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LED hallway lighting

Ray wrote:

Those nicad C cells are 1.2V, not 1.5V. Thus, your resistors should be a bit smaller. However, 30mA is pretty high for most LEDs, so perhaps a larger resistor will let them live a bit longer.

The other issue is that charging the batteries when they are in series is a potential problem, since if one of the cells fails to charge, the other cells will get toasted.

Nicads like constant current at 0.1C, so a constant current charger of

150mA is appropriate for these guys. However, you also need to notice when they get charged up, and turn off the current until they drop some amount.

A constant current source is easy to make out of an LM317, a reference, and a comparator. You turn off the LM317 by pulling the common output to ground using the comparator output. The comparator needs enough hysteresis to not turn on until it gets down to 4.6, and turn off at

4.8. A normal recharge circuit would wait until 4.4V, but for a system like this, seems like you would want to keep the batteries as high as possible without frying them with overcharge or trickle. 12V | Constant Current Charger 150mA V With shutoff at 4.8V, Turnon at 4.6V - | o--. .------------------------------------. | | | | .-. | | _____ | 1k| | | | | | ___ | | | '----------------)---------------|LM317|--|___|----. | '-' | |_____| 12R | | | | LM339 | ___ | | o----. | o-----|___|----o | | | | |\ | 10K | | .-. | '----|-\ | o--' 10k| | | ___ | >----------o | | | o------|___|---o----|+/ | 4.8V --- '-' | 1k | |/ | - | | K | | | | R - TL431A | ___ | | o----^ '-----|___|--------' | | | 22k | .-. | A | 11k| | | | | | | | '-' | | | | | -o----o------------------------------------------------' (created by AACircuit v1.28.5 beta 02/06/05
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You should also use a 12V relay, which is set up so that if the coil is powered, the LEDs are powered from a 5V regulator driven off of the 12V DC input; otherwise, they are powered from the batteries.

When the power goes out, the relay switches, and the LEDs are powered by the battery. The LEDs need their own resistors, of course.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.
Reply to
Robert Monsen
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What I want to do is use 6 white LEDs that are rated at 3.5V forward v and max forward I at 30mA. I am going to use 4 1500mAh NiCad sub c batteries (because I have a few of them)in series to power up the LEDs in parallel. I know that I'll need about 50-75ohm resistance to limit the current to about

25mA.

I plan on using a 12V dc wall transformer to power the LEDs and charge the batteries when the power is on. What I want to happen is when the house power is on, the batteries get charged and then switch to trickle charge to maintain them and power the LEDs all the time. When the house power goes out, the batteries will power the LEDs.

The system should work something like an exit sign from

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What I'm looking for is a schematic for the exit signs from the link above or a simular type circuit.

-Ray

Reply to
Ray

Ray wrote

Don't wire the l.e.ds in parallel and use a common R. Use a separate R for each l.e.d.

Reply to
dB

Robert Monsen wrote in news: snipped-for-privacy@comcast.com:

Thanks, that's exactly what I was looking for.

30mA is the maximum current rating of these LEDs. They can take up to about 100mA and then the genie is released. They probably won't last the 100,000 hours at 30mA. Your eye can't tell the difference in brightness when powering them with 20 - 30mA, so I will keep them down around 20mA or less. 
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-Ray
Reply to
Ray

I agree.

Reply to
Tom Biasi

What's the benefit of using seperate resistors as long as you keep the power thru the resistor below the watt rating?

Reply to
GotCoffee

The problem is that LEDs vary as to the amount of current they require at a given voltage, and the ratio of currents is exponential in changes of voltage. Thus, even a tiny difference in devices can cause a big current difference, leading to one LED which is much brighter than the others. Since this difference gets bigger as the device gets hotter, the bright LED will tend to get brighter as it heats up. This can cause it to fail.

The typical solution is to use separate resistors, because even a little bit of resistance will tend to compensate for this tendency.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.
Reply to
Robert Monsen

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