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>
X-Newsreader: MicroPlanet Gravity v2.60
Date: Tue, 10 Dec 2002 05:47:16 -0800
NNTP-Posting-Date: Tue, 10 Dec 2002 06:53:28 PST
Organization: InReach Internet
In article , email@example.com mentioned...
> Watson 'Atto Parsec' Name wrote...
> > firstname.lastname@example.org mentioned...
> >> Watson wrote...
> >>> email@example.com wrote,
> >>>> ,----
> >>>> |
> >>>> R3
> >>>> 2.2k white white
> >>>> | LED LED
> >>>> ,----+ x | |
> >>>> | | | |
> >>>> | R4 C C
> >>>> R2 +--- B ----- B
> >>>> 1.0k | E E
> >>>> | C | Q2 | Q2'
> >>>> +--B | +---
> >>>> | E Q1 | |
> >>>> R1 | R5 R5'
> >>>> 2.2k | 6.8 6.8
> >>>> | | | |
> >>>> '----+------+-------+----
> >> More accurately, R4 = R2/R1 * r_e for Q1, or 36 ohms.
> >> 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. :>)
> > Well, thank you for the improvement. I found one design deficiency(?)
> > with the original circuit. I found that if one LED fails open, the
> > other LEDs dim substantially. The 30 mA is no longer flowing thru the
> > open LED's 6.8 ohm emitter resistor, so there is no IR drop across it.
> > This allows much more current to flow thru the B-E junction of that
> > transistor, hogging the base drive current away from the rest of the
> > transistors. Hopefully, keeping the LED current at 30 mA will prevent
> > that open LED from happening.
> Nope, that's a problem or potential problem at one level or another
> with all of the shared-reference circuits, because they rely on only
> driving Ic/beta for each LED-transistor combination, rather than Ic.
> The circuit above, capable of driving 100mA into the base-bus line, is
> less vulnerable because in applications with say up to 100 LEDs it has
> left-over capability to handle the situation of a few missing LEDs.
> You do have the option of using more Q2 transistors, so that each one
> is responsible for fewer LED drivers.
> One "solution" is to add a separate base resistor with each LED-driver
> transistor, thereby limiting the current it can draw if the collector
> is open. But be careful in choosing the resistor's value, because it
> introduces an error related to the transistor's beta.
> For example, assume a beta of 150 creating a base current of 200uA.
> Adding a 470-ohm resistor would create a 94mV drop; we would correct
> for this by raising the reference by 94mV. However for a transistor
> beta of say 250, drawing 120uA for a 30mA output, and dropping 56mV,
> we'd see an emitter voltage of 238mV instead of 200mV and thus get a
> current of 36mA (actually 34.5mA, but you get the idea). So adding
> this resistor can create a likely non-trivial error. In the absence
> of an LED, the 470-ohm resistor limits the extra base current draw
> to about 0.4mA, which is nice. Perhaps one could be satisfied with
> a smaller base resistor, say 220 or even 100 ohms as a compromise.
> Alternately a different solution could be used, such as adding a
> second transistor to provide a current path in place of an open LED.
> - Win
I thought about the added stuff. But then I started out with the other
solution, which I posted previously. No base current, because no base!
No problem! Hey, it was Win who said that these 2N7000s make things a
LED Current Controller Basic (Block) Diagram:
+--------------------+------O + 4 AA Ni-Cd cells
| | 4V to 5.6V
| ----- White LED
| \ / 3.5V at 30 mA
| | | D
| Constant | |--+
| Voltage | | Enhancement
| Source |---------||<-+ mode MOSFET
| Equal to | G | | 2N7000, BS170,
| Vgs(th)+.3V | |--+ NTE490, etc.
| | | S
| \ R3 10 ohm
| / Current
--- \ Setting
GND / resistor
For FETs with Vgs(th) 2.2V and higher:
| | 4 to 5.65V
| | 4 AA Ni-Cd
\ R1 | Recharge-
/ 1.5k | able Cells
/ \ / White
| --- LED
| | | |--+ D
| | | | Q3
| TL431 \ R1 +------||<-+ 2N
| +--->/ 25k | | 7000
| K | \ Curr |--+ S
/---/ R | / Adj. |
/ \-----+ | |
--- | |
| A \ |
| / \ R3
| R2 \ / 10
| 47k / \ ohms
| | /
| | |
| | |
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you put NOSPAM in the
Subject: line. alondra101 hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html