LED question

[midwayfair]

You can not, ever, place diods/LEDs parallell!

Huh? Of course you can. I really don't know where the rumor got started that the diode with the lowest Fv completely takes over, but it's demonstrably false simply by USING LEDs so we don't have to use just out ears. If you put a green LED in parallel with a RED led on the same current limiting resistor, the green one will be dimmer, but it will still turn on. The red one will dim almost impercetably.

Now put them on two different LDRs -- they will work as normal. And it's a good thing, too, or we'd never be able to have parallel circuits with semiconductors -- including a rransitor-based pedal with an LED indicator in parallel.

I'm on a tablet and making dinner, or I'd go downstairs and put some LEDs on the breadboard for photo evidence.

It's possible I've misunderstood what you meant.

[RobA]

If you are using identical LED's, it works perfectly well too with both of them turning on just right.
I think maybe "the don't do this" comes from a couple of points. The first is that it's a bit weird to figure out what they are going to do since there is no parallel diode rule. The second and bigger point is that diodes don't stay the same over time and they don't age the same individually. If the characteristics change too much, then the circuit doesn't behave the same way and the failure isn't very predictable. If one gets too much out from the others, the brightness flops around drastically. If they each have their own resistor, then gradual changes will only cause small changes in brightness instead of major shifts.

[Vallhagen]

It's possible I've misunderstood what you meant.

To avoid possible misunderstanding; this is how i meant:

, where i would choose a Fig 2 design if i wanted to individually control broghtness/current flow through each LED. I would choose Fig 3 if the LEDs were "similar" or close to similar.

But i think we disagree on Fig 1?

I expressed myself a bit drastically; "you can not, ever"; which i maybe should change to: "if you are lucky, it might work". LEDs are - as we know them - components that we shall not really wear out at all, right. They work a lifetime and more. If you design as Fig 1, the risk is that you will wear out one of the LEDs early. The small difference (because there most likely is at least a small difference) in voltage drop cause big difference in current flowing through the LEDs. I'd say Fig 1 shows "bad design" and i put it on the "dont do this" list.

This link goes a bit deeper, and gives some calculated example as well (1 % voltage diff can cause 18% current diff): http://electronics.stackexchange.com/questions/22291/why-exactly-cant-a-single-resistor-be-used-for-many-parallel-leds

Cheers

[RobA]

Yep, it's figure 1 that's the question. I think if you just change "Forbidden" to Bad Idea, there won't be any more question. 

[Vallhagen]

Yep, it's figure 1 that's the question. I think if you just change "Forbidden" to Bad Idea, there won't be any more question. 

You're right. Changes made. After all, it aint really illegal;). And pragmatically; in cases like this, where all currents are small and voltages low, we cant really cause any dangerous damage by designing things wrong or bad. But if we should setup parallell diodes in - say - a 230VAC rectifier, the word "forbidden" would be suitable. It could cause a fire.

[RobA]

Really good point there. I was only thinking in terms of LED's and low voltage. It could definitely be a dangerous failure mode in the situation you describe.