Why does this circuit blow up LEDs?

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#21
At a quick guess I would say that when that LED is active it turns all the others off because you then have just the single forward volt drop of that one LED. That drops the available voltage to around 2 volts and so the series chain go out.

So its maybe a bit better thought out than first appears and achieves complex switching with the minimum effort.
 
#23
Absolutely. You may need to go much lower than that LED depending. Modern hi brightness LED's are as bright on 0.5milliamps as old LED's are on 20ma. So replacing those 10k's with a single resistor of perhaps 100k could be all that is needed.

(It is important that the volt drop of the LED's is similar so don't fit a white or blue one for D605 and then red ones for the others. Otherwise the series chain may not fully go out when D605 is lit. Keep all the LED's the same)
 
#24
Mooly said:
At a quick guess I would say that when that LED is active it turns all the others off because you then have just the single forward volt drop of that one LED. That drops the available voltage to around 2 volts and so the series chain go out.

So its maybe a bit better thought out than first appears and achieves complex switching with the minimum effort.
Click to expand...

The LEDs are all in series and the switches are across each. So all can be ON or OFF

The resistor 2k5 should stay in circuit.

Gajanan Phadte

Edit:ease of everything
 
#26
LEDs finally showed up!

I replaced them all. Worked great!

Now, after screwing everything back together, it has stopped working great. The line straight LED cannot lit up anymore. It is completely dark. But it's not dead! If I have another LED lit up (e g loudness), the line straight LED actually sort of works - it flashes briefly when I press the button. Why...
 
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#27
Until you have fixed this it might be worth pulling that 220uf cap out to save the LED's from getting spiked.

It sounds like you have a break or intermittent connection somewhere.

Pull the cap out, get it in the faulty state and then measure the voltages around the LED network. Its just a series path so should be easy to find.
 
#29
I can't kill the LEDs anymore. It's just the line straight one that behaves in this strange way. It just flashes briefly when it's supposed to stay lit. And again, only if another LED is already on - otherwise it remains completely dark.

I've looked over the connections and everything seems completely perfect, and even if I just semi-press the switches (trying to cause an intermittent connection), nothing bad happens.

It literally works perfectly now. Minus that weird line straight...
 
#32
With two t092 style transistors like the 2N3904 and one 50 ohm resistor you can make a bullet proof, fast, current limit that you could even short out and it won't blow. It will certainly protect a string of LED's. I've used it for protecting motors, LEDs, opto couplers, and audio output transistors. Put it after the cap, before the first LED. It is wired as a two wire, series module. You have plenty of V to keep it biased on. It will never let anymore than 12ma through it under any conditions. Changer the R to 40ohms and it will limit at 15ma.
If I can figure out how to find and point to a URL, I'll attach an image. Do I get assigned a URL as a member?
Anyone?
 
#33
Nico Ras said:
Agree with all Mooly's statements that a momentarily voltage spike kills them. I would support Jean-Claud suggestion further by clamping the LED supply with a zener replacing the C523 No need for constant current
Click to expand...

It's Jean-Paul but never mind, I like Jean-Claude better 😉 With a CCS or CLD it won't matter if one or more LEDs are switched in series as the current will be constant. It seems that spikes are then also solved if they were a problem to begin with.

From experience 2 mA is more than enough with modern LEDs. You won't have to wear sunglasses then if you switch the thing on.

edit: after checking the schematic again it occurs to me that C523 and C524 could be exchanged. C524 normally would be the 220 µF IMO.
 
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#34
Okay guys, I will try your ideas. But does anyone understand why this LED does what it does? How come it only lits up for a split second instead of staying lit as it is supposed to?

And why did it work perfectly first and then started doing this after I put the amp back together?

Note that it doesn't die: I can make it blink as many times as I press the button.
 
#36
rayma said:
Sounds like it's a momentary switch.
Click to expand...

It seems more like the line straight LED immediately drains out the power supply when it gets turned on and thus doesn't light up any more than a split second.

Because if I turn the amp off with the other LEDs still on, these will keep glowing faintly for a long time, due to there still being some power left. But if I press line straight, the power immediately runs out and all LEDs get completely dark.
 
#37
Wonderful you guys.

I replaced that weird LED. Turns out it was broken. New one worked fine. But it got killed again.

Removed the 220 µF capacitor and put in yet another new LED (weird, that capacitor was metallic shiny purple, looked real fancy-like, much more so than the other caps in this amp). 😉

And NOW we're talking. Everything works beautifully now. For real.

Thank you SO much. This was driving me mad! 🙂
 
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