gunrunnerjohn posted:JohnGaltLine posted:All the theory aside, Junior, use a 1.5k Ohm resistor in series with your LEDs and they should work well at voltages of 10-20 VAC on the input side. Assuming a typical LED designed for maximum output at 20mA of current.
JGL
Actually John, you can use a whole lot lower resistor than that as long as you have reverse voltage protection using a diode or you've connected two LED's in reverse polarity to protect each other. Let's just assume you have .707 of the RMS voltage to consider since the LED only see half of the cycle. I get around 14 volts, and I'm dropping at least two across the LED. So, we're talking about 12 volts or less, at 20ma, that's a 600 ohm or larger resistor. LED's will tolerate peak currents of multiple times their continuous rating, so this is a pretty safe number.
Perhaps I see the confusion now. in this example you are multiplying the RMS of the input to get the RMS of the output. It is entirely possible that .637 is the correct number to obtain the desired value. Unfortunately, to my way of thinking, anyway, this RMS value to the LED is a useless number. Now it may be that I am off base here and that there will be no harm done in repeatedly overdriving an LED, but it has been my understanding that even brief surges over current will damage LEDs over time. GRJ seems to suggest this is not the case, but I can't confirm it looking at the data sheets of various LEDs.
The figures I'm using instead of showing 14 volts passing through the LED show 25.5 volts on each peak though it is only about 13 volts RMS. If the LED will remain undamaged by being driven over current on each cycle than it's all academic and doesn't matter for a practical application.
JGL
