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Topic: Can switching regulators and transistors be smoothly PWM'd? (Read 2042 times) previous topic - next topic

Boardburner2

I mentioned 20 mA.
That's rather bright for a switch indicator.
2 -3 mA should be enough.
Try using a 470 ohm resistor to start with.

Wawa

OP wants PWM brightness control, so 10-20mA is ok.
Maybe this switch has to be seen in sunlight as well as in the dark.
Maybe wise to post a link to these switches.
They could already have inbuild current limiting resistors.
Leo..

TomGeorge

Hi,
I really think we need;

A copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

So we know exactly what the OP nw has in mind.

Thanks..  Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

TomGeorge

Hi,
Quote
I don't trust myself to PWM the lights directly - if I put the duty cycle a bit too high and an LED burns, ordering another switch will be $25. If I'm only playing with a safe voltage like 2.8, I can't screw it up. Limiting resistors are also out, they waste too much power.


Quote
Edit: Oh, okay. I guess I need a current limiting resistor in case the voltage out fluctuates a little bit. I'll add one. But I'm still okay to PWM the transistors as planned, right?
Have you thought of this?
http://www.ebay.com/itm/5-pcs-3W-5-35V-LED-Driver-700mA-PWM-Dimming-DC-to-DC-Step-down-Constant-Current-/252056674079

google      led driver pwm input

Tom.... :)

What are the specs of your LED?      What is its current?
Everything runs on smoke, let the smoke out, it stops running....

ElCaron

Are we actually discussing to use a constant current LED driver for a 0.05W indicator LED (that is inconveniently stuffed in a relatively expensive package) now? Come on, guys. Put a current limiting resistor in series with the LED on you PWM capable Arduino pin and be done with it. 5V/250Ohm=20mA, so put 280Ohm in there and there is no way whatsoever you LED burns. You will get less than 20mA, of course, but for an indicator LED, that should be fine - from want you write, I get that you are afraid for your LED, not want to drive it to it's limit.

jverive

Kinda need to know the LED specs, including Vf at It and Vf at Imax. It would also be helpful to know the luminous output vs. If, as well as some details about the environment (bright office lights, moderately lit warehouse, dimly lit corridors, etc.).

You really do want to use a current source or current limiting resistor with your PWM circuit. Vf for a given LED can vary a couple of tenths of a volt between manufacturing batches (and even within a batch), and this can make a big difference in brightness (especially at lower currents) and LED life (at higher currents) with a constant voltage supply. I have seen these differences many times in my 20+ years of design and failure analysis.

ChrisTenone

At first I thought this was some sort of fancy, high power LED that needed a special power supply. Now I understand it to be the same thing as covered by the fifth Example in the IDE's Example menu.
What, I need to say something else too?

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