On/off LED wiring question

[gary h]

Hi all,

I have the 5-0-5 taps off my preamp transformer available. I used one leg and the ground to illuminate a 5V blue LED as an on/off light. The other leg is floating. At first the LED was lit without visible intermission. Over the last couple of months the LED has started to blink on and off with decreasing frequency. Clearly this does not represent the 60 Hz pulse going to it. I suspect I am destroying the LED and possibly endangering the transformer by wiring it directly this way. (My thinking was that as long as there is no negative voltage going to the LED, the rapid + pulses would suffice. Now I'm thinking an RC might be necessary.

Is this a bad wiring scheme? What would be a simple circuit I could employ that would use the available 5V tap for this purpose? What possible damage might I be doing to the tranny or the preamp circuit?

Thanks,

gary

[bigwill]

Does your LED have a series resistor?

[nigelwright7557]

Are you protecting the LED from reverse voltages ?


Resistor - LED -!
- REVERSE DIODE -!

[tomchr]

I second the suspicion that the reverse voltage killed the LED. A small-signal diode (1N4148 comes to mind) in anti-parallel with the LED would do the trick. You'll probably need to replace the LED, though.

I assume the "5V LED" has a series resistor built in.

~Tom

[chrish]

If you type 'led series resistor' in to google you get a bunch of on-line calculators, like this one for example.

[gary h]

Thanks for the responses,

I did not use a resistor because I get 5V from that tap and I ordered a somewhat uncommon 5V LED from digikey for that purpose.

If I understand correctly by attaching it's + terminal to one of the 5V taps and it's - terminal to the ground I don't get any negative voltage, but rather a bumpy half wave 5V that oscillates between 5 and 0 volts. Please correct me if I have this wrong.

I wonder if I shouldn't put a cap in front of it to smooth it out. Problem is then I will be losing voltage and underpowering the LED.

Any other suggestions?

gary

[Eli Duttman]

Any other suggestions?

Yes, use the available "10" VRMS, a series current limiting resistor, and a protective series PN junction diode, along with the indicator part.

Your thinking about "5" VAC is erroneous. First and foremost, the max. voltage present is (sic) 1.4X the AC RMS value. Another consideration is that the voltage actually present varies with loading.

[llwhtt]

My Manley amp runs the input and phase inverter tubes on 12VDC however the power LED is run on AC right off of the heater winding with a 511 Ohm series resistor. If you are running 6 volt heaters try a 250 Ohm resistor in series with the LED.

Craig

[gary h]

Thank you gentlemen.

[Miles Prower]

Originally posted by gary h
I have the 5-0-5 taps off my preamp transformer available. I used one leg and the ground to illuminate a 5V blue LED as an on/off light.

Blue LEDs really, really, really do not like reverse voltages. You can easily get away with feeding other LEDs AC if you include a ballast resistor, and they won't mind. Those blue LEDs are quite a bit more sensitive, and much easier to poof.

You need to feed them DC, and make certain you include a ballast resistor. Even then, no guarantees they won't eventually poof.