Accessories that run on 3v how can you hard wire them?

I use 12 vdc from a computer power supply to power all my accessories and lights. I just use a DC to DC buck converter to reduce the voltage for items like Miller signs that use 4.5 vdc. There are similar ones to convert AC, if that is what you use.

Here's a solution that will power a bunch of them. Connect the input to transformer power, and adjust the meter to read 3.0 to 3.2 volts. Connect your accessories to the output.

eBay: 124608667462

Thank you so much for the help!!!

Enjoy.

One thing that bugs me about the AC-to-DC converter that GRJ shows is it wastes a lot of power to generate the low-voltage 3V DC when starting from, say, 14V Accessory AC from a train transformer.  In round numbers, for every Watt of 3V power going to the LEDs, it draws almost 5 Watts from the train transformer.  That's a somewhat anemic efficiency of less than 20%.  A Watt of power is about 20 LEDs.  So while house AC power is "cheap" compared to replacing batteries, all the wasted power demotes the benefit of LEDs in the first place as the overall power consumption is like that of incandescent bulbs!

To be sure, you can't beat the single module, easy knob control, integral voltmeter, and reasonable price.

But OGR being a discussion forum, here's a different tack:

An AC-to-DC converter module at left generates "raw" unregulated DC...somewhere around 20V.  Then DC-to-DC converter modules can be used.  There are many DC-to-DC converter modules (with integral voltmeter) that use more efficient conversion technology than the module shown earlier in thread.  Yup, you need two modules (albeit no soldering) which is understandably an issue.  And the adjustment "knobs" on these converters requires a mini-screwdriver rather than just turning a knob.

But the power conversion efficiency in these modules is many times that of the earlier module...say 75% instead of 20%.  So for every Watt of power to the LEDs, this method would draw ~1.5 Watts (instead of ~5 Watts) from the train transformer.

And if the application requires a second (or third, or fourth) separate low-voltage DC to power LEDs, you only need to add the lower-cost DC-to-DC converter as a single AC-to-DC converter can be shared (within reason).  For example, Menards, Miller, Lemax and other vendors have layout lighting/buildings that use 4.5V DC.  So, as illustrated, you could use two DC-to-DC modules, one set to 3V DC to replace the 2 x 1.5V batteries, and a second set to 4.5V DC to replace an LED accessory previously powered by 3 x 1.5V batteries.

The two module types are widely available on eBay, Amazon, elsewhere.  The example below is from Aliexpress.com.  Prices vary all over the map.  But if willing to deal with the delays/hassles of shipping from Asia, for the same ~$10 for the all-in-one module shown earlier, you can get one AC-to-DC converter and two DC-to-DC converters.

There are more efficient AC-DC switching modules, I just couldn't turn one up quickly when I posted that.   I even have them in my part drawer, but you have to find them on-line to really recommend them.

Here are three different AC->DC buck mode switching power supplies that run on track power voltages.

I use the same thing that John H. posted. Matter if fact  I use a 12v power supply and power 4 of them and here is what I get out of all that power! I hope it helps! I am not saying its right, but it works for me!

For whatever reason I was thinking the question was to "hard wire" the 3V DC LED supply to the layout wiring such as from the 14V AC Accessory output of a train transformer.

If it's "OK" to introduce a new connection to the house 120V AC wall-outlet, then using a 12V computer power supply (that plugs into the wall-outlet) followed by one or more DC-to-DC converters as discussed above is probably the least expensive method.  You get DC power measured in the hundreds of Watts so we're talking powering hundreds or thousands of LEDs.  You can get a re-purposed PC power supply for next to nothing at Goodwill or wherever.  You might need to mess around with connectors, perhaps crimp some wires, and the like... but the price is right!

But if you are only trying to eliminate a couple of AA batteries powering some small building with, say, a dozen or so LEDs, then you only need a few Watts of DC power.  In which case, a simple "battery eliminator" or "AC adapter" gets the job done too!  It plugs in the wall outlets and typically has a coaxial-barrel style connector.  There are adapters to convert the plug to flying-leads (aka bare wire).  The one below comes with such a cable so you end up with 2 wires to replace the + and - connections to the 2 x 1.5V battery holder.

@gunrunnerjohn posted:

There are more efficient AC-DC switching modules, I just couldn't turn one up quickly when I posted that.   I even have them in my part drawer, but you have to find them on-line to really recommend them.

Here are three different AC->DC buck mode switching power supplies that run on track power voltages.

Right.  Actually, I figured you suggested that module because of the integral voltmeter and the large adjustment knob!  I can't count the number of times I've mistakenly assumed availability of a voltmeter (and knowledge on how to use it).  2 wires in, 2 wires out...no special tools required.  Doesn't get much simpler than that!

As for that tiny screwdriver adjustment to set the voltage on all 3 modules you show above, it's gotten to the point where I replace the "stock" potentiometer with one that is finger-adjustable.  My eyesight just ain't what it used to be!  And while for most applications setting the voltage is a one-and-done, I seem to use these modules in applications where the occasional adjustment is made.  And when the module is upside down, in an awkward location, and dimly lit, fussing with a jeweler's screwdriver is a nuisance.  While I can't imagine anyone else doing this, here's what the modification looks like.

Replacing (rotating) the blue potentiometer also allows me to fix the annoying characteristic on some of these modules where turning the knob CW decreases the output voltage. 

Really appreciate this topic, since I'll be installing several Menards accessories as I move forward, and a compact and economical method for powering them is needed.  The wall warts are huge, and their splitter cables are expensive.  The options presented above look great.

Questions regarding a couple of potential mistakes that would be easy to make:  For LED lights, will incorrect DC polarity cause damage?  Can incorrect voltage (too high or low, within reason), cause damage - i.e., 3v when the accessory uses 4.5v, or vice versa?

@stan2004 posted:

As for that tiny screwdriver adjustment to set the voltage on all 3 modules you show above, it's gotten to the point where I replace the "stock" potentiometer with one that is finger-adjustable.  My eyesight just ain't what it used to be!  And while for most applications setting the voltage is a one-and-done, I seem to use these modules in applications where the occasional adjustment is made.  And when the module is upside down, in an awkward location, and dimly lit, fussing with a jeweler's screwdriver is a nuisance.  While I can't imagine anyone else doing this, here's what the modification looks like.

Replacing (rotating) the blue potentiometer also allows me to fix the annoying characteristic on some of these modules where turning the knob CW decreases the output voltage. 

I like that mod with the knurled knob, and I agree with the thought about many of these having the pot's work backwards, very annoying!  What's really needed is a truly tiny voltmeter model that has the voltage display but doesn't make the module huge.

@Mallard4468 posted:

...

For LED lights, will incorrect DC polarity cause damage?  Can incorrect voltage (too high or low, within reason), cause damage - i.e., 3v when the accessory uses 4.5v, or vice versa?

Applying the incorrect DC polarity (i.e. + to -, - to +) to an LED can damage it.  It depends on the LED.  In general, an LED can tolerate small backward voltages of, say, 3V or 4.5V; they just won't light up.  In the OP's example of 2 x AA batteries, the manufacturer has to assume some percentage of users (larger than one would think) will install the batteries backwards.

It stands to reason that low voltage is OK since batteries decay.  The LEDs just get dimmer and then go out.  On the higher end, it's another one of those it depends on the LED circuit.  If you apply ~4.5V directly to an LED meant to operate at ~3V, it will light up brilliantly but is not long for this world.  LED operating voltages are in the 2-3V range, depending primarily on the color.  So LED circuits that operate from 4.5V will have some method to limit the voltage that the LEDs themselves see.  A few years ago there was a spate of reports on OGR that Menards LED buildings would work for a while but then die when using Menards wall-wart adapters.  Turns out their adapters were "unregulated" and might apply something like 8V (IIRC) at the connector.  They pulled the adapters from the shelves and re-designed them to apply a regulated, constant 4.5V DC.

There have been many OGR threads using similar if not identical methods as those shown in this thread to supply 4.5V DC for Menards buildings.  It appears most guys prefer to operate Menards buildings around 4V...or even lower to dim the lights.  That's one advantage of the voltage converter modules that allow you to adjust the voltage.

What a great and informative thread!

John,  if I have a transformer putting out 7 volts AC, and I use the first regulator you listed, can I use the regulator to adjust the output  up and down between 0 and 7 volts?  This would let me use the device to adjust the speed of my Atlas oil pump.

I know that this is a silly question, but I thought that such a device would be called a rheostat, not a regulator.

Thx, Mannyrock

I do apologize for some of my posts due to me being a new comer to this hobby. But from what I have seen and heard it is always good to have a Re fresh on some of these topics. Plus there is always newer technology coming out that makes any hobby more advanced and more efficient.

@chefmarkt69 posted:

I do apologize for some of my posts due to me being a new comer to this hobby. But from what I have seen and heard it is always good to have a Re fresh on some of these topics. Plus there is always newer technology coming out that makes any hobby more advanced and more efficient.

No need to apologize, I think you are spot on!  I have been in the hobby since the late '60s, and worked in electronics (in the electric power and telecom industries) for 43 years before retiring a year and a half ago.  I always see new products or new applications on these topics, and this topic is no exception.  Keep your questions coming!!! 

@Mannyrock posted:

What a great and informative thread!

John,  if I have a transformer putting out 7 volts AC, and I use the first regulator you listed, can I use the regulator to adjust the output  up and down between 0 and 7 volts?  This would let me use the device to adjust the speed of my Atlas oil pump.

With an input of 7 VAC, you would probably come close to being able to get 7 VDC out of it, but you'd be better off with 10-12 VAC into the switching power supply module.

@Mannyrock posted:

I know that this is a silly question, but I thought that such a device would be called a rheostat, not a regulator.

A rheostat is just a large potentiometer that  carries higher current than the typical potentiometer.  However, it's not all that useful for anything that might have a variable current draw as the voltage also varies with the current.

Thanks John.  I can definitely pump 12 VAC into the regulator, I was just afraid that that might burn it up.

Mannyrock

Most of the AC-DC power supply modules we use will accept up to around 20-25 VAC, they're loafing at 12 VAC.

The power moduels we typically use are buck mode switching supplies.  Buck mode means that the output voltage is always lower than the input voltage, so having more input voltage available is a good thing.  Also, switch mode power supplies don't waste tons of power if the input/output voltage differential is large, they actually work better at larger differential voltages as a rule.

The linear mode power supplies, OTOH, convert any voltage differential between the input voltage and output voltage to heat.  Needless to say, at higher output power and a large input/output voltage differential, there's a lot of wasted energy all turned into heat with a linear power supply.

@chefmarkt69 posted:

I have a few accessories that are led lights that run on two AA batteries. I would like to hard wire them. What would be the best way to do so. I did search the forum but had not luck and also I am new in the hobby. All of you have helped me so much and I very much appreciate it very much!!

Any train show or swap meet ought to have used AC track transformers For Sale at reasonable prices(less than ten dollars).  When you have one, set the control handle for three volts and remove the handle. Cut a hole for the stem in a piece of cardboard, plastic, foam or wood.  Tape that piece over the stem with duct tape or packaging tape.  Don't spare the tape, wrap it around the top, sides and bottom several times - it will be there for years.  Mount the now permanent 110v to 3v step down transformer inverted on the underside of your layout.  Run wires to your Menards buildings.

A second transformer could be used for the Miller signs.

John

@rattler21 posted:

Any train show or swap meet ought to have used AC track transformers For Sale at reasonable prices(less than ten dollars).  When you have one, set the control handle for three volts and remove the handle. Cut a hole for the stem in a piece of cardboard, plastic, foam or wood.  Tape that piece over the stem with duct tape or packaging tape.  Don't spare the tape, wrap it around the top, sides and bottom several times - it will be there for years.  Mount the now permanent 110v to 3v step down transformer inverted on the underside of your layout.  Run wires to your Menards buildings.

A second transformer could be used for the Miller signs.

John

You need DC for the applications discussed, so AC track transformers need not apply!  Also, very few train transformers will actually output voltages as low as 3V, and certainly not precisely.