Pulsing LEDs to achieve maximum output?

without burning

illumination, but want to

to be applied

type?) handling

ie: current,

what transistor

that weren't

applications such as

these crossing

while.

priced a wee bit

Read the last thread and you will see that pulsing leds REDUCES the output compared to that obtained by the same average current at dc.

I guess the theory is you could supply more V at the junction, letting it cool in the off period, and get higher output, but you're also switching it on and off (quite possibly reducing output than if CW)

The idea is to get the LEDs to last long, ideally, my simple illumination project would consist of nothing more than a simple bank of LEDs, and a voltage regulator! (skipping any of the pulsing)

Why not use a few more LEDs then run at a lower setting.

More light less loss!!

Regards, Gordon.

output

voltage

far

just

LEDs

expect

output

cool in the off

(quite possibly

project would

regulator! (skipping

cool in the off

(quite possibly

The problem is the eye sees the average brightness. Put simply if you double the power into the LED but for only half the time you get no change. (Even that assumes that the LED power efficiency is constant for all power inputs.)

This trick does work under certain conditions but typically the frequency and mark space ratio has to be low enough for the flashing to be seen. For example 0.25 second ON, 2 seconds OFF.

project would

regulator!

Actually you need a current regulator not a voltage regulator but that's being picky.

Colin

Hi Colin,

Curious, lets say I have 10 LEDs that draw 20mA each, so 200mA total at lets say the breakover voltage is 2.0V, wouldn't a 7802 (if they even have regulators that small, 7805 common enough) be able to regulate it?

With the "7802" regulator, you'd be able to handle 50 LEDs (1A) I suppose, if my calculations are correct.

I just don't want to burn out a ton of LEDs cuz I could have done something different, research first, then try it ;). Carpenters measure twice, cut once.

lets say the

that small, 7805

I did say I was being picky....

LED's act like a "constant voltage sink" so if you want to connect them to a voltage source you need to put something in series to limit/regulate the current. You can use a resistor in series to limit the current.

Using your examples...

R = (Vs-Vled)/I R = (5-2)/0.02 = 150 R.

and the power dissipated in the resistor is

P = 3 * 0.02 = 60mW

so 1/8th W resistors are OK.

You could try putting two LED in series with each resistor and recalculate the value needed as...

R = (5-4)/0.02 = 50 R

and then

P = 20mW

Basic maths says that if you want an accurate answer to an equation (eg accurate control of the current) you should avoid subtracting one number from another similar one (eg avoid Vs-Vled approaching 0!).

On the other hand the larger the voltage drop across the resistor the less sensitive the curcuit is to resistor and LED tollerance - so you may not need a voltage regulator at all (but watch the power dissipation in the resistor if it's very large).

Overall you need to balance these issues.

What is the source voltage? (eg input to the regulator).

Colin

Essentially, Vs can be just about anything at this point, I will design around the load requirements. It will be stepped down from AC, rectified and filtered.

Thanks for the info, I'm trying to source out a local supplier for some white LEDs of good quality and of the output I require (the more mcd the better).