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Challenge: Simple power-line LED

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Hello again Carbon,

Oh did you tell us that already? I might have missed that fact about the standard mechanical switch. My 'bad'.

However, you're still going to be placed under arrest for violating the electronics fashion police good design practice policies :)

Let me ask you this anyway:

If the LED runs for a year from now would you really want to do it this way in the future for an indicator that you really had to depend on or even a general purpose indicator, or would you prefer to use a tried and proven safe technique to run the LED?
 
Why do you seem offended by this? Is it really that egregious an infraction against the Electronics Police?

Hi CZ. I'm not offended by it. It just kind of bugs me that you think that just because this method works for a few LEDs, it is an ok way to light an LED. Yes, we all understand that it might work, but it is a method that should not be used on purpose. It just isn't the ideal way to drive an LED.

Der Strom
 
However, you're still going to be placed under arrest for violating the electronics fashion police good design practice policies :)

But ossifer, ossifer!

Let me ask you this anyway:

If the LED runs for a year from now would you really want to do it this way in the future for an indicator that you really had to depend on or even a general purpose indicator, or would you prefer to use a tried and proven safe technique to run the LED?

Well, I'm tending to reply that while I wouldn't use a single LED, for reasons that have been explained here (power factor and wasted power in the reverse direction), I would go ahead and use 2 LEDs back-to-back, with no intervening resistor even.

I'm thinking of the case of a commodity-item power strip; I can now buy these things in my local dollar store. They're practically throw-away items. (Not that I like throwaway items; in fact, I hate them.) So ASS-U-MING that back-to-back LEDs with a capacitor would pass muster with the UL, then yes, I might consider doing just that.

We're not talking about "mission-critical" components here. I would certainly never suggest doing this in, say, a heart-rate monitor used in a hospital or some such.

I believe someone in that other recent thread on this pointed out that they saw just this in a commercial power strip or something.

Please, let's keep a little perspective here.

By the way, I nominate the circuit in this post as the best example of overkill in lighting a LED directly from the poweline. I mean, four diodes, two capacitors and a resistor to drive one lousy LED? Heck, why not go all the way and build a dedicated, regulated SMPS for that one little diode?

I suppose that's the way NASA would spec it ...
 
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Hi Carbon,


These days two back to back white LEDs is definitely a possibility well worth considering, but back some years LEDs where not that cheap so other ideas such as bridge rectifiers have been used. They are cheap too, and four diodes are very cheap (28 cents). The electrolytic cap is there to prevent some brightness fluctuation as the full wave rectified sine alternates up and down between peak and zero volts. In some cases where a 120Hz brightness fluctuation would be annoying that cap would be mandatory. That cap also helps keep down surges through the LED when the product is plugged into the line with no charge on the capacitor. Cant use back to back LEDs as simply if a cap is needed here, although it can be done.
 
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I sense you are gradually coming around. :)
You calculated the impedance of the cap at 60 HZ, but now think about when you throw the switch and the sine wave happens to be at its peak. What would the impedance of the cap be if the rise time of the switch was say 100 NS? Assuming a cap with zero ESR, what would the current be?
 
The only reason it works is because the LED fails once every cycle, which allows reverse current through the cap. With that particular LED the failure just happens to be something like a "zener effect" and has not yet destroyed the LED. Do you have a 'scope photo of the voltage across the LED? That would something more interesting than just arguing.

Anyone who designs things made to last would add a 1N4007 across the LED, and a series resistor with the cap. That raises the circuit from "just hasn't blown up yet" to something that a professional would expect NOT to blow up. Then someone who likes reliability would also add a cap across the LED itself, as I said in an earlier post. In production quantities a diode and resistor are maybe 1 or 2 cents each, it's a good example of "false economy" trying to save 4 cents and the result is a really poor design that relies on it "just hasn't blown up yet"... :)
 
it's a good example of "false economy" trying to save 4 cents and the result is a really poor design that relies on it "just hasn't blown up yet"...

Oh I could say so many things about vehicle designs and home appliance design right now it all most hurts! :rolleyes:
 
Yeah I hear you on cheap home appliances designed to blow up after a while, but they also have to last a reasonable time, to get them out of warranty.

I don't think Carbonzits circuit is even suitable for that as it would fail almost immediately for a significant percentage of users (those with mains spikes for instance)!

It would be more typical to use a low wattage resistor that slowly cooks and lasts 6 to 12 months, something where the manufacturer would reasonably expect it to always work for X amount of time and slowly increase chance of failure over time. Or maybe parts pushing their current or voltage ratings.
 
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