LDD-300L with LEDs in parallel?

E

eddiesylas

Premium Member
I've always wired in series so this is new to me...

I would like to drive Qty.10 (3.3V, 30mA) LEDs from Unique-LED (pn:560VW6C) in parallel using the Meanwell LDD-300L.

I think I will use the 12V, 1.5A input from a power supply and an Arduino 5V PWM dimming input from I/O pin. The Output should be 6V@300mA.

So each LED (if wired in parallel) would see 9V @ 30mA if run 100% correct?

Can I do this?

Do I use a resistor and if so where (other than the 10kohm I/O pin to Gnd)?
 
Running all in parallel and you need 6.3V ps minimum 12V works fine.
30mA x 10
so yes 300mA Meanwell is fine. Because it is limiting current for you , no resistors are needed.

So each LED (if wired in parallel) would see 9V @ 30mA if run 100% correct?
Click to expand...
no, they will see the voltage that produces 30mA current. 3.4V per spec sheet.

Even dimmed they will each see 3.4V only for shorter and shorter periods..

Keep in mind if any diode fails open, the current will readjust to feed a total of 300mA.
300mA into 9 ( 33mA each w/ an increase in voltage a bit).
Say 1/2 of the diodes fail.. Then 300mA into 5 diodes 60mA each

somewhere around this point (sooner or later) all diodes will fail as they get too much current..

All diodes, to assure success, should have the identical electrical specs.. Imbalances in any parameter add to the probability of diode failure..
In other words the mA will be unbalanced across the LED's
 
All the LEDs will see the same voltage. The total current will be the sum of the currents that each LED draws. The problem is that you don't know how much current each individual LED will see - the current-voltage relationship for LEDs is not completely consistent from one device to the next (i.e one diode may draw 25 mA at 3.4 V, then next may draw 35 mA at 3.4V.) Since the LDD is a constant current driver, it will adjust the voltage to make sure it's pumping out 300 mA (or whatever it's set to.)

The other problem is that the resistance of semiconductors tends to drop with heat. The more current a device draws, the more heat gets generated. More heat means lower resistance and more current drawn. you can see where this leads. If there's nothing in place to limit the current draw of a given LED it's very easy for a positive feedback situation to occur and for it to burn out.

This is why LEDs are generally run in series with a constant current supply. The amount of current that the devices see is automatically set so the situation I describe above doesn't occur.

Why do you want to run them in parallel instead of in series anyway?
 
If you look at the really cheap LED flashlights on the market you will see the biggest issue with this clearly. Even a couple of mA difference (with such a low max current, its percentage is high) in the LEDs will make some barely light up compared to the others, and that will compound with heat.

It that is okay i would ditch the driver and get an accurate voltage regulator instead and just put 3.3V or something into them. Low risk of them going thermal if they never have access to too much voltage like with 12V behind 10x rated current driver.

If you instead want them all to be driven properly get a driver capable of ~30mA, run them in series. Something like this ON Semi current regulator, part number NSI45030AZT1G. Its 30mA, up to 45V with 1.8V overhead, so 10x 3.4V LEDs is fine... Obviously it doesn't do PWM dimming, but you can flip a switch really fast =D, or at least the arduino can a transistor or something able to switch ~35V at 30mA on and off.
 
Thank you for the lesson and input!

Gorgok said:
Something like this ON Semi current regulator, part number NSI45030AZT1G. Its 30mA, up to 45V with 1.8V overhead, so 10x 3.4V LEDs is fine...
Click to expand...
Good recommendation, but...
Gorgok said:
Obviously it doesn't do PWM dimming, but you can flip a switch really fast =D, or at least the arduino can a transistor or something able to switch ~35V at 30mA on and off.
Click to expand...
this is the goal... Arduino controlled dimming.

sleepydoc said:
Why do you want to run them in parallel instead of in series anyway?
Click to expand...
The problem is matching the correct equipment with the goal, ill explain a bit better below.

oreo57 said:
300mA into 9 ( 33mA each w/ an increase in voltage a bit).
Say 1/2 of the diodes fail.. Then 300mA into 5 diodes 60mA each

somewhere around this point (sooner or later) all diodes will fail as they get too much current..
Click to expand...
Got it! Reliability is important... and I have a bad habit of "set it and forget it..."

I am interested in programmed dimmable moonlight.
Goal:
Arduino PWM control
Very cool white look(anywhere ~8000K-12000K is fine)
~1W total power (20000mcd OR 25lm etc.)

Problem:
1) Finding PWM driver to match LEDs (also many have been low voltage limited preventing 0-100% dim)

2)Most LED are 6000K+ and/or cheap and less reliable (If I could find a single "quality" 1W LED I'd drive that at 300-350mA etc.) I'd even be willing to learn how to mount it....

So....... Im looking for a quality driver (0-100% capable with NO flicker) I just happen to be building with LDDs.
 
With low power LEDs I'd just use resistors (one per series string or 1 per LED if all in parallel) and then just PWM a transistor (or Mosfet).
Using a resistor for each LED eliminates any possible problem with running them in parallel with a single driver..

No need for a driver at all until you get into the hundreds of mA IMO..
 
Well sure, and that is the way I may go if a better answer doesn't present itself. Now maybe this doesn't matter, but the signal in a driver (even PWM fans) are cleaned up before the load sees the signal and at longer off periods on the PWM cycle are more visible.

If I use a transistor is it OK to go with a 5V,3A power Supply? What size resistor?
 
I would lean towards the adjustable regulator method to ensure you can't over drive any of them. For dimming all you would need is a transistor to accomplish pwm dimming with arduino. An arduino compatible fan speed control circuit board that is capable of running two wire fans would work well also.
 
eddiesylas said:
Well sure, and that is the way I may go if a better answer doesn't present itself. Now maybe this doesn't matter, but the signal in a driver (even PWM fans) are cleaned up before the load sees the signal and at longer off periods on the PWM cycle are more visible.

If I use a transistor is it OK to go with a 5V,3A power Supply? What size resistor?
Click to expand...
5v 1amp would be more than enough but amps doesn't matter as long as it's over what the string will draw by a margin of at least 20%, use what you have.


http://led.linear1.org/led.wiz

plug in your specs to this handy little calculator.
 
Yes, overly expensive, like most neat things... and yes, just the IC. I am going to have many board options to solve as I haven't built (or bought) anything yet. The excel spreadsheet is getting long.
 
It is neat that it has built in current mirrors along with dimming, too bad it dosn't come in a model that handles hundreds of mA.....would save me from building current mirrors out of transistors for big arrays when I don't feel like taking up the space for a bunch of LDDs.......
 
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