From: Winfield Hill
Subject: Re: Best way to power array of LEDs?
Date: 8 Dec 2002 12:31:17 -0800
Organization: Rowland Institute
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <2Qan9.3129$cS4.firstname.lastname@example.org> <3DC843FE.4A25FFF3@fanwap.com> <3DC99AD4.2AB0FE52@fanwap.com>
X-Newsreader: Direct Read News 4.20
> email@example.com wrote,
>> Generally a transistor beats a few diodes, and takes about the same
>> amount of space. In the circuit below a Vbe-multiplier reference
>> has replaced the diodes. The adjustable nature of this type of
>> reference allows us to run the current-sense resistor at 0.2V, and
>> a properly-chosen collector resistor R4 allows us to cancel out
>> the effects of changing supply voltage. To a first approximation,
>> R4 = R2/R1 * r_e for Q1 at its 1mA operating current, or 36 ohms.
>> 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
>> | | | |
>> A quick calculation shows the voltage at point x should be about
>> 0.95 volts, setting the value for R2/R1 at 0.95/Vbe - 1 = 0.46
More accurately, R4 = R2/R1 * r_e for Q1, or 36 ohms.
> But Win has given us a simple solution (above) that can be built with BJTs
> available at Radio Shaft. And no pots to adjust(!) The only problem I see
> with his circuit is that with three or more LEDs, the base currents for the
> drivers may add up to more than the constant voltage source can supply. So
> I have to look at increasing the current to a couple mA, or else use more
> than one constant voltage source. Because just about everything that I've
> built has a half dozen white LEDs or more in it.
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. :>)