LEDs are basically diodes. Could I power an LED with AC power, with the appropriate resistor? Reason I ask is because I have a couple dwarf signals with incandescent bulbs in them with one of the bulbs burned out, that are connected to 12 volt AC power. I have a couple dwarfs in the spare parts box that are leds. It would be easier to connect the new LED dwarfs to the existing power, than to run DC power 10 feet over to the dwarfs. All of my switch motors and signals are powered by a 12 volt AC power transformer from Radio Shack, with the 12 volt ac power available from several terminal strips around the 24 foot layout.
Hi hobo9941
Basicaly no LED's are DC and polarity sensative.
That said go to radio shack where you got the LED"s and get a 12V bridge rectifier.
It might be called something else in the US esentialy it will turn AC into DC
run your signal feeds through that then the resistor to the signal problem solved.
regards John
Hobo9941,
What You need is a fullwave bridge rectfier and an Electrolytic Cap. for a filter. Spec's on Your AC source would be helpful. If You have the know how to built it would be great....If not..getting another power supply would be best. And if You wanted to keep Your other incandesants in the circuit, that's also fine...they work on AC or DC.
Read thru this link and it will tell You about everything You need to know about building one Yourself, with diagrams:
Or just get a converter AC to DC, already made for 12v.
http://boards.straightdope.com/sdmb/showthread.php?t=649863
Take Care!
Frank
I agree with John.
Radio Shack doesn't carry much in the way of electrical components anymore.
Something like this would do the job—and then some!
http://www.ebay.com/itm/20-PCS-DB107-DIP-4-DB-107-1000V-1A-BRIDGE-RECTIFIER-/291170406435?hash=item43cb1bd023:g:WCMAAOSwq5lTnuQe
This is just an example of the first one I stumbled across. I mention it due to its small size and you can clearly see where the AC goes in (both pins marked with ~) and, of course, DC out at + and - .
These are rated one amp. That's enough for about 50 LEDs.
Good Luck, Ed
Yes, an LED is a diode, but it isn't designed nor intended to be used as a rectifier. They can't handle reverse voltages in the same way a rectifier diode can.
A simple option is to put another diode in series with it to protect it from reverse voltage. The better option is to use a direct current power supply.
The limiting factor is the peak inverse voltage. Even a basic common rectifier diode has a PIV of 50 volts or more, which basically means the diode junction cna handle, without failing, voltage up to 50V trying to go through the 'wrong' way - think of it like a damn, holding back water, it will hold so much until it fails.
An LED though, has a PIV usually not much more than it's common forward voltage. So for a common red LED that works on 2.2V, much more than that in the opposite direction will fry the junction. Current plays a part in that, and people have been using a diode with a resistor across DCC power just fine, the resistor limits the current in both directions so while the reverse voltage is high, the current is rather low. This still does shorten the lifespan of the LED, but we're talking maybe 10 years instead of 20. A cheap (they used to sell packs of 100 for like $2.99 even at Radio Shack) diode connected antiparallel to the LED (LED cathode to protection diode anode, led anode to protection diode cathode) with the current limiting resistor connected between one junction and the power source (doesn;t matter which side, as long as there IS a resistor) will limit the reverse voltage across the LED to one diode drop, or about 0.6V, which is under even the LED's meager limit.
Since there isn't a lot of room inside the signal - the protection diodes do NOT have to be right next to the LEDs, they can be connected to the leads under the table.
If you are doing a lot, it might be easier to just provide as DC power supply to run them. You could even use a 5V DC power supply for the signals, meaning a much smaller current limiter resistor.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
there doesn't need to be a regular diode in each LED path. Just one in series with the supply can be used for multiple LED circuits.
greg - Philadelphia & Reading / Reading
Thanks for all the replies. I was under the impression that an LED with the proper resistor would work on AC, and just not be as bright, due to only using half of the AC cycle. I was trying to avoid running a new DC line just for a couple dwarfs. I have a fairly large layout.
hobo9941 Thanks for all the replies. I was under the impression that an LED with the proper resistor would work on AC, and just not be as bright, due to only using half of the AC cycle. I was trying to avoid running a new DC line just for a couple dwarfs. I have a fairly large layout.
What else are You using the AC12volts for? If it is just for incandesants, You don't have to install another line to power them. Use the same line that is there...just switch the power supply to DC.....incandesants will work on AC or DC.
LED's will work on AC with resistor, but be on half the time, but flicker, constantly and won't last long.....read the link!
If You don't have the LED's yet and what to spend a little money on getting some LED's that will work on AC/DC/DCC take a look at this link...they have just that. It is a five pack, but I believe they also come single. Also any color You want and size. Do a little navigating on the site and You should find what You need.
I personally use the NANO size LED's.
http://lights4models.com/index.php/led/non-flashing-led/1-8mm/red/small-bright-red-led-pkg-of-5.html
Out of ignorance more than anything, I installed three signals on my layout that are LEDs and run on AC power .... only because the AC line was nearby and the closest DC tap at the time was on the other end of the layout.
Aside from the usual resistor (at least I knew that much !), there is no bridge rectifier, diode or other extra components. Guess what ? .... nearly fifteen years later, they are still doing their job.
Guess it's a case of "they work because I didn't know they shouldn't" !
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Mark,
I had a similar experience...but I did know better, by accident I connected LED's to a 12.6vac Transformer by Radio Shack and installed Led's with resistors along with incandesants in the circuit and they still work...It plainly states 12.6vac on the transformer. I also just used, two Grey wires...You got Me........and they don't flicker. LOL
Hey Mark!
Mark R.Guess it's a case of "they work because I didn't know they shouldn't" !
You're good at that! IIRC you said the same thing about your constant lighting circuit for cabeese.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
hon30critter Hey Mark! Mark R. Guess it's a case of "they work because I didn't know they shouldn't" ! You're good at that! IIRC you said the same thing about your constant lighting circuit for cabeese. Dave
Mark R. Guess it's a case of "they work because I didn't know they shouldn't" !
You only know about the ones that WORKED ! The percentage is probably higher on the ones that blew up. I just tried things .... once in a while you beat the odds and it works.
I've had a couple people look at some of my projects scratching their head, claiming "that shouldn't be working" !
I just shrug my shoulders .... "I dunno" ....
Mark R. Out of ignorance more than anything, I installed three signals on my layout that are LEDs and run on AC power .... only because the AC line was nearby and the closest DC tap at the time was on the other end of the layout. Aside from the usual resistor (at least I knew that much !), there is no bridge rectifier, diode or other extra components. Guess what ? .... nearly fifteen years later, they are still doing their job. Guess it's a case of "they work because I didn't know they shouldn't" ! Mark.
zstripeMark, I had a similar experience...but I did know better, by accident I connected LED's to a 12.6vac Transformer by Radio Shack and installed Led's with resistors along with incandesants in the circuit and they still work...It plainly states 12.6vac on the transformer. I also just used, two Grey wires...You got Me...
A couple of things about LED's and reverse voltages: they generally do not die immediately with the voltages and currents we are dealing with, and what effect it has on the LED will vary greatly depending on the LED itself, the voltage, and the current.
Exposing an LED to reverse voltages greater than what it is rated for will usually degrade the LED instead of killing it right away. If you have an LED that has been working off of AC for many years, I would not be surprised if that LED was noticably dimmer compared to the same LED when brand new. The color and composition of the LED will have an effect on it's tolerance for reverse voltages, for example, red LED's tend to be much more tolerant than white LED's. With the righrt combination of LED, voltage, and current, you might get away with years of use on AC, but you also run the risk of getting a much shorter life from the LED. The only way to be really sure is to protect the LED from reverse volatges.
zstripe..and they don't flicker. LOL Take Care! Frank
No, actually it is flickering, it's just flickering at 60 Hz so you're not noticing it.
CSX Robertzstripe ..and they don't flicker. LOL Take Care! Frank No, actually it is flickering, it's just flickering at 60 Hz so you're not noticing it.
I totally agree with everything You just posted....I just didn't feel like typing it all in....most people would not read it anyway!
BTW: Everything that We just discussed, was in the link I posted in the beginning......how many people do You think read it all or even looked at it? LOL.
hobo9941I was under the impression that an LED with the proper resistor would work on AC, and just not be as bright, due to only using half of the AC cycle.
ignoring the reverse voltage, you're correct in that since a positive voltage is only applied half the time, the LED is half as bright. One way to correct for this is to use half the resistor value for a DC voltage.
This approach of controlling the duty-cycle, the percentage of time that there is a voltage applied, is a common way of varying LED brightness. Controlling the duty-cycle is a lot easier that controlling the constant DC current which requires very small adjustments to voltage.
zstripe Mark, I had a similar experience...but I did know better, by accident I connected LED's to a 12.6vac Transformer by Radio Shack and installed Led's with resistors along with incandesants in the circuit and they still work...It plainly states 12.6vac on the transformer. I also just used, two Grey wires...You got Me........and they don't flicker. LOL Take Care! Frank
They do flicker, but at 60 times a second you just can't see it.
Thanks again for all the replies.