# l.e.d light

Discussion in 'LEDs and Optoelectronics' started by mogy, Apr 10, 2011.

1. ### mogy

7
0
Apr 10, 2011
i have a
' LED Number: 42
Total light generated: 200 lumen
LED Feature: wide view angle (120 degree)
Emission color: blue (465-470nm), red (630nm)
Color ratio: red LED/blue LED = 2:1
Dimension: 18.9" x 0.4" (479 x 10mm)
Power cord length: 10"
Working voltage: 12V DC
Power requirement: 2.5W
and a 12v 7ah batery. How do i safely connect them together? i also have a 1300mah 12v nicd.
any idea,s

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Read this and then come back with any questions you have.

3. ### mogy

7
0
Apr 10, 2011
re. l.e.d

i might be wrong,but, does that information suggest a current limiting resistor? (dyslexia)

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
You might be right.

Some form of current limiting is *essential*

5. ### mogy

7
0
Apr 10, 2011
any ideas? (12v 2.4 watt, 12v 7ah lead acid batery)

6. ### mogy

7
0
Apr 10, 2011
i looked at resistor calculator, (voltage drop defeated me!)

7
0
Apr 10, 2011
insperation:

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
If that is the device you are using, then it has resistors incorporated in it and can operate from a specified voltage.

Otherwise, you need to determine the following:

a) LED forward voltage drop (or sum of these if you connect several in series)
b) LED current
c) Power supply voltage

Add from these calculate the series resistor value.

(a) and (b) are typically specified for a device, often in terms of a graph showing one vs. the other. You will want to choose a LED current less than the maximum, and use this to determine the forward voltage drop.

You have already told us that (c) is 12V

If you don't have specs for the LEDs then there are some rough figures for forward voltage drop in the article I pointed you to.

9. ### mogy

7
0
Apr 10, 2011
yep i just checked, resistors already calculated, any joy for betwean unit and batery?
(12v 7ah lead acid, or, 12v 1300mah nicd)

10. ### mogy

7
0
Apr 10, 2011
my circuit will determine the actual current! (didn,t seem obviouse to me...)
anyway thanks for the help! usefull

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Yes, your circuit determines the current. Your choice of battery should be based on current, discharge characteristics, capacity, weight, size, required run time, etc.

The 7Ah battery has just over five times the capacity of the 1.3Ah nicad pack. You can find the approximate run time by dividing the Ah rating by the current in amps.

So if your strings of LEDs draw 120mA (0.12A) then the 7Ah battery will last 7/0.12 = 58 hours and the nicad pack would last just under 11 hours. However it is generally considered poor form to discharge a lead acid battery below 50% on a routine basis, so 28 hours for the lead acid battery vs, 10 for the nicad pack are more reasonable figures.

Of course these figures are based on a totally made up current draw that almost certainly does not correspond with what you have. You should repeat the calculations with your actual figures.

It may also be a good idea to show me how you plan on connecting the LEDs so that I can double-check that you have everything correct.