LED hallway lighting

R

Ray

Guest
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 www.dual-lite.com

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


-Ray
 
Ray wrote:
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 www.dual-lite.com

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


-Ray
Click to expand...
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 www.tech-chat.de)

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.
 
Robert Monsen <rcsurname@comcast.net> wrote in
news:pJCdnfp16deUKNLfRVn-sg@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.



--
-Ray
 
I agree.


"dB" <dmb06851@yahoo.com> wrote in message
news:1757808.0504031302.7219a747@posting.google.com...
Ray <ray.straub@gmail.com> wrote

I am going to use 4 1500mAh NiCad sub c batteries
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.
-Ray

Don't wire the l.e.ds in parallel and use a common R. Use a separate
R for each l.e.d.
Click to expand...
 
GotCoffee wrote:
What's the benefit of using seperate resistors as long as you keep the
power thru the resistor below the watt rating?
Click to expand...
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.
 
Ray <ray.straub@gmail.com> wrote

I am going to use 4 1500mAh NiCad sub c batteries
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.
-Ray
Click to expand...
Don't wire the l.e.ds in parallel and use a common R. Use a separate
R for each l.e.d.
 
What's the benefit of using seperate resistors as long as you keep the
power thru the resistor below the watt rating?
 
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