Help with simple LED project?

Thread Starter

wrybread

Joined Jul 3, 2005
10
I'm working on a project that powers 6 LEDs, all 10mm super bright (.5 watts) red.

I'm powering them with a single 18650 LiOn battery (3.7 volts).

The LEDs are each separated by about 6 inches of wire.

A few questions:

- ideally I'd like for the LEDs to be dimmable. I know this should be done with PWM, but given the low voltage (and a desire for simplicity) can I just increase the resistor value?

- and would it be possible to use a variable resistor to get variable dimming?

- and can I use a single resistor (variable or not) for all the LEDs (assuming they're all identical)?

- I'll be daisy chaining the LEDs together. So 2 wires will connect to each LED. Is there a neater way to connect the wire to them than soldering?

Thanks for any help.
 

mcgyvr

Joined Oct 15, 2009
5,394
Need forward voltage and current specs on the LEDs..
More than likely due to the low supply voltage you need to put each LED in parallel with its own current limiting resistor..
That "could" be a variable resistor to offer dimming but you need to be very careful about resistor dissipation.. Typically variable resistors are low wattage and may burn up if not properly calculated.
However since you need each in parallel that would mean 6 resistors (or 6 variable resistors to adjust)

I think you should read a bit on how to properly size a resistor for a single LED and take what you learned there to complete your project..
 

Bernard

Joined Aug 7, 2008
5,687
If Vf is about 2V then I will be about 300 mA each or 1.7 A total. As an approximation, use 5 ohm @ 1/2 W in series with each and drive all in parallel with an emitter follower power transistor or better, a darlington like D1326, 8A,60V, B 2000, isolated case, or TIP127 ?
 

Thread Starter

wrybread

Joined Jul 3, 2005
10
Thanks so much for the input, comments below.

> Need forward voltage and current specs on the LEDs..

Oops sorry about that. Forward voltage is listed as "2.0-2.3v", current is "100-120mA".

> how to properly size a resistor for a single LED

Using the resistor calculator here (http://ledcalc.com), I should use a 15 ohm resistor for each LED. Or of course a 5ohm as Bernard suggested would be fine too.

Is it possible to use a single resistor for all the LEDs? So I'd place the resistor just after the battery +, and run the LEDs in parallel from there? If so, any advice on what value to use? (Guessing 15 ohm * 6 = ~100 ohm?)

> use 5 ohm @ 1/2 W in series with each and drive all in parallel with an emitter follower
> power transistor or better, a darlington like D1326, 8A,60V, B 2000, isolated case, or TIP127 ?

Could you elaborate a little on what the emitter follower power transistor or darlington would do?
 

hp1729

Joined Nov 23, 2015
2,304
Thanks so much for the input, comments below.

> Need forward voltage and current specs on the LEDs..

Oops sorry about that. Forward voltage is listed as "2.0-2.3v", current is "100-120mA".

> how to properly size a resistor for a single LED

Using the resistor calculator here (http://ledcalc.com), I should use a 15 ohm resistor for each LED. Or of course a 5ohm as Bernard suggested would be fine too.

Is it possible to use a single resistor for all the LEDs? So I'd place the resistor just after the battery +, and run the LEDs in parallel from there? If so, any advice on what value to use? (Guessing 15 ohm * 6 = ~100 ohm?)

> use 5 ohm @ 1/2 W in series with each and drive all in parallel with an emitter follower
> power transistor or better, a darlington like D1326, 8A,60V, B 2000, isolated case, or TIP127 ?

Could you elaborate a little on what the emitter follower power transistor or darlington would do?

Does he have the voltage to lose across the transistor? Maybe use the transistor as the ballast resistor?

How about something like this? (see attachment)
The value of the resistor for dimming depends on the transistor you use. It doesn't have to be a Darlington I guess. What do you have on hand? Most any power transistor would probably do.
 

Attachments

Last edited:

mcgyvr

Joined Oct 15, 2009
5,394
Is it possible to use a single resistor for all the LEDs? So I'd place the resistor just after the battery +, and run the LEDs in parallel from there? If so, any advice on what value to use? (Guessing 15 ohm * 6 = ~100 ohm?)
No.. LEDs in parallel without current limiting each string is a no-no.. They don't share current equally
 

hp1729

Joined Nov 23, 2015
2,304
No.. LEDs in parallel without current limiting each string is a no-no.. They don't share current equally
What McGyvr says is good advice. That being said, my LED flashlight is just batteries and four LEDs in parallel. No resistors. :)
 

mcgyvr

Joined Oct 15, 2009
5,394
:)
An acceptable cost. Agreed, not a good design. Do you have an LED flashlight? What is it like?
No flashlight.. But I've built numerous high powered (50 to 200+ Watt) LED fixtures for saltwater aquarium systems using everything from 3W Cree diodes to 100W clusters
ALL feature full dimming and proper constant current control..

My time is worth far more than a few resistors.. :p
 

Bernard

Joined Aug 7, 2008
5,687
If you have a bag of LEDs you could select 6 that matched Vf to +- ..025 V & a 2.5 ohm @1W series resistor, otherwise use 15 ohm for each LED Or use transistor as amplifier as in thumbnail. The emitter follower will work but base would need about 5V for full output.NPN 00000.jpg
Base V from dim to full output is about 1.54V to 1.7V.
 
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