General > General Technical Chat
LED Lighting (350 mA). Is there any standard for # of LEDs is a string?
<< < (6/12) > >>
Zero999:

--- Quote from: Benta on August 10, 2022, 10:18:32 pm ---
--- 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.
-------
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:

--- End quote ---
If you're set on strings for four LEDs, then why use the TLV431? The LM317 would do and still have plenty of headroom for cable losses.

I'd say 19V is a bit on the high side, for strings of four + a linear regulator. Have you considered hacking the power supplies? It's highly likely changing a resistor will get the voltage down to between 14V and 15V, which still gives enough headroom for a linear regulator.
mariush:

--- Quote from: Benta on August 10, 2022, 07:57:13 pm ---
--- 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.

--- End quote ---

The built in sense resistor of around 95 ohm (for the 10mA current limit) will be less precise, and will allow for +/- 10% as the datasheet says, so 9..11 mA.

You parallel an external resistor with that built in resistor, for example on Page 11 , Figure 7... it shows  2.1 ohm used for 350mA current.
So you could choose a good quality low ppm resistor to get let's say 320-340mA of current, then even with all the variation of the internal 95 ohm resistor and external resistor due to temperature change, the current in the series won't go over 350mA of current.

Basically... I'd look at a 2.2 ohm or a 2.4 ohm resistor ... for example https://www.digikey.com/en/products/detail/koa-speer-electronics-inc/SR733ATTE2R40F/9845654

Or you could even have two resistor footprints by the driver chip, and for example parallel two 4.7 ohm resistors (common value, mass produced, pennies, and less heat per resistor)
example : https://www.digikey.com/en/products/detail/vishay-dale/CRCW08054R70FKEAHP/2226710


Keep in mind that the TL431 will also drift with temperature, as will resistors and capacitors in same circuit, and if you have these on same PCB with the LEDs the pcb will probably be heated up to 60+ degrees Celsius
So account for that when you think other solution will be more precise at keeping current at 350mA.



nctnico:

--- Quote from: mariush on August 10, 2022, 01:18:28 pm ---You can bin the leds so that the forward voltage of every series ends up very close to the forward voltage of other series that are in parallel. Super easy to make a test fixture of some sort to measure the forward voltage and brightness at various current levels.

--- End quote ---
That is way too much hassle. Typically you buy the LEDs from the manufacturer binned for color & brightness but you still have to expect differences large enough that make parallelling several strings problematic.


--- Quote ---They do this parallel thing with led strips for monitor backlights and in other places and nobody complains about differences in brightness.

--- End quote ---
They likely have a way to deal with proper current sharing otherwise you will see difference in brightness in the panel. Been there, done that.
Zero999:
A few transistors can be used to make a switching regulator with current limiting, which can be used to drive a string of LEDs. The zener diode can be removed, or you might want to keep it and increase it to a higher voltage, such as 15V,
https://www.romanblack.com/smps/a04.htm
Benta:

--- Quote from: mariush on August 11, 2022, 08:13:29 am ---Keep in mind that the TL431 will also drift with temperature, as will resistors and capacitors in same circuit, and if you have these on same PCB with the LEDs the pcb will probably be heated up to 60+ degrees Celsius
So account for that when you think other solution will be more precise at keeping current at 350mA.

--- End quote ---
I think you missed my replly #19 and the LED current plot.
I'm really not worried about TLV431 temperature drift. Even if if was 10 times worse, it's still good.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod