From: Watson 'Atto Parsec' Name
Subject: Re: Best way to power array of LEDs?
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <2Qan9.3129$cS4.firstname.lastname@example.org> <3DC843FE.4A25FFF3@fanwap.com> <3DC99AD4.2AB0FE52@fanwap.com>
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Date: Mon, 9 Dec 2002 00:34:57 -0800
NNTP-Posting-Date: Mon, 09 Dec 2002 01:39:55 PST
Organization: InReach Internet
In article , email@example.com
> Winfield Hill wrote...
> > The circuit can be easily scaled to handle many LEDs. One possibility
> > is to operate Q1 at higher currents, scaling R1 to R4 appropriately.
> > Another is to add a buffer transistor between Q1 and the bank of current-
> > sink transistors. We can do this because the Vbe reference is adjustable
> > and can easily accommodate another Vbe drop.
> >= 1.69V power supply
> >= ,----+-- R3 --+---- 3.4V to 6V or more
> >= | | 1.8k | multiple 30mA
> >= | R4 R6 white LED driver
> >= R2b 62 27 using 2n4401
> >= 68 | | white white
> >= | | 1.63V | LED a LED b ...
> >= | | C | |
> >= R2a +----- B | 30mA | 30mA
> >= 3.3k | Q2 E | |
> >= | C | 0.96V C Q3a C Q3b..
> >= +--B +------ B ------ B ------- many many more...
> >= | E Q1 | E E
> >= R1 | R5 | | <- 0.20V
> >= 2.2k | 1.0k R7a R7b
> >= | | | | |
> >= '----+--------+---------+--------+--- etc
> >= all 6.8 ohms
> > As before R4 cancels changes in Vbe-Q1 with supply voltage. If you
> > desire, R2 can be trimmed for exactly 30mA with your transistors,
> > this worked out to 3.368k for Spice's 2n4401 parts. R6 protects Q2
> > by limiting the transistor-array base drive to 100mA, which should
> > be enough for perhaps 500 LEDs. :>)
> Or better yet, use a trusty LM358 (miniDIP version of the 324) for
> a simple yet accurate low-dropout multiple-LED current source.
> = power supply
> = ,-----------------+---- 3.3V to 6V or more
> = | | multiple 30mA
> = R3 | white white white LED driver
> = 22k | LED a LED b
> = | LM358 | | 30mA | 30mA
> = | _| | |
> = +-- R2 --+------|+ \ C Q1a C Q1b..
> = | 36k | | >--- B ------ B ------- many many more...
> = | | ,--|-_/ E E
> = | LM385- | | | | | 2n4401
> = \_|_1.2 R1 '----|--------+ |
> = /_\ 4.7k | | | <- 0.14V
> = | | | R5a R5b
> = | | | | |
> = '--------+--------+--------+--------+--- etc
> = all 4.7 ohms
> The LM385-1.2 is one of my favorite parts, a jelly-bean reference
> IC that should be in every parts cabinet. If more than 50 LEDs
> are driven, Q2-style buffer can be added as in the previous design.
> - Win
Thanks for the latest, Win. I built your upper circuit and it's working
okay with four LEDs, on a set of four AA Ni-MH cells.
I can get the TL431s or TL430s, but I've never seen the LM385-1.2, and
probably the closest I could come is the following. Maybe you'd like to
comment on this one. It sems to give constant voltage, but it
apparently doesn't have temp compensation. I guess it works similar to
your upper circuit, but wih two transistors?
--+----- Unreg V
/ To give a few mA, maybe 1k
+--------> Constant V, about 1.4V
3906 /| |
/ | |
+--/\/\/\---+ Less than 10 ohms
| | /
| |/ NPN
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