Re: Driving large numbers of LEDs
- From: "Bob Eld" <nsmontassoc@xxxxxxxxx>
- Date: Thu, 15 May 2008 09:39:41 -0700
"Guy Macon" <http://www.guymacon.com/> wrote in message
news:zZOdnUY8rb7YxbHV4p2dnAA@xxxxxxxxxxxxxxx
I have a new client with an interesting product. They are driving
large quantities of LEDs (in the hundreds, with future products to
be in the thousands) in banks of red, blue, and green (with, of
course, different voltage/current requirements for different
colors). They want be able to dim each color using a microcontroller
-- PWM is OK as long as the human eye doesn't see any flicker.
Maximum efficiency is important: they want it to run cool and to
save energy. They are looking at a first run of 100 units to
test the market.
How would you go about meeting these requirements? An off-the-shelf
constant-current power supply and some uC-controlled switching
transistors is the first thing to come to mind for such small unit
quantities, assuming that I can find a power supply that is happy
having the load go from zero to max and back every few milliseconds.
Anyone have any better ideas?
--
Guy Macon
<http://www.guymacon.com/>
Each color should be separate. Each color should be made up of series,
parallel strings operating on as high of voltage as reasonable, say 140
Volts. Each series string would connect to this 140 V. rail through a small
ballast resistor. The number of LED's in series depends on their operating
voltage at the current of interest.
For example, Say the voltage of each LED was 2.2 volts at 20 mA., and, you
want 10 Volts of ballast voltage. Then the string voltage would be 130 Volts
and there would be 130/2.2 = 59 (60 is ok) diodes in series. The ballast
resistor would be 10/.02 = 500 Ohms.
As many parallel strings of 60 diodes each are used to make the total
required. Each series string has its own ballast resistor. Each color is
made up similarly but has its own number of LED's in series and ballast
resistor values for their specific voltage and current
An appropriate high voltage FET or power transistor of sufficient current
rating connects each color group to ground when on. These FETS or
transistors are driven from microprocessor ports. They are switched on and
off in a PWM fashion to control the brightness. Code in the processor
controls this at a frequency high enough to avoid any visible flicker, say
above 1kHz.
The only regulator necessary is for microprocessor power. The ballast
resistors perform the current equalization function, no regulation is
required for the LED's. Current feedback could be applied to the micro to
control the PWM for current regulation if necessary.
.
- Follow-Ups:
- Re: Driving large numbers of LEDs
- From: Guy Macon
- Re: Driving large numbers of LEDs
- References:
- Driving large numbers of LEDs
- From: Guy Macon
- Driving large numbers of LEDs
- Prev by Date: Earthquake hits Mexico this morning
- Next by Date: Re: 0.1 Volts output voltage regulator
- Previous by thread: Re: Driving large numbers of LEDs
- Next by thread: Re: Driving large numbers of LEDs
- Index(es):
Relevant Pages
|
