In school, I learned that all diodes behave similarly despite their main function. Even though rectifying diodes are specifically made to run on higher current and usually used in an AC to DC rectifying situations, it doesn’t mean that these diodes can only be used for that situation. The same thing applies to other diodes too, including light emitting diodes (LEDs). I had learned that theoretically, any diode will function as a half wave rectifier but never really tried to test this. I decided to try this on a breadboard with a few of our parts.
Selecting a Safe Transformer
First, I had to find a relatively safe way to experiment with AC voltage, so I used the list of wall mounted transformers I listed in this post: Wall Mounted Transformer . Digi-Key part number 237-1880-ND ; Triad Magnetics part number WAU12-200 is a 120VAC input to 12VAC output with a maximum current output of 200mA.
12V is low enough to be considered safe on a breadboard and 200mA is pretty reasonable current capacity without being overly excessive (1A or more would be a little more hazardous). The part also has a convenient connector that has many through hole mates available. The mating connector that I chose for connecting to a solderless breadboard was Digi-Key part number 486-3381-ND; Shurter Inc part number 4840.2201 .
I simply soldered regular wires from a wiring kit I purchsed a while back (20-24AWG solid core wires) to pins one and two as seen on the datasheet on the part page. Remember to still practice safe handling of these wires though, 12VAC and 200mA is still a lot more hazardous than a DC level!
Selecting LEDs and Resistors
When I designed the experiment, I treated it like it was a DC application. I just assumed that the overall average/root mean square (RMS) output was 12V and used that as my starting voltage in my equations. Since I half expected the LEDs to burn out due to the negative 12V phase exceeding the breakdown, I bought three LEDs of different colors and two of each just in case things decided to burn up. First, I didn’t want too much of a voltage drop and wanted a relatively low current draw, so I used a baseline 1-4V forward voltage and a pretty standard assumption of 10-30mA per LED based on common values. Here are the part numbers I chose for the LEDs:
Green LED Digi-Key part number 160-1659-ND; manufacturer part number LTL1CHKGKNN
Red LED Digi-Key part number 160-1661-ND; manufacturer part number LTL1CHKRKNN
Blue LED Digi-Key part number 365-1201-ND; manufacturer part number OVLLB8C7
The red and green LEDs I picked both have similar enough forward voltages that the same resistance value can be used. The blue LED has a higher forward voltage so the resistance is smaller. Here is the formula I used to calculate the resistance needed:
The value that comes closest to actual resistance values for the red and green LEDs is around 500 ohms. The blue one requires around 430 ohms. To calculate the wattage capability of the resistors, I took the test current times the supply voltage which is 0.24W or 240mW. I used a pretty decent design factor and found a 0.5W rated 500 ohm resistor: Digi-Key part number CMF500HY-ND; manufacturer part number CMF55500R00FEBF
and a 0.6W rated 430 Ohm resistor: Digi-Key part number BC4161CT-ND; manufacturer part number MRS25000C4300FCT00 .
Wiring
The wiring is extremely easy in this case, all that is needed is the LED and the appropriate resistor in series. One cool benefit of using AC, in this case, is the LED can be inserted either way. Remember the 430 Ohm works for the Blue LED and the 500 Ohm works for both red and green.
Here are pictures of the breadboard setup:
The red will be the same as the green, so I didn’t take a picture in that case.
Electrical Behavior - Advantages and Disadvantages
So the lights will appear to be on all the time, but you can certainly tell at times that the light is flashing as moving your eyes quick enough will detect change. The light is flashing with a frequency of 60Hz since the input signal comes from a US Outlet. (This will be different in other countries) I took a slow-motion video of the blue light at 240FPS to show this:
There are advantages and disadvantages to powering LEDs this way.
Some advantages include:
- LED Placement Works Either Way
- Design is Simple and Cost Effective
- LEDs Will Eventually Wear Out Faster
- Flicker is Annoying and Noticeable
For convenience, here is a link to the cart for the parts I used: https://www.digikey.com/short/pdrvct