LED on AC

[Zanius]

Hey guys, I'm set on trying to build a schumman pll. (yeah, good luck there)

While searching and setting up for the build I found out that LEDs won't work on AC as they do on DC. (the pll uses 12VAC power)

So I've been searching around but I haven't found a proper answer on how that will work properly. Some say you need some diodes in serial, others say you need one diode parallel.
I have a ac wall wart and maybe I could run some tests, but I'm kinda afraid since I don't know my way around AC  

Is there a safe way to power them or should I look for AC mini bulbs? I want to have 3-4 leds on the box so even a circuit that would transform ac to dc would be nice.

[Andy Miller]

DC power supplies for audio circuits usually contain elements of filtering, smoothing, etc.  But if you just need DC to power your LED's, maybe you can get away with something simpler like this?

I'm not sure.  Someone else who's more certain will probably chime in.

[Ciaran Haslett]

I've done this converting guitar amp pilot lights to take LEDs.  2 easy ways to do it, with pros and cons.

1. Slap a diode in anti-parallel (top to tail) with the LED.  This blocks reverse voltage on the LED.  Pro...it's really simple to do and only takes 1 component.  Con...your LED with flicker at 50/60Hz which you may find annoying..or as with some guitar amps...can introduce hum.  Your resistor will still need a CLR remember

2. Buy (or build) a rectifier bridge.  This will convert your AC LED supply to DC.  Pro...no flicker.  Just wire your LED as normal.  Con...you need more parts.  I've had to do this with some amps to stop hum, even to convert the heaters to DC (an absolute MUST on Epi Valve Jrs by the way)

Just to stress...I've only done these on guitar amps with 6.3VAC heater supplies.  As you're using 12VAC I'd imagine you'd need a much larger CLR than you're used to.  Also, with point 1, you'll need to wire one side of your AC to the footswitch where you would normally have wired GND.  i have no idea if this will cause loud popping from the LED when kicking the effect in.

Good luck

[Travis]

If all else fails, there's pretty much no chance in hell you won't notice the PLL is on

[Zanius]

Haha, that's true.
I'll try building a rectifier then, and test it with another circuit to see what will happen pop-wise. Ok thanx!

[strewel_peterson]

I think this might be less complicated than it first appears. Looking at schematics for the Schumann, it looks like the feed for the indicator light is taken from the -12 volt DC supply rail - after rectification, regulation and smoothing, so you'd just need to work out an appropriate clr value as usual (but with the led the opposite way round to how we normally wire them, because it's a negative supply).

LEDs will work on AC fine (flashing at the AC frequency, but 50 or 60Hz will most likely) look like they're continuously on because of persistence of vision - they're just diodes, after all, and won't conduct at all one way, and will conduct above Vf the other way (until your voltages/currents get high enough to destroy the diode).

If I rember correctly, bicolour leds are used in some applications to indicate dc polarity and ac - eg red for on polarity, green the other and orange (both lighting up) for ac.

The pulsing current through the led might mean more hum - but I'd guess that this will be negligible with those smoothing caps in the circuit.

[Zanius]

I noticed that as well, without ofcourse being able to explain it as well as you did :)

I build and tried a bridge rectifier and it works fine. Once I am done with the circuit I will try a led on that output. It is marked as "indicator lamp" though, something that leads me to believe it is destined for use with a "panel indicator" lamp.
Something like this


Anyway, I'm sure that if I can get the rest of the effect to work, I will work out something for indication.
Thanx!

[motterpaul]

That is cool that you worked it out with a bridge rectifier. It makes sense, but I would not personally know how to implement it. I guess my mind would have gone to "why don't I use a small lamp?"