# Silly LED question 2*12Vl ead acid supply

I want to turn n a number approximately 20 ultrabright LED's from 2 lead acid 12V batteries in series, easy, but I want to maintain the same brightness (current) whatever state the batteries are in, so I might have 20V I might have 28.2 volts, my thinking is 2 series chains of say 10 LED's in parallel, but how to keep the current stable.
I could stick a zener across the LED's and a series resistor, then a series resistor to supply but the value of that resistor is tricky (for me at least) to decide on
or is there a better way I'm missing

Thanks in anticipation

## Replies

• sauravgoswami
well the simplest way is use an LCD!! watever you should consider Zener voltage and the current you require use ohms law to figure out the resistence pls ensure tat much current is available form the source
• pmcleod
It needs to be LED's as they are to provide illumination.I'd use the zener and a resistor to the LED's from the clamped voltage is easy with ohms law, but the value from supply, which could be anywhere between 20V and 28V would need to be chosen such that it'll drop the right volts whatever the supply from the battery, I guess I use a R value suitable for the lowest voltage and rate the zener so it takes any excess current from the highest voltage?
Thanks
• sauravgoswami
try it and say ur observation i will expecting it,in the meantime i will consult with other guys and try to find the solution
• pmcleod
Thanks, I haven't actually got the hardware yet, just trying to lay a pcb out ready, I guess the safest way is to buy some leds and build a breadboard, but I imagine the way I have described will work, it just looks an inefficient use of the battery power.
thanks for looking at it, I'll go play
• raithrovers1
This site has a calculator for your resistor values etc.
#-Link-Snipped-#

Hope it helps!

Visit #-Link-Snipped-#
• reachrkata
I have a simple solution.

1) Drive the LEDs through a NPN transistor (or many depending on number of LEDs and current required) with LED(s) connected to the collector and battery connected to LED(s).

2) Connect a resistor Re from emitter to GND.

3) Choose a regulated supply Vr for the base drive (which can be derived from the battery itself). Also choose a base resistor enough to drive the transistor to saturation.

The principle of working is -
The emitter current Ie is given (Vr-Vbe)/Re. This is constant since Vr & Re are fixed and Vbe varies very less.
Since the Base current will be negligible, Ie = Ic. Therefore Ic is also fixed.

Since Ic is the LED current, the LED current also gets fixed for any variation in battery.

Hope you could understand this, because I don't know how to attach an image on this post. I drew a circuit in MSPaint and could not attach it.

-Karthik
๐
• reachrkata
I perhaps got the reason -

I am not allowed to post attachments. Big K - HELP !!!! ๐
Sorry if this is off-topic !!

-Karthik
• Ashraf HZ
reachrkata
I perhaps got the reason -

I am not allowed to post attachments. Big K - HELP !!!! ๐
Sorry if this is off-topic !!

-Karthik
We can't attach anything on CE yet. For now, you can use an external image host such as imageshack. One you upload there, they will generate a link and embed code. Just paste in the img embed code in the post ๐
• Kaustubh Katdare
Yes, please use -> #-Link-Snipped-# <- to upload images. Once you upload an image, right click and copy the image path and insert it into a pop up box that opens up when you click button in text editor on CE.

You are reading an archived discussion.

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