Linear LED driver very accurate due to emitter degeneration?

[treez]

Hello,
This linear LED driver puts 50mA into each string, even though only one string is regulated....and even though the led strings are different lengths.

Is it the emitter degeneration which is equalising the led current in each string?

schem and LTspice sim attached

 

[Audioguru]

As a few us said on the other website forum, the transistors are not the same and have different temperatures so their currents will be different.
The current will increase as the transistors warm up and one transistor warms more than the other one.

 

[dick_freebird]

Degeneration helps, but never perfectly. It may be the
cheapest way to "good enough". What is "good enough",
is something you have to figure out - then you can do the
analysis to determine whether the circuit you have, is.

 

[treez]

the transistors are not the same and have different temperatures so their currents will be different.
The current will increase as the transistors warm up and one transistor warms more than the other one.

...Again I see what you are getting at, but the one that is regulated is the 2_LED string one, and this is the one that will be heating up more, and its current will try to grow more, but it is regulated , so it will be ok.....the one with 3-leds will thus suffer less current in its leds, which is better than too much current in its leds, so having the regulation in just one string is not ideal, but at least the correct string (the shortest one) has been regulated...would you agree?.......in the actual application, there are ten extra strings of three_LEDs which are like the one shown here, (ie not "directly" regulated).

 

[FvM]

It's not clear to me what you are asking beyond obvious circuit behaviour?

 

[godfreyl]

It's not clear to me what you are asking beyond obvious circuit behaviour?

He's asking if we agree with him. Note:

...would you agree?...

The model answer is obviously "Yes, I agree".

- - - Updated - - -

This is by no means the first such thread. For example, you may remember a few previous threads where he first asked us to agree that connecting LEDs directly in parallel is OK, then asked us to agree that connecting LEDs directly in parallel is a bad idea and should never be done.

As a rule-of-thumb in this kind of thread, only clear-cut black & white answers are acceptable. There's no space for any middle ground or rationed responses like dick_freebird gave in post 3 - either you agree or you don't.

Same story with threads asking questions like: "Do you agree that there's absolutely no reason for anybody to ever buy xyz LEDs, given that abc LEDs have similar specs and are currently 5c cheaper according to the only catalog I bothered to look at?"

 

[Audioguru]

The SIM program shows an input of "1" which I assume is 1V and not 1 can of beer.
The datasheet for the BC547B shows that at 50mA (will the LEDs survive?) the base-emitter ON voltage is typically 0.85V but could be as high as 0.9V.

The datasheet for the old metal-can 2N2222 shows that its typical base-emitter ON voltage is typically 0.75V at 50mA but could be as low as about 0.71V.
Then the current in the 2N2222 could be (1.9V - 0.71V)/20 ohms= 60mA.

The difference in brightness between LEDs at 50mA and others at 60mA is insignificant. But will the LEDs survive 60mA? datasheetarchive.com never heard of them.
I wonder why ALL the transistors are not the same?

 

[treez]

The LEDs are power topleds by osram.
(yes the "1" is for "1V" in the above circuit)

This alternative circuit is even cheaper....and uses a vbe to regulate the led current to 50mA. (the 2.5V reference sets the voltage across the vbe and the emitter resistor, if the regulated string goes open, the lower BC847 turns off, and allows the upper bc847 to turn on which shuts down all the strings...I don't know why this feature would be wanted in a car rear light)
I tried changing transistors to see the effects of transistors with different parameters in the different strings......in the simulator its not much difference...the simulator appears "kind" to this circuit.

The 2 Power BJT's in the actual circuit are BCX56-16
**broken link removed**

I really cannot see how accurate current regulation can be garnered with a BJT VBE......but this is happening inside cars today.

Would you agree that the VBE of the BJT = BC847B could be anywhere between 500mV and 900mV depending on temperature and batch?
**broken link removed**

if someone presented this to you as a automotive led light regulator, would you agree that the led current accuracy is insufficient?

LTspice sim and schematic of alternative attached

 

[Audioguru]

Replace all those parts with one LM10 adjustable voltage reference and opamp all in an 8-pins package.

 

[treez]

https://www.ti.com/lit/ds/symlink/lm10.pdf

..would you recommend an LM10 for every string?

 

[Audioguru]

https://www.ti.com/lit/ds/symlink/lm10.pdf

..would you recommend an LM10 for every string?

No, only for the control string for its negative feedback. The bases of all the transistors connect together.

 

[treez]

the LM10 is nice, its got me thinking, and the (similar) TL431 is very cheap.....

Could you possibly contrast the following two alternative versions of the LED string regulator? (attached below)
In the actual circuit, the designer , for some reason, just used the TL431 as a 2.5V zener diode ("LINREG 1_TL431REF").
Was this because using the TL431 in a negative feedback loop could engender oscillations?
The version using the TL431 as a negative feedback loop opamp is called as follows....."LINREG 1_TL431REF".

Schematics and LTspice simulations of the circuit with the TL431 used as a zener diode, and as an opamp with negative feedback.
(please give me a shout if you wish to receive the .asy file and .sub file for the TL431)

Here are schems in jpeg
LINREG - TL431REF:
https://i42.tinypic.com/17qtsi.jpg
LINREG - TL431REG:
https://i44.tinypic.com/33dzitx.jpg