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Changing DC Circuit to Pulsed DC Answered

I am interested in changing my fixed current DC circuit to pulsed to increase the brightness of my LEDs. I really have no direction when it comes to making this pulsed circuit. I have looked into precision full wave rectifiers that I think are what I would be looking for. I am not sure what type of pulse width I would want. The one that I think would be ideal would be a Mouser DF01M, I am not sure if a pulse width of 200 microseconds is fast enough.



25 days ago

To give everyone a better chance of helping you, please give the following info:
What voltage are you working with now`?
What current are you feeding the LED?
How do you limit current now?
What is the LED voltage drop, and what is the max specified current?
Is it running on a battery supply?
This is crucial info, to give you a solution that you'll be satisfied with (although, the LED won't be any brighter than its spec says (like nn Cd in nn degrees), without shortening its life to some extent.
If you can accept a more narrow beam, you can get an apparent increase in light output, as the light will be concentrated in a smaller spot - this takes an optic lens or a mechanical change of the LED itself (/or rather the part of the LED that act as a lens).

But you can get a seemingly brighter LED, by pulsing it with short pulses of a higher current, but the pulse/pause relationship (the Duty Cycle) has to allow the LED (actually the crystal radiating the light), to cool off sufficiently in the off period of the pulse train.
The reason that works, is the slow response of the eye, which will see it as constant light.

To get directly usable help on the topic, please answer the questions on top A.K.A. Help others to help you :)

(I just designed an LED driver good for around 10 MHz - with regular red LEDs running at 60..90 mA, so anything below should be a walk in the park)
Have a nice day :)


26 days ago

PWM or Pusle Width Modulation is NOT to increase the brightnes sof a LED. It is for the contrary: To DIM THE LED.
See, the human eye cannot see a flicker if it flickers fast enough (lets say 100Hz+ or 10ms and below). But if you toggle the LED every 10ms, it stays off 50% of the time. We see this as dimmed. The percieved brighness is NOT 50% with 50% PWM thou... The human eye is not linear in terms of brighness...

Anyway: It looks like you have a "nice" DC voltage and want to switch over to a fully rectified AC.
No problem with that. You simply have to be sure your LED can take the rather long pulses in betwheen the pause-persiods.
See, normally one has 50 or 60Hz. Thats around 20ms pulse width.
The LEDs i work with, specify pulses of high current to around 1ms and below.
So, your peaks have to fit the DC-realm of your LED and resistors.
This in turn leads to another problem: The peaks are the new DC-regime. The pauses in betwheen the peaks are BELOW the DC-operation-mode and therefore dimmer than before.
And tadaa... We have a PWM with ON = your previous DC. This means, you will automatically dim your LED if you are running it on fully rectified AC instead of DC.

You can add a capacitor to smooth out the bumps thou.


26 days ago

It is far easier to replace the LED's with high brightness types ;)
A pulsed LED does not always appear any brighter because you still need to cater for the off time of the pulse.
And either way, going over the rated limits for the LED will have a negative impact on the lifespan.

For example the LED in my little projection clock kept failing after a few months and I kept replacing it.
One day I just grabbed a super bright one and added a resistor to run it at about 80% of the rated current.
Still much brighter than the original and I have not replced the LED in over 8 years now...
And it is on 24/7....