Questions about 5mm LEDs put in series

[AlexGT]

Hi everyone!

I am doing some modifications to a HID spotlight and would like to know if I connect 8 red 5mm Leds in series if they could take the 12v from a SLA or 14.8v of a li-ion battery pack, I figured since each red led can take between 1.8 and 2.1v according to spec 8 would be between 14.4v and 16.8v, a fully charged SLA is around 13.5v and a fully charged li-ion pack would be 16.8v.

Is my thinking correct or did I miss some important detail? Will the leds be overdriven in DD? Any tips you could give me on connecting them would be much appreciated.

Thanks!

AlexGT

 

[AlexGT]

Oh! By the way my project consists of a spotlight that has:

- 8 Red leds
- 1 White R2 DX dropin (Up to 18v)
- 1 White Fluorescent area light
- 1 35watt HID
- GITD sheeting

This would probably cover all my lighting needs LOL!

AlexGT

 

[spencer]

Just use a resistor.

 

[AlexGT]

Can you be more specific? Do you mean use 8 resistors? 1 for each led? that is what I have now for all 8 white leds installed, I want to DD from 12-16.8v that is why I am asking about putting them in series.

AlexGT

 

[spencer]

Either or.

 

[lctorana]

8 x 1.85 = 14.8V

Now LEDs are non-ohmic concuctors, with the characteristic that a small change in voltage causes a relatively large change in current.

With 8 LEDs, I would suggest that on the charger, they will just about light up the room, and off the charger, they will be barely visible.

They should cope with 2.5V each, so since the voltage won't go even near 20V, the LEDs will come to no harm, but the light production curve will be near useless (unless you want a nice bright "on-charge" indicator!).

How about, as a compromise, 2 strings of 4 series LEDs, each with a resistor?

 

[kurni]

Hi AlexGT, I've been looking for a bright red led (preferably SSC); mind me asking from where you got it?

Thanks,
Kurni

 

[AlexGT]

Mine are 5mm leds not SSC I bought them on ebay these are the 35000mcd kind in red.

Thanks for the replies so far, I didn't understand what Ictorana said, so 8 in series driven by 12v-16v will not be bright? I tought red leds need 1.8 to 2.1 fv to operate 2.5v would be overdriving it?

AlexGT

 

[TigerhawkT3]

You can get red XR-Es from DX.

If you don't mind using four or five red LEDs instead of 8, you could just use a cheap CC regulator like an LM317T (or the version that's smaller, cheaper, and still more than adequate for this application, but whose model number I don't remember). For very close to 20mA, I'd recommend a sense resistor of 60-70Ω, which could be obtained with a pair of 100Ω resistors in parallel, in series with a pair of 10Ω resistors in series. If you want closer to 12mA, just use a single 100Ω resistor.

 

[kurni]

You can get red XR-Es from DX.

Thanks, I just found www.mouser.com; they have all sort of SSC.