnects the circuit, and when the power is removed to the power adapter, the relay connects the circuit and thus the LEDs turn on. You can pick up a good relay for 3 bucks all over the place. If you want to buy online you can try the surplus centre at: http://www.surpluscenter.com/.

You don't need to get a high amperage relay since the LEDs don't use that much power.

About the charger - If you have an old UPS (preferably a small, 12V one <500W, if you have a bigger one, you can use that too, but your wasting a good inverter, unless you hook it up to a big enough battery). The UPS is designed to charge 12v batteries, and so that should work perfectly. You can also use a 13.5V power adapter from radio shack or one of those places.

Or how about using a small solar panel to keep the batteries topped up?

Cheers,

Stephen.

[betwixt]

I've done all of the things mentioned earlier and taken a few more steps too. My home needed complete renovations so while putting in the main electrical cables, I fitted a secondary lighting cable which feeds white LEDs fitted inside some of the normal lamp fittings.

The LEDs run from a motorcycle battery (6V/10Ah) and are switched on automatically by a small control board. I originally used the relay trick to switch the LEDs on when the relay coil was NOT mains powered and to route the output of a small mains powered charger to the battery when it WAS powered. The trouble with this strategy is the LEDs wasted power by being on all day if the main power failed and the battery discharged prematurely.

I got round the problem by dispensing with the relay and using some simple electronics instead. Now the LEDs are turned off during daylight by sensing the voltage from a PV cell on the roof. The PV also charges the battery when the LEDs are off.

I have the design on www.atv-projects.com if anyones interested.

Brian.

[RC in FL]

LED lights are pretty fair for lumens output for actual LED power consumption.  The tough part is getting a electronic driver design that does not waste a lot of power.

White and Blue LED's require between 4.2 to 4.5 vdc.  All LED's act as a diode meaning that the current goes up exponentially around this voltage so driving LED's from a voltage source is not appropriate. (you will likely blow out the LED)

The simplest circuit is to add a ballast resistor in series with the LED but this is very inefficient with a lot of power loss in the resistor.

The most efficient way to run LED's is with a switching power supply that uses an inductor and current sense control circuit that effectively creates a current source with very high conversion efficiency.

[ghurd]

I've done a lot of these lights. Some are over 5 years old with no maintance or problem.

A decent EXIT sign costs well over the $50, and the battery only lasts about 2 years (constantly overcharged). Same with the emergency floodlights.

Most battery chargers overcharge, no matter what the box says.

The ballast resistor is insugnifigant in this application, dollarwise.

A $20 PV, $25 battery, $2 controller, and the light should be plenty.

A 2w PV, 2 to 7ah battery, PCB137ACV ($1 charge controller chip), and LED bulb.

Good LEDs are cheaper in the long run. They allow for a smaller battery, PV, less LEDs, etc.

G-

[tawa]

> Why not use solar panels?

Because solar output during the 6 months of winter in Michigan is about nil.

[tawa]

I've never used a relay before. Is there a web site on how to wire it?

How many leads does it have?

Is there one for negative, then 2 for hot/positive?

Are relays rated by the max volts or amps they will support?

[srnoth]

Relays are basically switches. There are two main leads, which, when voltage passes through, switches the switch. Then there are usually about 12 other leads, each a separate switch, whith the main hot, hot when on and hot when off, for each. This may vary, as the relay may only act as two switches, and therefore have the two main and six other leads for the two switches. It is actually quite simple to figure out without a wiring diagram. Just use the 'continuaty' mode on your tester, and that way you can figure out which lead is hot when there is not voltage applied to the main leads, and then apply voltage to the main leads (for most relays 12v is fine), and test to see which leads are hot when voltage is applied.

Relays most likely have a max voltage rating, but the more important rating is the maximum amps rating. (Actually, there will probably be two voltage ratings, one will tell you how many volts to use to activate the relay, and the other will be how many volts the relay can switch). So basically, what you need to be concerned about it how many volts you use to activate the relay. I used an old automotive relay, rated for 12v to activate it, but I had an old 6v power adapter lying around, so I used that instead, and it worked fine. THe other thing is how many amps the relay will need to switch. I presume that you will the LED circuit that you will be switching will be no more than 12v, so voltage will not be a problem. What you need to figure out is how many amps will the circuit be running at. Once you have figured this out, you must make sure that the relay is rated at a higher amps than that. The reason for this (of which I can bear testiment) is that if you overload the relay (pass too many amps through it) the relay contacts will get so hot that they will weld together, rendering the relay useless.

Keep on having fun,

Cheers,

Stephen.