Simple power source

[JRP]

So, I just want to light 36 LED's under a layout board.  They are each 3 volt, draw no more than 10-20ma and come with resistors. Whether I do this in series or parallel, I just want to know if I can use one power source?  One reader told me I could do this with a 3V 1 amp wall wart that would drive all of them.  That seems too simple for all 36 lights.  Do I need more power than that, say a 12V wall wart?  I also have a 10 and 6 pole terminal block.  Will they come in handy for wiring this?  Please, keep your suggestions simple as I am not that experienced with layout lighting, just enough to be dangerous.  

JRP

[gregc]

36 LEDs at 0.01A (10ma) requires 0.36A

but are they really 3.0V.  a little too much voltage can damage them and wall warts may not be regulated.

adding a series resistor would make it less likely to damage them.   an adjustable step-down regulator with a 12V supply would provide better control.   

3 LEDs and one series resistor with a 12V supply would be safe.  300 Ohm ((12V - 3*3V)/0.01.   if the supply voltage were a little higher, the current would be a little higher, but within the 10-20 ma range 20 ma ((15V - 3*3V) / 300)

wire up a few on the bench and get comfortable with what you have to work with, hopefully you have a few unused wall warts lying around

 

[RR_Mel]

I use buck converters for fixed voltages on my layout.  My main source is the old standard 12 volts.  I use the buck converters to drop the 12 volts to the lower voltages needed.  By starting out with 12 volts I can adjust the voltage from 1.2 volts to just under 11 volts.

I have several fixed voltages on my layout, 1.3 volts for micro bulbs, 4.5 volts for Miller Engineering animated signs, 5 volts for Arduinos and 8.5 volts for Grain of Wheat bulbs.

The buck converters are available from just under 2 amps to 12 amps.  As you will only need .72 amps for 36 LEDs I would go with a 2 amp allowing for a bit of future expansion.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=Buck+Step-down+LM2596+Power+Converter+Module+DC+4.0%7E40+to+1.3-37V+LED+VoltmeterBuck+Step-down+LM2596+Power+Converter+Module+DC+4.0%7E40+to+1.3-37V+LED+Voltmeter&_sacat=0



Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California

Growing old is a bummer, aging is definitely not for wimps.

[JRP]

Beginning to udnderstand, but does this mean I can only operate 3 LED's with one 12V supply?  If so, I would have to have twelve 12 V supplys in order to operate all 36 of my LED's?  Gosh, I was hoping I would need only 1 or 2 drivers for all of these lights.  Am I understanding this correctly?

[RR_Mel]

You only need one power supply capable of .8 amps to light up 36 LEDs.  Preferably a voltage 5 to 12 Volts and a series resistor for each LED to set the max current to .02 amps or 20ma each.

36 x 20ma = 720ma or .72amps

EDIT:

You mentioned that the LEDs came with resistors, included resistors are normally configured for 12 volt operation.  36 LEDs with included resistors would draw somewhere in the vicinity of 20ma each from a 12 volt 1 amp wall wart at .72 amps.

Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California

Growing old is a bummer, aging is definitely not for wimps.

[hon30critter]

Hi JRP,

No, you can operate all 36 LEDs at 20ma each (0.020A) on one 2 amp wall wart. I believe what gregc was saying is that you could put 3 LEDs in series with a single resistor so that you only need 12 resistors instead of 36. In other words, you would have 12 groups of 3 LEDs each, all on one 2 amp, 12v power supply. The 12 groups of LEDs would be wired together in parallel.

Personally, I wouldn't bother with putting any of the LEDs in series with each other. I would use a 1000 ohm resistor in series with each LED, with the 36 LED/resistor combos wired in parallel. If you use a 12v power supply, the resistors will reduce the current sufficiently to keep the LEDs from burning out. If the LEDs are too bright you can use higher value resistors (3500 - 5000 ohms) to dim them down. The higher value resistors will also cause the LEDs to draw much less current. That is often desireable because at 20ma a typical LED is very bright and often hard on the eyes.

Here are the two wiring options. The top shows all the LEDs in parallel (RRMel's suggestion is the same) and I believe that the second one represents what gregc was suggesting. Note that I am guessing at the 350 ohm resistor value. Maybe somebody can give a more accurate value. (I don't wire LEDs in series at all). Note also that I may have the + and - reversed. I can never remember which way the diodes should point.

RRMel's method of using inexpensive buck converters works well too. In his case he needs a variety of different voltages to feed the different types of lights. The buck converters allow him to get all those voltages from a single 12v power supply. In the diagrams above, the buck converters would be in line between the 12v power supply and the circuit. All the bulbs in each circuit would be operating at the voltage put out by each converter so you would have separate converters and circuits for each different voltage requirement

Note that Mel is using a much larger power supply than what you need. He is powering several hundred bulbs whereas you are only starting with 36 LEDs. There is nothing wrong with starting with a large power supply if you anticipate adding a lot more lighting down the road, but I would get one that breaks the power output into 2 amp fused circuits just for safety's sake.

EBay is a great source for cheap resistors as well as LEDs.

Don't hesitate to ask if you have more questions.

Cheers!!

Dave

 

[RR_Mel]

Well put Dave!!!

Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California

Growing old is a bummer, aging is definitely not for wimps.

[CSX Robert]

Everything Dave said is correct, I would add to it, however.  The advantage of the series/parallel wiring (the second diagram in Dave's post) is it draws a third of the current.  It probably won't make a difference in this situation - 36 parallel LEDs at 10ma (which is likely bright enough) would only be .36 amps, and just about any off the shelf power supply you get will handle at least half an amp.  It could certainly make a difference if you plan to expand down the road, though.

[hon30critter]

CSX Robert

Everything Dave said is correct, I would add to it, however.  The advantage of the series/parallel wiring (the second diagram in Dave's post) is it draws a third of the current.

Hi CSX Robert,

I didn't know that! Thanks.

I couldn't understand why people would use a series/parallel circuit but now I do. Makes sense.

I guessed at the 350 ohm resistor value for the three LED/single resistor circuit. Is that value correct?

Thanks,

Dave

[gregc]

hon30critter

I guessed at the 350 ohm resistor value for the three LED/single resistor circuit. Is that value correct?

depends on the desired intensity.

but it's good to have a relatively close value to start with.   for 3.0V LEDs and 12V supply, the current is 8.5ma ((12-3*3)/35) which may be on the low side (i.e. dim)

[CSX Robert]

hon30critter

I guessed at the 350 ohm resistor value for the three LED/single resistor circuit. Is that value correct?

 

Oops, I forgot to answer that.  I see Greg already has, but yes, if aiming for 10 ma, 350 ohms was pretty close.  The formula for current is the same, current = supply voltage - LED voltage drop / resistance, but you have to use the voltage drop of the three LEDs combined.  330 would be closer [(12-9)/330=0.009] and is a common resistor value (I don't know the common values off the top of my head, but I have a set sitting on my desk).

One area where this makes a huge difference is LED Christmas tree lights.  They normally have groups of 35 LEDs in a series, if they were all ran in parallel they would draw 35 times as much current.

[JRP]

OK, this is much better.  I think I know what to do now and am very glad to hear that I can operate all my LED's off of one 12V 2 amp wall wart.  Question: can my terminal blocks be of any use here? 

Thanks Dave, Mel and Greg.

JRP 

[gregc]

JRP

Question: can my terminal blocks be of any use here? 

if your LEDs are dsitributed around the layout and wired in groups of 3 with a resistor, i would think it would be conveninent to have a 12v(?) bus running around the layout that each group is connected to.    the two connections for each LED group can be several feet apart on the 12v bus

• suitcase connector could be used just as they are used for DCC feeders,
• you could solder the LED connections to the bus or
• you could run long lengths of wires back to some central point with terminal blocks

[JRP]

I think I have this.  Thanks again to Dave, Greg, CSX Robert and Mel.  

Warm Regards,

JRP

[MisterBeasley]

Think ahead.

When I started my layout, I used quite a few low-power wall warts, even power supplies from old electric shavers.  Eventually, I built out all of Phase 1 of the layout like this.  It was a rat's nest of sloppy wiring and terminal strips powering it all.

For Phase 2 and beyond, I wised up.  12 volt, 4 or 5 amp DC supplies driving common bus lines, plus on-off switches and appropriate fuses for each bus.  I like mostly incandescent bulbs for my Transition Era structures and street lamps, so I need a bit more power, and I also drive Miller Engineering signs and grade crossing signals from the same bus.

A 12 VDC supply online costs about $8, and they pay shipping.