LED burning up

[txstrat]

Hey everybody,

I'm having some trouble with the pilot light in one of my builds.
I've used the standard Fender 6.3 volts AC light before, but it burned up every couple months.
I think because of the vibrations. I had to bend the lamp holder due to very little room in the chassis.

I was sick of the lamps burning up and changed the pilot light to a blue LED wich I fed from the heater winding.
This led to more hum.

I changed to a different winding, wich is 17 volts @ 60mA. No more hum, but...
First try was a 680 ohms resistor in front of the LED. It burned up after a couple hours of using the amp.
I thought there might be too much current and changed to a 820 ohms resistor.
But the LED also burned up after a couple hours of use.
Don't know what to try next.

Any help would be appreciated.

Thanks in advance

Matt

[mozz]

Try a lot bigger resistor, you don't need to run the led at 20ma. How high actually is the 6.3v tap? Mouser sells a #47 replacement led lamp which works great. EDIT: I assumed it was rectified already.

[Dave H]

And put a diode in series with the led if it doesn't already have one.

[txstrat]

Try a lot bigger resistor, you don't need to run the led at 20ma. How high actually is the 6.3v tap? Mouser sells a #47 replacement led lamp which works great.

I tried a series resistor calculator which led me to 1.5 K with a then current of 7.1 mA. LEDs start to glow at 8mA. I'll give it a shot.
I don't think to place an order at mouser would make sense for me. The shipping would be much higher than the LED.

[txstrat]

And put a diode in series with the led if it doesn't already have one.

I thought of that too, but what benefit would it actually have?

[bob p]

prepaid orders ship free at digikey.

[Tom Phillips]

prepaid orders ship free at digikey.

Can you tell us more about the terms of this deal and how to access it?
I cannot find anything about this on the Digikey ordering site.
Thanks,
Tom

[bob p]

Tom, the policy is buried in the fine print of the Mail Order policies in their print catalog. I don't know if they advertise it on the web site.

I know that they still offer this deal, as I have a small order that's enroute to me right now.

[bob p]

Just checked their web site -- it's printed right on their mail order form.

[nickb]

Hey everybody,

I'm having some trouble with the pilot light in one of my builds.
I've used the standard Fender 6.3 volts AC light before, but it burned up every couple months.
I think because of the vibrations. I had to bend the lamp holder due to very little room in the chassis.

I was sick of the lamps burning up and changed the pilot light to a blue LED wich I fed from the heater winding.
This led to more hum.

I changed to a different winding, wich is 17 volts @ 60mA. No more hum, but...
First try was a 680 ohms resistor in front of the LED. It burned up after a couple hours of using the amp.
I thought there might be too much current and changed to a 820 ohms resistor.
But the LED also burned up after a couple hours of use.
Don't know what to try next.

Any help would be appreciated.

Thanks in advance

Matt

I always add a 6.8 ohm 0.5W resistor is series with the regular incandescent bulb. You get the authentic glow and they then last for ever.


On the LED, LED's generally can't handle much reverse voltage and so should not be fed from an AC supply. Use a single rectifier (average current is halved) in series or FW bridge.

[olddawg]

An LED needs a DC voltage. I would use a small, cheap bridge rectifier package and a smoothing cap on a piece of perf board. If you google LED landscape lighting it will give you a full explaination.

[Dave H]

I thought of that too, but what benefit would it actually have?

It prevents the reverse voltage/current damaging the led.

[R.G.]

As Dave H says - it keeps the reverse voltage from destroying the LED.

LEDs are unidirectional parts. They glow proportionate to the current through them in the FORWARD direction. That is current flows from anode/+ to cathode/-. They glow brighter and brighter with higher and higher current until they can no longer get rid of the waste heat and immediately change into Darkness Emitting Diodes - DEDs. For the T 1/34 "standard" package, this happens at about 20ma.

LEDs break over when fed voltage in the reverse direction: cathode positive and anode negative. But they cannot stand much reverse voltage. A 1N4007 will not break over in the reverse direction at less than 1000V. A nominal LED will break over - and usually become DEDs - with reverse voltages of 5V or less. The prevailing wisdom is as follows:

1. Don't feed an LED AC.
2. Don't feed an LED AC.
3. See 1 and 2 above.

One way to do this is to use a series diode in the positive current direction, so the added diode stops all current flow in the reverse direction, and holds off all the reverse voltage. Another is to use a SHUNT diode that conducts in the reverse direction. This clamps the reverse voltage the LED sees to the diode's forward voltage, usually under 1V, which is usually safe for LEDs.

The series diode in effect half-wave rectifies the incoming AC. This is good for the LED, but as you note, it can cause hum by causing a DC offset on the AC due to loading if the winding is used for other hum-sensitive stuff. The shunt diode is usually better for this.

An even better way to do the AC to LED setup is to use a tiny DIP-8 full wave bridge with the LED across the output + and - terminals. Now the LED pulls equal currents on both AC half cycles, and minimizes any hum out into the winding. The limiting resistor for the LED can go on the "outside" before the FWB or on the "inside" just in series with the LED. Doesn't matter, same current in both cases.

[Jazz P Bass]

Use a diode in parallel with the LED.

But hook it up with the reverse polarity.

This will prevent the reverse break down voltage from going above 0.7 volts.

[txstrat]

Wow, thanks everybody. As a first try, I'll put a diode in series with the LED and report back after a couple of hours playing time. Maybe I'll let the amp running on stand by over night.