led's as pilot lights

[phsyconoodler]

I just built a little amp with a skull on the faceplate and I want to use 2 led's in the eyesockets.(no it's not halloween,I know)
  Can I wire them using the 117v AC in voltage from the on/off switch? I'm not an led guy so I'm at a loss to say if it would work. I have 2 led's with a forward voltage of 2.4v each and will put them is series.

  I think a diode to the plus side,and a 2.2k 1 watt resistor top ground on the tail end would do it.

Someone correct me on this one if I'm about to blow something up here.

Edit: after doing some reading I think 117 v is too high.I will use my heater voltage to do it.

[DummyLoad]

117v AC in voltage from the on/off switch?

conventional LED? no.

you need at least 1/2 wave recto, some filter, and volt divider. some LEDs bundle this in a housing or same (larger) envelope. newer, AC powered LED strings form part of a bridge ckt.

bear in mind any circuity connected directly to the MAINS (no isolation) is usually not a good plan.

best solution is to tap the filament string with a small DC PS.

--pete

[HotBluePlates]

Also be sure to look at what current draw is typical on your proposed LEDs. You will not usually need to allow it to draw the full typical current for full brightness.

I think you are wise to attached the LEDs to the heater circuit. That is where I put mine on my Standel amp.

[kagliostro]

May be I'm wrong but I'm convinced that a LED can function also if connected at an AC line (of proper voltage and with a proper in series resistor) after all a LED isn't a diode ?

it self rectify the AC voltage and bright

K

[phsyconoodler]

I added mine to one leg of the heaters and now it hums.I obviously unbalanced the heaters.I might have to add a balance pot or?????

[HotBluePlates]

Place the LED from one heater leg to the other, not from one side to ground.

[phsyconoodler]

Ah I see! So two 2.4v led's in series and across the heaters?

Edit: I put the two in series and across the heater wires.They work and the hum is gone.Even my dull mind figured it out with a push in the right direction!!! Thanks guys!!

[HotBluePlates]

Yep.

Ideally, there is a current limiting resistor. Assume your LEDs are specified to draw 20mA for full brightness. You note they have a forward voltage of 2.4v.

6.3v - 2.4v - 2.4v = 1.5v to be dropped.

The circuit current is 20mA, as the LEDs are in series. 1.5v/0.02A = 75Ω

100Ω is probably close enough, and I promise the LEDs will still light and with near-full brightness with somewhat less than specified current. 1.5v * 0.02A = 0.03w, so a 1/2w resistor is more than enough (or 1/4w or 1/8w, whatever you have on hand near that value).

[phsyconoodler]

Thanks HBP!

Here's a shot of the little amp chassis.It's gonna be a 'hanger' with the tubes down,going in a small head with a matching 1-12 cab.
  It has an enormous amount of gain on tap but with my tele it can get lovely cleans too.No squealing feedback at all unless you put the headstock on the amp.Sustain for days too.

[DummyLoad]

after all a LED isn't a diode

yes, with a poor peak reverse voltage (PIV) spec. they shouldn't be used as such. if it works though...  

--pete

[HotBluePlates]

I like the skull!

In the pic, the plexi panel looks an inch thick. I always liked how the gold paint behind the plastic really gives it an impression of depth.

[Willabe]

Hey what's on the 2 preamp tubes?


           Brad     

[phsyconoodler]

There are a couple of halloween finger puppets on the preamp tubes;a ghost and a goblin.

Actually the faceplate is Gold aluminum.

[PRR]

> 117v ... 2 leds ... 2.4v .. is series. 2.2k 1 watt

117V-2.4V-2.4V is 112V.

112V across 2.2K is 51mA.

LEDs are usually 20mA max.

Also 112V, squared, divided by 2200 ohms is 5.7 Watts.

This "can" be made to work (with different values, maybe better plan).

But with 2.4V parts you really should be looking for a lower voltage. 6V heater supply is a lot closer than 117V.

> a LED isn't a diode ?

Yes, but the Reverse Breakdown is low, and not well specified.

Often it is speced as >5V. Now if the supply voltage is much greater, then the resistor will offer about the same current either way. 20mA on the 2.4V forward connection is as much heat as it will stand. If the reverse path gets 20mA (or 19mA), at double voltage (5V instead of 2.4V), it will run twice as hot as designed, and may melt.

We could use lower current. (That's also less heat in the resistor.) However the ">5V" is not a critical spec. Nobody cares. It could be 10V or 20V, depending how long they left the LEDs in the oven. If we flow 15mA at 20V, that's _6_ times the rated heat ini the LED.

Also half-wave LEDs flicker. If you move your head fast you see the dotted-line effect. This might even be good for a "skull". For a normal pilot-light, I find it annoying.

Now that we are on 6V power, the ~~9V peak is only 4V above the LED reverse breakdown. You should run the math, but it is pretty unlikely to flow big reverse current or heat.

For a 2-eye skull, I'd run the diodes opposite polarity. Not only balances the DC-from-AC, now the skull eye flicker alternates left-right....

[Madison]

Dude, that's ferkin' awesome!
 
Good job!

[Heinz]

Using the heater voltage is certainly the best choice. However, for the sake of completeness...

Powering LEDs from mains voltage can be done using a capacitive reactance instead of a big resistor. I have done this several times to indicate the status of a toggle relay in my light switches. The circuit is small, works great and does not dissipate any heat.

Here's a good description of the circuit: http://www.marcspages.co.uk/tech/6103.htm

Keep in mind that this was made for 230V.

[DummyLoad]

heh! love the tube ghoolies...

 

another option for AC LED PS attached. seems workable. from http://www.turbokeu.com/myprojects/acled.htm

i love the oscillating eyes though...

--pete

[jojokeo]

I have done this also on a number occasions w/out any problems to date. I've mainly used the first one of the three and w/out seeing DL's looks to be the same? Anyway, here's throwing out there three ways to do it.