do stars always have to have current regulators?

[oscillator]

I have a voltage converter that outputs a very constant voltage in between 5.05 and 5.04 volts max 2amps, if I were to put some 1/3w red Star leds @ 2.5 volts onto this current, would they be safe?

 

[VidPro]

you mean put 2 reds on your 5v supply in series? (its DC right
---|>---|>----

it will "work".
If you underdrive it , any changes that occur would have it work forever with no problems.

if you put a bit of resistance in, even if things change they wont change as MUCH for the led.
--{~}-----|>---|>----
and you could add resistance in till you get to the exact drive current you want anyways.


so if you put resisters on and ran them parellel
|---{~}---|>----|
|---{~}---|>----|
it would be way more flexable to changes, but waste power out the wazoo.

I (myself) would easily run it with voltage control, I do a LOT of that, but usually i keep it a teeny bit lower than max specs, so anything that changes is covered.
With a Current driver, things can change quite a bit, and the drive current would stay the same.

things that can change: the LED voltage, the power source , temperatures. <- those things.

 

[Mr Happy]

In an ideal world, all lamps both incandescent and LED should be driven to a specified current disregarding voltage. This ideal situation is often unnecessarily elaborate and expensive though, so it is often not used.

However, the non-linear characteristic of LEDs makes them much more sensitive to voltage than incandescent lamps, and current regulated drive is more important. Connecting a power supply direct to an LED without any kind of current limiting arrangement gives a very good chance of blowing it up.

Note that the classically simple approach of putting a resistor in series with a voltage source is close to a constant current supply, and so a current regulator doesn't have to be a complex electronic circuit.

 

[NextLight]

(For your application consider Vout of your converter = Vbat in my comments, below)

If you want to use them at "star" typical power levels, YES stars need a current regulator, or current limiter (such as a resistor or PTC thermistor) or a power limiter (if using a converter) to avoid thermal runaway.

One can run LEDs including stars, at voltages below the LEDs design Vf (applied Voltage forward) point, but with current so very low, the output is pitiful. For example, one white LED with nothing to limit the current might just begin to glow at 3.0V, cold. At 3.35V it might be at full rated output at room temp. At 3.5V warmed up, it would destroy itself in a few seconds. The current of a LED goes up exponentially (very steeply) around the specified Vf. The only thing limiting this is external resistance or other external current limiting-regulation.

I have a couple high Vf 5mm white LEDs running direct, one from a RCR2A LiIon, which I never charge above 3.85V, and one running direct from a tiny 3.6V (rated, ~3.9V fresh no-load) primary stack. These run around ~17mA max. The output is about half of an SMJLED. (From memory, my little LEDs are 2 year old, 10CD@30theta rated Nichias.) These setups rely on the higher Vf of the LED and the small but important internal resistance of the cell(s) and flashlight contacts. If the LEDs were to burn up, I would simply put in another 20 cent LED. (Way cheaper than a Mini-Mag lamp!) It hasn't happened yet.

Trying the no current limiting or regulation technique with power (1-5W) LEDs, either the output is as pitiful as my little LEDs above (when Vbat is well below normal specified Vf of the LED) or the LED burns up, going into thermal runaway (when one uses a Vbat at or near specified Vf, depending on starting temperature, heat sinking, and exact battery characteristics.) This failure is accelerated because as the LED temp rises, the Vf value of the LED drops, meaning for a given applied voltage, the current will be much greater. POOF!

SOMETHING outside the LED must limit the current (or Power, thus somewhat limiting current) or the LED will act like a fuse, after a short, bright, run.

 

[oscillator]

Thank you, I appreciate the advice everyone! it is a lot of help.

the current regulator I have is fluctuating 5.03-5.05V @ 2A max which is perfect for a couple of strong red lights in series, and two converters in series makes 10 V, which is perfect for three white lights in series at 3.33V... so I hope I will be able to run K2 leds with a good heat sink just straight through a 1ohm 10W resistor

 

[TorchBoy]

two converters in series makes 10 V

How are you (planning on) running those things in series? If being run from the same power source they might not like plus being connected to minus on their outputs.