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From: Lizard Blizzard
Subject: Re: Best way to power array of LEDs?
Date: Fri, 04 Oct 2002 17:54:54 -0700
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <3D9D32DE.email@example.com> <3D9D5939.CD795C11@kth.se>
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Well, since everyone is doing ASCII schematics, so will I.
LED Current Regulator Using Two Transistors
ASCII schematic Oct 4, 02 (view with courier font)
--- 1 or
\ / more
k / Q1
| |/ NPN GP
Q2 \| |
NPN GP |-470---+
/| | Sets LED
/E 3 current = 0.6V / R
| 3 33 ohm gives about 18 mA
Since the 33k current also goes thru the LED, the lower I limit is when
the emitter resistor approaches infinity, when the only current thru the
LED is what is going thru the 33k. So the 33k can be connected to the
positive supply instead of thru the LED. This 33k value may have to be
changed if the circuit is regulating low voltages or high currents.
One other thing I just thought about. If you need multiple LED strings,
you don't have to duplicate the Q2 transistor and 470 ohm resistor. You
would divide the 33k by the number of strings, so 4 strings would be
about 8.2k (connect it to +V). Then connect all four Q1 transistor
bases together and use a separate 33 ohm resistor for each of the four
Uwe Zimmermann wrote:
> Tilmann Reh wrote:
>>>Have assembled an array of LEDs for use as a 3rd brake light in my car,
>>>configured as 4 rows of 9 jumbo LEDs, as of yet unwired. Of course the power
>>>supply will be 12 volts nominal (13.8 volts, maximum).
>>>What is the best method to power these from an intermittent (whenever I press
>>>the brake pedal) power source?
>>Better use many LEDs in series to avoid unnecessary power loss.
>>With 12V supply, you can use about 5 LEDs in series and a smaller
>>resistor for each chain. These chains are then connected in parallel.
> The series connection of as amny LEDs as the supply voltage allows is
> indeed the way to go. As others stated correctly the bus voltage in a
> car is far from constant and therefore the use of a resistor might
> result in significant current variations through the LEDs, but here we
> actually can use the simple constant current source I suggested in a
> different thread recently.
> For a min voltage of 12V in a car and a typical LED voltage of 1.6V you
> can wire up 6 LEDs in series, giving exactly 6 identical chains in your
> case. Even if the LEDs should drop 1.8V, this would be sufficient...
> Now take one BF245B/BF245C JFET transistor for each chain and connect
> source and gate together. This will give a constant current source with
> the characteristic current Idss. Since this current spreads from
> transistor to transistor you should get yourself some more of those and
> select 6 which have a current which suits the parameters of your LEDs.
> You then connect the drain of the transistor to the positive supply and
> the gate/source to the anode of the first LED in each chain. The cathode
> of the last LED is connected to the negative supply.
> 12V o---o o----o---|>|--|>|--|>|--|>|--|>|--|>|----o 0V
> | | |
> ------ |
> | |
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