LED burning up

[txstrat]

Here's the report.
It works great. LED is running at 7.1 mAmps. Let it running for hours (actually it was almost a day) with no issues so far.
Thanks everybody.
Positive side effect: It's not as bright as before. i.e. doesn't hurt your eyes when looking at it.

[Jazz P Bass]

So what was the circuit like?

Series diode?
Parallel diode?

[txstrat]

So what was the circuit like?

Series diode?
Parallel diode?

Series.
17 v winding -> diode -> resistor -> LED -> ground

[Chuck H]

Using a couple of on line calculators, that were easily found in the first five offerings of a Ooogle search, it looks like a resistive load on your half wave rectified 17VAC winding yields 7.65VDC. Using that as a supply voltage and assuming a red LED another online calculator indicates a series resistor of 820R as the nearest standard value for the circuit. It's typically done with the resistor to ground rather than the LED. Not sure why.

You can also follow jazz's advice and put a diode circuit parallel to the LED as an over voltage safety measure. Though it doesn't seem necessary, but I don't know. What are the odds the 17V wind will suddenly spike up high enough to damage the LED?

P.S. If you have a capacitor filter on the half wave circuit that bumps the supply voltage to 24VDC and that would require about 2.9k of resistance in series with the LED. So a 3.3k would be the nearest standard value.

[Tom Phillips]

...You can also follow jazz's advice and put a diode circuit parallel to the LED as an over voltage safety measure. Though it doesn't seem necessary, but I don't know. What are the odds the 17V wind will suddenly spike up high enough to damage the LED?...

The maximum reverse voltage specification for LEDs is surprisingly low. You do need to follow JPB's advice in post #14 or your LED will have a short life. You can see examples of the use of the parallel diode in the existing published amp circuits that use front panel LEDs powered from AC sources. Also check out the LED manufacturer's data sheets linked at the supplier's sites, such as mouser, and you will find the LED spec in question.

[Chuck H]

Even taking into account that the winding is rectified? I assumed (perhaps wrongly) that eliminates the possibility of inverse voltage. I was considering the potential for overvoltage. With a parallel diode arrangement set up like a shunt regulator. Which I thought might be an overkill measure.

[Mike Sulzer]

The maximum reverse voltage specification for LEDs is surprisingly low. You do need to follow JPB's advice in post #14 or your LED will have a short life. You can see examples of the use of the parallel diode in the existing published amp circuits that use front panel LEDs powered from AC sources. Also check out the LED manufacturer's data sheets linked at the supplier's sites, such as mouser, and you will find the LED spec in question.

See RG's earlier post; one diode either series or parallel should do it. (But I think the parallel might be better because the current in the two half cycles is better balanced) But he suggests four, a bridge; both efficient and balanced.

[Tom Phillips]

Even taking into account that the winding is rectified?

Yes. Because, as described the existing circuit is just a half wave rectifier with no filter. Therefore the diode is seeing the full reverse voltage every half cycle. Edit: See correction in post #25.

I assumed (perhaps wrongly) that eliminates the possibility of inverse voltage...

If a full wave bridge were used then the diode would be protected. However, the single shunt diode is usually used in commercial applications because is is a simpler implementation.

[Chuck H]

I'm just stuck in rote observations rather than actual study, but this was my perception:

[Tom Phillips]

Chuck,
Your perception is correct. I got off on the wrong track. I was thinking that about the situation where someone tried to use just the LED and a resistor connected to an AC voltage source. In that case the LED is exposed to the full reverse voltage every half cycle. There are still examples of circuits that use the extra parallel diode for balancing. (Mike mentioned that) I believe that helps prevent induced noise on the heater circuit.
Tom

[txstrat]

Does it mean I could use a diode in parallel to the LED -> both on the heater winding and end up with no increased hum? (provided I've used the right resistor)

Matt

[Mike Sulzer]

Chuck,
Your perception is correct. I got off on the wrong track. I was thinking that about the situation where someone tried to use just the LED and a resistor connected to an AC voltage source. In that case the LED is exposed to the full reverse voltage every half cycle. There are still examples of circuits that use the extra parallel diode for balancing. (Mike mentioned that) I believe that helps prevent induced noise on the heater circuit.
Tom

One series diode is safe because the reverse drop occurs across the diode. Please read RG's post. The parallel diode alone is better in the sense that the current is nearly the same on the two half cycles, but it is less efficient. If that matters, use a bridge which is good in both ways.

[Tom Phillips]

Does it mean I could use a diode in parallel to the LED -> both on the heater winding and end up with no increased hum? (provided I've used the right resistor)

Matt

That may help. IF the hum/buzz was increased due to adding the LED circuit. I'll look for one of the Fender schematics that shows the balanced LED circuit.

[bob p]

If anyone is still looking for a schematic, I recently fixed a hum problem on an Epiphone Century 1939 that uses an LED for a status lamp, and a 1N400x protection diode on the heater supply.

This is a very hummy amp due to the way that the heater wiring was poorly implemented by the Chinese ghost manufacturer. I fixed the amp's hum problem by revising the heater wiring and shielding the inputs. I don't think the LED/diode pair contributed to the hum problem.