>From time to time I read dire warnings about the death of a LED by reverse >polarity. It does not happen. A LED is a diode, and is used normally in >forward bias, with a resistor (or some other circuit) to limit the current. If >the LED is reversed, it simply blocks the current, and precisely nothing >happens. You can make a cool bridge rectifier with 4 LEDs, but although >technically OK, that is not very practical in most applications due to their >high forward drop and low current tolerance. Two wire, bi-color LEDS use this >principle where two different color dies are connected back to back, depending >on the polarity, one color or the other lights. A red and green LED back to >back make therefore a great polarity indicator. Interestingly, if fed an AC >current, a third color can be produced, i.e. a green / red combination will >shine yellow on AC. RGB LEDs have 3 dies, but can display many colors by >lighting one or more dies at varying intensity. This is done through PWM, >something a LED is very much suited for. Dimming of LEDs, as well as >multiplexing of LEDs is predicated on PWM. Many of the newer Ultra-bright LEDs >will still light pleasantly on 5V with series resistors as high as 1.5 kOhm.
The series diode does nothing to protect the LED, but does indeed drop the voltage slightly due to its forward drop. From an efficiency perspective it makes no difference, the diode also dissipates the drop Voltage times the current in heat, just as the resistor does. Since the circuit has a diode it is probably fine to leave it there, but adding one in a new design has no use at all. My apologies to most on the list since you already know all of this. I just felt I had to respond here since I still see this incorrect proposition about reversed LEDs too often. Bill From: [email protected] [mailto:[email protected]] On Behalf Of gregebert Sent: Thursday, March 24, 2016 10:34 AM To: neonixie-l Subject: [neonixie-l] Re: Off topic, but it has an orange display.. Heathkit ID-1590 anemometer As long as you dont exceed the RMS current rating of the LED, they will outlast any other type of illuminating device being replaced, especially incandescent. >From the schematic, there are already diodes in the path, so if an LED was >installed backwards there's no possible way it can get damaged from >reverse-polarity. However, the LED wont illuminate if backwards. LEDs have roughly a 2V drop, so you *will* need to add a series resistor; just need to calculate the rough value. The supply is 6.3 VAC (about 9V peak) , and there are 2 diodes in the path. So, the resistor's voltage drop must be about (9-2-0.7 - 0.7 = 5.6 volts). If you get larger diodes that operate at 20mA (should be a good replacement for incandescent lamp), your resistor will be around 270 ohms. Use a 1/4 watt resistor. Obviously you will need to experiment with a few different resistor values to get the desired brightness, so try some values between 220 and 470 ohms. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/6ccc7136-e272-442c-a81e-43f93556783c%40googlegroups.com <https://groups.google.com/d/msgid/neonixie-l/6ccc7136-e272-442c-a81e-43f93556783c%40googlegroups.com?utm_medium=email&utm_source=footer> . For more options, visit https://groups.google.com/d/optout. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/005101d185f3%24a5e80b10%24f1b82130%24%40com. For more options, visit https://groups.google.com/d/optout.
