Voltage and current regulator.

[vasilios]

Hi everybody.
I finally started to build my first divelight (a handheld backuplight)

The lightsourse is a ostar LE UW E3B (1120lumen, 6500kelvin)
The powersourse is 8 li-ion in series (29,6v, 900mah)
The main component for the powerregulator is a LM200 voltageregulator with current limiting.

I need help to calculate the size of the resistors so the output will be 21v and 700mah.

 

[georges80]

Hi everybody.
I finally started to build my first divelight (a handheld backuplight)

The lightsourse is a ostar LE UW E3B (1120lumen, 6500kelvin)
The powersourse is 8 li-ion in series (29,6v, 900mah)
The main component for the powerregulator is a LM200 voltageregulator with current limiting.

I need help to calculate the size of the resistors so the output will be 21v and 700mah.

Umm... you really don't want to use a linear regulator unless you have power to burn - literally....

You have 8 x 4.2 = 33.6V (hot off the charger) and want to drop 33.6 - 21 = 12.6V and provide 700mA to your LED, so 12.6 x 0.7 = 8.9W needs to be dissipated (wasted = heat).

As the batteries settle to around 3.7V per cell you have 3.7 x 8 = 29.6V and still have to drop 29.6 - 21 = 8.6V at 700mA, so now wasting 6W (heat)

If you use a linear regulator you are better off removing at least 1 li-ion battery from your pack - it's just being turned into waste heat...

Better yet, use a LED driver with a switcher that will do a DC to DC conversion and run around 90% or better efficiency and still have constant current going to your LED.

cheers,
george.

 

[vasilios]

I am not good with electronics, please help me to figure this out.

I believed the L200 worked with a pwm signal and did not produce so much heat.

 

[lucca brassi]

For your needs (simplicity) I recomend much more simpler and safer linear (L200 I have some problems in past with self-oscilating ) regulator LM317T in TO220 package
all you need you have described here : as current regulated circuit.
http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm

Important it is that in your case input voltage is cca 1,5V higher than output voltage (but not too high-because too much voltage difference convert in to the heat )
You'll need 7 li-ion in that case; (even you can try with 6 li -ion), LM317T , 1,8 ohm resistor at 1W or better more(3W).

 

[Klem]

Unsure what you mean by Li-ion's, "settle" at 3.7V

As I understand it, 3.7V is the average voltage over the usable voltage range of the cell. When fully charged they should be about 4.15V, and when used will discharge down to about 2.5-3V, when the protection circuit kicks-in to stop the drain. If not they will drain below 2.5V and this is when they become permanently damaged.

Manufacturers note the wide voltage range and have decided to call Li-ion cells by the average usable voltage... 3.6V or 3.7V.