Simple LED help please

[DiodeDave]

I'm about to my witts end on a simple project.

I'm trying to install Radio Shack red blinking LEDs on my Lionel layout.

They are rated at 20ma, 5 volts and I have a 13 volt AC power supply.

From what I have learned, I need a 400ohm resistor and a DC power supply.

Correct?


Here are my questions,


1. The LED is rated at 2.5 to 5 volts.

In one setup, I had 2 LEDs in parallel and a 680ohm resistor in series before the first LED.

It worked ok, but then I accidently applied 15v AC which blew the LEDs with pop!

Now, I can't get one (new) LED to work with the same resistor, which worked in the past.

Any ideas?

2. Will 13vAC with the LED in series with a 680ohm resistor damage the LED?

3. Is it possible to power the LED with a 680ohm resistor and a diode in series using 13vAC?

Thanks

 

[davelectronic]

Hi welcome to the forum,
Its not ideal but you can place a diode across the led for AC but its not the best solution as the led is only getting half rectification, but it wont blow it.

 

[DiodeDave]

By saying "across the LED" Is the diode in parallel with the LED and current going in the same direction?

Will this limit some of the current and affect the resistor size?

 

[davelectronic]

no it wont matter about current keep the resistor above 420R for 13 volts.

 

[Resqueline]

I've never worked with blinking LED's so I'm not sure, but I'd be concerned that the internal blink timer in the LED would be upset by losing power 60 times per second.
I'd suggest doing it this way to ensure a reasonably constant power supply to the LED. The resistor value may need to be tweaked, better to start with a higher value.
You can also use a full-wave bridge rectifier instead of just a single diode. The resistor value would have to be doubled but the capacitor value can then be halved.

 

[Laplace]

Since these are blinking LED's one concern would be whether the voltage across the LED would exceed the rated value during the OFF period when the current is low and very little voltage is dropped across the resistor. How might that affect the LED? I think I would try a 4.7v Zener diode across the LED and place the capacitor on the other side of the resistor so it can charge to full voltage of 17v (13x1.414-0.7).

 

[DiodeDave]

Thanks for the replies and the values shown in the suggestions.

I think I'm getting closer to my target.

 

[Resqueline]

I agree with LaPlace, it's a better (failsafe) solution.

 

[computerlen]

Just for future information. If you have one blinking LED, any connected in series will also blink at the same rate.