🚀 go-pugleaf

In article <3DC99AD4.2AB0FE52@fanwap.com>, tong@fanwap.com talked about...
> Yes, I mean in series.
> But two strings can not use the same R values.
> So sad it very hards to calculate the current and R.
> the forward voltage drop does not identical in each LED.
> and it changes when current change too.

> ex:

> strings 1 :   ( 12 volt - ( 3.453 + 3.6324 + 3.833 ) ) / 40 ohm = 27 mA

> Now I want 20 mA ,  How to calculate the resistor ?
> Since forward voltage change when the current change too.

Early this year or last year we had this newsgroup discussion about making 
a simple current regulator for LEDs.  You will have to put your LEDs in 
series in place of the single LED and adjust the resistor values for the 
higher voltage.  For a start you will probably want to put a pot in place 
of the 2.2k resistor.  Here's the simple version of the schematic.  To view 
this you must use Courier font.


<<
Rich Grise wrote:

> If I were building it, I'd lose the 2N3904, both of the 1N914's,
> the 3.3K, and the 22K, and just connect the wiper of the pot to
> the base of the 2N4401. You might want to add a zener across the
> pot to regulate its voltage as the batteries droop, but I'm way
> too lazy to calculate that for you. In fact, I've used silicon
> diodes in series, forward biased, as more-or-less of a voltage
> regulator. Oh! I see - you've already got D1 forward-biased.
> Yeah, the 2N3904 has way too much gain for what you're trying
> to accomplish. Put a couple or 3 more 1N914's in series with
> D1, and put the rest of the circuit back in your parts bin. :-)

Thanks for the idea, I just might tack solder a couple parts together to 
see how it works.  I'm really not concerned with the current regulation 
being precise.  All I want to do is eliminate the wide variation of the LED 
current with the changing battery voltage abd minimize the dropout.  In 
other words I don't wanna use a fifth AA cell to increase the 'stiffness' 
of the current using just a resistor to limit the current.

Do you mean something like the following???


> Watson A.Name wrote:

> >      Current Limiter, Low Dropout, for LEDs
> >      by Watson A.Name  Jan 16, 02 rev 020116A
> >
> >      + = connection   ) = no connection
> >     Must be viewed with monospaced font such as Courier
> >
> >         +---------------+--------------+
> >         |               |              |
> >         \               |              |
> >     2.2 /              --- White       |
> >      k  \              \ / LED         |
> >         /              ---             |
> >         |               |              |
> >     +---+               |              |
> >     |   |               |              |
> >     |   |            |  /              |
> >     |   \            |/              ----- four
> >    ---  / 470   +----|  2N4401        ===   Ni-
> >    \ /  \       |    |\              -----  MH
> > D1 ---  /       |    |E \             ===   AA
> > 1N  |   |       |       |            -----  cells
> > 914 |   +-------+       |             ===   4.0
> > D2  |   |               +-o T.P.     -----   to
> >     |   |               |             ===   5.5V
> >    ---  \               \            -----
> >    \ /  / 2.2k          / 10           |
> >    ---  \               \ ohms         |
> >     |   /               /              |
> >     |   |               |              |
> >     +---+---------------+--------------+


> --
> Cheers!
> Rich

Jan 20, 2002  Measurements taken with B&K 2707 DMM.

At 5.5V, T.P. V = .325V.   At 4.0V, T.P. V = .271V.

This is about 20 percent variation in LED current over the full range.  
This is not as good as the original adapted circuit that had about 13 
percent over the full range.  But it represents a considerable improvement 
over a single resistor, which had a 3:1 variation in current over the full 
V range.  I think that this circuit is a good compromise between 
simplicity, parts count and parts availability, and compactness since there 
will be a circuit for each LED in a multiple LED light.

-- 
>>


> Lizard Blizzard wrote:
> > 
> > Nucharin W. Jansen wrote:
> > > What's happen when voltage drop across LEDs are not same ?
> > > I have problem with white LEDs in series.
> > > I can't control the current.
> > > I have to adjust in real work one by one in each branch.
> > 
> > You say the LEDs are in series, so the current is identical in each one.
> >   All you have to do is put a current limiting resistor in series with
> > those, and use a value that will give you the right current thru all.
> > 
> > If you made a mistake and meant parallel, then yes, you should not
> > connect LEDs in parallel without a resistor in series with each LED to
> > limit and equalize the current.
> > 
> > [snip]


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