Your LEDs won't light on AC so you would either need a DC supply or (not a good method) have a rectifier at each location where the LEDs are connected.
JRP I also have an old (but working) MRC Tech 11 model 2500 transformer with both DC Direct, DC variable and AC terminals.
I also use an old MRC Control Master 20 power supply for some of my LED lighting and I use the variable DC (normally for track) side to feed the LED lighting. This gives me a bit of control over the overall brightness of the powered lamps. Most often they look better at a slightly reduced intensity.
You will still need resistors at each LED or sometimes, groups of series LEDs.
Your other option is to use the Fixed DC output of the 2500 but unless you use something like a DC/DC regulator on the lighting feeds you will not have any control over the brightness of the LEDs without changing the values of the resistors on the individual LEDs.
If you decide to use the variable (track) output of the 2500 I'd suggest placing a piece of tape over the reverse switch. If you inadvertantly reverse the polarity after the LEDs are wired they won't light. You won't hurt them but you might wonder why they arent lighting up and forget that the reverse switched was bumped.
Good Luck, Ed
You can buy a variable voltage wall wart for a little more than the price of a 6 pack of craft beer.
Henry
COB Potomac & Northern
Shenandoah Valley
Hi Henry,
Your link was bouncing directly back to this thread page for some reason and I'm not entirely sure why. I copied the URL, re-coded the highlighted text with bracketed urls, and that fixed it.
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
Can someone explain to me why LEDs aren't supposed to light on AC? They do just fine passing the equivalent of half-wave as long as you respect the peak and not RMS voltage in both directions when figuring what not to exceed...
This is NOT to advocate that you actually use AC to drive a string of LEDs, just noting that they will in fact produce light, however briefly, on the right AC voltage.
OvermodCan someone explain to me why LEDs aren't supposed to light on AC?
A bi-color one will. depending on the frequency it will give you kind of a blend of the two colors.
For the "AC" output of a DCC booster an LED will light with the appropriate resistor. I use one for testing track continuity and in some locomotive decoder installs I will wire the numberboards directly to the track pickup. Yes, they're always on and that's OK with me.
I also use LEDs off the DCC track pickup without a rectifier (but one zener diode) and a cap for "flicker-free" passenger car lighting. The LEDs are only seeing "half" the cycle as far as I can tell.
P.S. Henry's link doesn't work for me.
Regards, Ed
You "can" run LEDs from AC, but it's not a best practices method. A single bridge rectifier right at the power supply with an amp rating to match will convert your entire line to DC. White LEDs consume between 10 to 20 milliamps each, so be sure whatever power supply you use will supply the total current required. Some of my buildings have upwards of 20 to 30 LEDs each .... it can add up fast.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Guys, thank you all for the suggestions. I think I know now what to do here.
Mark R.You "can" run LEDs from AC, but it's not a best practices method.
compared to an equivalent DC voltage, the resistor would need to be < half because the polarity only matched half the time and the voltage is only hjigh enough for a portion of the cycle
greg - Philadelphia & Reading / Reading
gmpullmanHenry's link doesn't work for me.
Try this https://www.amazon.com/Belker-Adjustable-Universal-Household-Electronics/dp/B07NKZCWT1?tag=namespacebran131-20&keywords=wall+wart+power+supply
gregc Mark R. You "can" run LEDs from AC, but it's not a best practices method. maybe not the best, but why not use it if handy compared to an equivalent DC voltage, the resistor would need to be < half because the polarity only matched half the time and the voltage is only hjigh enough for a portion of the cycle
Mark R. You "can" run LEDs from AC, but it's not a best practices method.
maybe not the best, but why not use it if handy
I have a few spots where I've attached an LED across the track legs of a reversing section on my DCC layout which is a form of alternating current. A red LED on either side of the insulated joiner of the same rail is a quick indicator if the "polarity" matches going in to the reverse section.
it's just easier to run a bridge and a cap, then run DC from there ... and it's in the best practise range, lol
JRP Guys, thank you all for the suggestions. I think I know now what to do here.
Just to clarify, since you've gotten some conflictig information: Yes, LEDs will light on AC, but no you do not want to, and it's not just a "not a best practice," it will likely seriously shorten the life of your LEDs, especially wihte ones. Yes, LEDs are diodes, but unlike rectifier diodes they are only designed to withstand a few volts of reverse voltage. To run LEDs off of AC current, you really need to either have regular retifier diodes to block the revere voltage or wire the LEDs in parallel but oppostie polarity pairs to limit the reverse voltage the LEDs see to the forward voltage drop of the LEDs.
Ed, for what ever reason, I tested a 3 volt LED with a 470ohm resister and it lit fine using AC, also fixed DC, and variable DC. Again I used the power from an old MRC Tech transformer.
JRP I tested a 3 volt LED with a 470ohm resister and it lit fine using AC,
OK, but... I really wish any mention of AC had never entered the thread. Someone asked about it and I replied with my rationale for using it in certain, limited aspects of DCC wiring. I'm not an EE but I've used hundreds of various LEDs in different model RR situations and figured out what works and what doesn't along the way.
Yes, I wrote "your LEDs won't light on AC" in my initial reply. I SHOULD have said your LEDs won't like AC in stead.
You have DC, both variable and fixed. Stick with DC. The LEDs will be happier and your future lighting projects with LEDs will be more successful with a DC supply.
JRP Ed, for what ever reason, I tested a 3 volt LED with a 470ohm resister and it lit fine using AC, also fixed DC, and variable DC. Again I used the power from an old MRC Tech transformer.
Yep, like I said, they will light, but it's not good for them.
i'm curious if anyone has ever had an LED stop working because of excessive but reasonable (not >100V) reverse voltage?
gregc i'm curious if anyone has ever had an LED stop working because of excessive but reasonable (not >100V) reverse voltage?
I haven't had any quit, but I have noticed they have gotten considerably dimmer over time. Possibly the reverse voltage of the AC was gradually breaking it down. If the AC voltage was higher, it would probably have broken down sooner over time.
I haven't personally (I also don't expose them to reverse voltages), but I do know of people who have tried using white LEDs, which are known to be more sensitive to reverse voltage, for locomotive headlights without revserse voltage protection and had them die relatively quickly.
Note that most LEDs list a reverse breakdown voltage of 5 volts, so even 12 would not be "reasonable."
When I first got into HO scale in January, 2004, my first mentor and LHS owner, took control and masde sure that I had everything that I needed to successfully run trains. My sole power source was an MRC Rail Power 1370, an 18VA power pack. It was a DC power pack with an AC output for accessories. I only used the DC output to run trains.
When I shortly thereafter converted to DCC, my LHS guy signed me up for an NCE PH-Pro 5 amp system. He told me to use DCC power solely for running trains and keep the MRC 1370 in place to power accessories such as control panels and trackside lighting. The only time that I can recall that I have used the AC output was to run my Walthers Cornerstone 130' non-DCC turntable. The Walthers instruction manual directed that AC power should be used for this purpose.
So, to me, I am baffled that the OP wants to use AC power for his control panels, trackside signals, interior structure lighting or whatever. My advice is to stick with DC power and regulate your LEDs through the informed use of resistors. In my experience, AC power has little to no place on a small scale model railroad layout. I am willing to be educated ottherwise.
Rich
Alton Junction
Ed, sounds like DC is the way to go, and you certainly have more experience with this than I do. Thanks and I will let you know how it turns out using DC. Last question, use DC direct or DC variable?
JRPLast question, use DC direct or DC variable?
My layout is fairly large. I have four DC supplies for LED structure lighting and a fifth one for signals. On one of these I use an old MRC CM-20 as I mentioned above. This circuit also has a couple dozen incandescent lamps on it.
I like to have them on the variable side of the supply as I can lower the voltage just a bit especially to extend the life of the incandescents. While LEDs have a very limited "dimming" range it wouldn't hurt to run them at slightly less than 12 V. depending on the resistors you chose. Most of mine have 1KΩ but a few have higher values to make the light less intense.
From my reply above:
You can see that even the common LED household lamp uses DC for the LEDs. The black square labeled DB1 (Diode Bridge) converts the AC line to DC for the LEDs.
LED_A-lamp by Edmund, on Flickr
Good choice. I've got dozens of the wall warts around.
If you really want to get fancy you could use one of these "distribution blocks" as an option. The barrel plug goes in one end and you have ten spots for branch lines out to your structures.
https://a.co/d/3LqrF3y
There are lots of variations of these. I only show the above as an example.
Be cautious with wall-warts. They come as regulated and non-regulated. Be sure to use a regulated supply as it will provide the rated voltage constantly. An un-regulated supply will only provide the rated voltage under a pre-determined load that wall-wart was designed for. If you apply a higher or lower current load, the voltage output will be inversely different.
JRPEd, I think what will work for me is a 120V wall wart with an output of 12V DC. I only have about 46 LED lights I need to connect and already have 2 power wires installed to accept accessories. I tested one of my LED's (3V with a 475 ohm resister) and got a nice bright light. I'll send photos once I have a section done. Thanks again.
If you run your LEDs at full potential (20ma) you will need a minimum of a 1 amp supply in that wall-wart. You can run them at 10ma and still be plenty bright for structures. Just be sure the supply is capable of handling the load.
BigDaddy Can I run lights without resistors if I use this in D/C 3V mode.? gmpullman Henry's link doesn't work for me. Try this https://www.amazon.com/Belker-Adjustable-Universal-Household-Electronics/dp/B07NKZCWT1?tag=namespacebran131-20&keywords=wall+wart+power+supply
Can I run lights without resistors if I use this in D/C 3V mode.?
gmpullman Henry's link doesn't work for me.
to the forum. That was the same as my link.
The forum is not what it used to be. Promised upgrades never happened and glitches where your replies disappear or you can't get into the forum at all, happen.
Rick in Cassablanca "One comes in and one goes out"
Big Daddy,can I have street lights that work without resistors, If I use the 3V dc in the add you shared from Amazon. Thank you