From: Keith Wootten
Subject: Re: Best way to power array of LEDs?
User-Agent: Turnpike/6.02-U (<7r9OMDU2ooRJCQmjnYJxyjtHfm>)
Date: Fri, 4 Oct 2002 09:14:21 +0100
NNTP-Posting-Date: Fri, 04 Oct 2002 09:21:38 BST
Organization: ntl Cablemodem News Service
In message <firstname.lastname@example.org>, "Wouter van Ooijen
>>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?
>The crucial point is that when you put a resistor and N LEDs in
>series, the LEDs will take roughly a constant voltage so all voltage
>variation from your battery will be over the resistor, thus causing
You can mitigate this to a useful extent by using a small filament bulb
instead of a resistor. The positive temperature coefficient of the bulb
will help to keep the current variation reasonable as the supply voltage
changes. The bulb should be selected to run at more or less normal
brightness at maximum supply voltage while passing maximum LED current.
My bicycle rear light is an array of 50 LEDs from RS components which
are arranged in a series/parallel configuration with a nominal Vf of 9V5
IIRC. Using two small paralleled 5V lamps (can't remember the type -
selected from a junk box) the current swings 2:1 for a 14V-10V supply.
In my case, it's also a useful low battery warning - the filament lamp
goes out long before the LEDs.