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Topic: LEDs without the use of current limiting resistors (Read 5 times) previous topic - next topic

Hexadec

This is starting to look like my threads.... :smiley-mr-green:


Sponge.
I'm allowed to have it 'cos it's not sharp.

dhenry

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The thing is that I would love to know wich rules/relations/equations does this guy use to get the current average just with the amount of time that the led is on and off, in this part of the code:


From the datasheet for the mcu and the led.

Leds, especially high brightness leds, behave far more like resistors (at high current levels) than like diodes, in that their voltage drops go up significantly with the current through them - as a north-eastern-ish V-I curve.

That, coupled with diodes surprising resistance to pulsed current, makes them fairly resistant to current shocks.

Together with the mcu's output resistance, this "no resistor" approach works.

Engineering isn't about "you cannot do that", or "that's wrong". Engineering is about knowing why you cannot do it so you know when / where you can do it.

People holding a simplistic view just lack common sense to understand that.

retrolefty

Quote
Engineering isn't about "you cannot do that", or "that's wrong". Engineering is about knowing why you cannot do it so you know when / where you can do it.

People holding a simplistic view just lack common sense to understand that.


You know dhenry that is probably one of the few bits of wisdom from you that I can agree with. But even then such wisdom when responding to beginners to electronics is bound to do more harm the good. Too much cargo electronics going around these days as it is.

Lefty

Hippynerd

Mike, I dont have that equipment, nor the expertise to use them, but i have used a microscope before to examine dies. You can probably just use a regular microscope for this scale, and The LED even has a clear lens to see right into it. I still wont be bothering to do that. Instead I built the thing, and have been running it as a test to see how long it will take to fail, and it has shown no signs of failure yet. Its been months with no sign of dimming, no smoke... it just keeps working.

Dhenry, thats some sound theorizing. I remember before when you mentioned batteries having an internal resistance, and shift registers possibly working as current limiting devices. Your theories have merit, but I have no idea how to test the theory.

its also possible that parts are under rated or materials or manufacturing have changed, and made improvements that never got re-factored in or tested.

CrossRoads

If you have steady state current, open a LED wire connection up and put a meter in series, see what the current flow is.

If its multiplexed, put a shunt in series, and with a scope see the voltage is across the shunt, calculate the current.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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