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LED Lighting (350 mA). Is there any standard for # of LEDs is a string?
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tom66:
I don't know of many monitors that don't have independent current balancing for each parallel string, but even then the most common approach is all LEDs in series.  Yes that does mean if one fails, the whole panel goes dead but hey, e-waste is not their problem.
Benta:

--- Quote from: Zero999 on August 10, 2022, 10:39:18 am ---How many strings are you planning to have per luminaire?

Consider having a lower reference voltage, then you can have an extra LED or two and cut down power dissipation in the transistor.

--- End quote ---
Not decided yet. I'll need to experiment a bit with the subjective optical power, which also has to with materials, optics and enclosures.

I though about using a lower reference voltage, but the simplicity and incredibly good current stability of the TLV431 solution led me to that choice.
Also, the output of the laptop power supply is marked as pulsating DC, so I'll need a bit of headroom to be sure of correct operation.

Thanks for Your inputs All.
Benta:

--- Quote from: mariush on August 10, 2022, 11:01:31 am ---You could use a basic linear driver like  BCR421 which can do up to 350mA (absolute max rating somewhere around 500mA) using an external resistor (defaults to 10mA without one)
32 cents if you get 100, 20 cents if you buy 1000

--- End quote ---
Nice little device, but not very precise. With the LEDs being 1.50 Euro a pop (=6 Euro), I'd rather be certain of my LED current.

But Thanks.
Zero999:

--- Quote from: Benta on August 10, 2022, 02:59:43 pm ---
--- Quote from: Zero999 on August 10, 2022, 10:39:18 am ---How many strings are you planning to have per luminaire?

Consider having a lower reference voltage, then you can have an extra LED or two and cut down power dissipation in the transistor.

--- End quote ---
Not decided yet. I'll need to experiment a bit with the subjective optical power, which also has to with materials, optics and enclosures.

I though about using a lower reference voltage, but the simplicity and incredibly good current stability of the TLV431 solution led me to that choice.
--- End quote ---
I was thinking use the TLV431 as a reference for an op-amp. If you used a quad, such as the LM324, it would actually work out cheaper than four TLV431s.


--- Quote ---Also, the output of the laptop power supply is marked as pulsating DC, so I'll need a bit of headroom to be sure of correct operation.

Thanks for Your inputs All.

--- End quote ---
That's odd. Does the label/manual say what frequency it's pulsating at? What does the waveform look like on an oscilloscope? I think it's likely you're mistaken. The symbol for steady DC is a solid line over a dashed line.


Oh and just to be pedantic, there's no such thing as pulsating DC. It's really AC with a DC bias, but I know what you meant.
Benta:

--- Quote from: Zero999 on August 10, 2022, 09:25:42 pm ---
I was thinking use the TLV431 as a reference for an op-amp. If you used a quad, such as the LM324, it would actually work out cheaper than four TLV431s.
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That's odd. Does the label/manual say what frequency it's pulsating at? What does the waveform look like on an oscilloscope? I think it's likely you're mistaken. The symbol for steady DC is a solid line over a dashed line.


--- End quote ---

To your first point: that would defeat the purpose here. The idea is to have the controllers as POL regulators, one regulator per string of four LEDs. This allows me to place the strings in any physical configuration. Consider it as a "19 V, 350 mA, 4-Led module".
Imagine a "splitting box" with twelwe 19 V outputs powered from one laptop supply. The LED strings will do the rest themselves.

The voltage headroom on my design also allows me more freedom in cabling (losses).

Concerning the "pulsating DC": of course you're right, bad choice of words on my side, sorry. I interpreted it as "mediocrely regulated DC". ;)

-----------------------

Something that might interest you (you seem to like playing with circuits).
The TLV431/Darlington power BJT is dangerous, which I only found out after having serious problems with my first circuit. An emitter follower (which this circuit basically is) can oscillate with significant amplitude at >1 MHz frequencies. In a discrete circuit it's usually not a problem, but add in a TLV431 which has a lot of gain, and it gets very hairy.
The TLV431/BD679 combination proved impossible to tame, which made me go back to the BD437 plus a relatively large base resistor. This is stable and works reliably. R1 as 1 kohm and R2 as 5.6 kohm works even better.
You live and learn ;)


PS: (added later). Do you remember the "backlighting" thread where temperature stability was brought into the discussion?
Well, look at this:

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