Ac to Dc Buck Converter Voltage Question

Yes.  Presumably your train transformer has a 14V AC Accessory output.  Or 18V AC fixed output from a brick is fine too.

Your module says 2.5 Amps output max.  2.5 Amps at 9V DC is 22 Watts.  I'd guess a sound module from a boxcar might be rated at 1 or 2 Watts max.  So should be no problem driving two (or more).

 Thanks Stan! I'm using a180w brick thats reduced to 10v coming out of a TPC unit.  

Since the majority of my accessories are rated for 10-12v. I figured going on the lower side might extend the life of any bulbs.

So It would be 10v AC going into the converter set at 9v DC coming out.

Heres photos of the components for reference if it helps( the control board is probably not close enough):

That's a 1/2 Watt speaker...so we're "only" talking maybe 1 Watt max for the two units.

Since this is well within the capability of the Voltage Converter, providing 10V AC is probably enough.  That is, even if it was a 10V AC pure-sine, the voltage reaches ~14V peak which ought to be enough to maintain 9V DC output at light loads.  Plus, if the 10V AC is from a chopped-sine controller fed by an 18V AC brick, the peaks are much higher.

I'd just try it.  If you start to hear the tell-tale AC "hum" that will be the voltage converter output dipping from lack of enough peak input voltage.  If it's just a little bit of hum you could turn down the Converter to, say, 8V DC.  This will drop the volume a few dB but the voltage converter might be able to maintain a steady 8V so no hum.  A simple tradeoff. 

Thanks again Stan!

BTW, my MTH crossing bell sound has worked flawlessly since I added one of these buck converters to power the audio board, and you recommended setting the voltage at 5v DC.

Theres not  been a single cutout and there is no sacrifice in volume. 

Received the converter and it works great Stan. Goodbye batteries!

My next question is. Is there a volume pot that could be spliced in for each sound generator?

I find the sound louder than I expected. If I could dial it down a bit it would be ideal.

Can the sound be lowered without a loss in sound quality? Or would it be an increase in static with lowered sound.

@RickO posted:

...Is there a volume pot that could be spliced in for each sound generator?

I find the sound louder than I expected. If I could dial it down a bit it would be ideal.

Can the sound be lowered without a loss in sound quality? Or would it be an increase in static with lowered sound.

Adjusting the fixed volume of a layout accessory comes up fairly often.  For example, in this Mel's Diner OGR thread a potentiometer is spliced in.  This is a Radio Shack part that is apparently no longer available but if this is the route you want to go we can track down an available alternative.  When all said and done it might be in the $5-10 range.  The idea of course is the potentiometer (aka variable-resistor, rheostat) steals some of the voltage going to 8 Ohm speaker.  So if you adjust the 25 Ohm pot to 8 Ohms, it will steal 1/2 the power which amounts to a 6 dB reduction in volume.

Alternatively, since this might be a one-and-done adjustment, you could buy a handful of fixed-value resistors and likely same some money.

Let's say you have some 5 Ohm resistors (10 for $2 on eBay w/free shipping from a US Seller).  Splice in 1 between the sound board and the 8 Ohm speaker and you get a ~4 dB reduction.  Splice in, say, 3 (as shown) totaling 15 Ohms and you get a ~9 dB reduction.  Splice in 5 totaling 25 Ohms and you get a ~12 dB reduction.  ~12 dB would be the maximum reduction from the 25 Ohm potentiometer.

Of course the rub is knowing how many dB of reduction you need.  You'll know it when you hear it but that's not much help in ordering parts!  It's hard to know what a dB is but if you have a TV with a bar-graph volume adjustments, each bar is typically around 1 dB.

Reducing volume in this manner will not affect fidelity/distortion.

If you're only looking for a small reduction you might try dialing down the voltage out of the AC-to-DC converter to, say, 8V output.  You'd think a 9V gadget should still operate at 8V and in most simple sound circuits this will reduce output volume by ~1 dB.  If the gadget still operates at 7V, this should effect a ~2 dB reduction.  Reducing volume in this manner may affect fidelity/distortion but then again it costs nothing to try!

Your right about the one and done. Since your add shows 10 resistors for 2 bucks that would allow me up to the maximum 5 each for my 2 sound generators so maybe I'll go that route.

The wiring diagrams are super helpful too Stan, just what I needed. 

Theres a number of folks on this forum that offer helpful information that often fall a bit short telling the "whole story" leading one asking more questions.

Not so in your case Stan. 

This is probably very basic, but...

You can wire the transformer AC through the buck converter, run the transformer to max AC and measure the output at the DC end of the buck converter.   Adjust the DC Buck Converter output voltage down to 9.5 volts (or the maximum you would ever want).   That prevents you or others from grabbing the wrong output handle on the transformer, and smoking your LEDs.

Then gradually turn down the transformer AC voltage until you reach the lighting where everything works and you get the level you like.  Obviously, using a fixed output like a power brick, or wall wart, that is already at max output.

Realize that LEDs (a) like DC better than AC, (b)  turn "on" at some minimum voltage,  and (c) burn up at very slightly above the rated maximum voltage.  In the middle there is some dimming or brightening though not like with an incandescent bulb.

The lower you run the building lighting etc. and still get your results, the easier on the accessories/ longer life.  I almost always run everything at the minimum.

Roger-that on the one-and-done.

If you go the fixed resistor route, you can gain some variability with a simple on-off switch.  In the example below the switch shorts/shunts 2 of the resistors.  Thus you can select between 5 Ohm (switch "on") or 15 Ohm (switch "off).  I figure you're removing the 9V battery snap connector shown in your photo and perhaps even the on-off switch...in which case you could re-purpose the switch into a hi-lo volume selector! 

Otherwise, on-off mini toggle switches are fairly inexpensive even from US sellers.  The eBay listing above even has pre-soldered wires.

Why all the switches when you can just add a 50 ohm pot and have continuously variable volume?  Here's one I use on an MP3 module, it does a nice job of low volume to full volume.

CT6EP500 50 ohm 1/2W Trimmer, 78 cents.

Agreed those screw-driver adjust trimpots would work in this case.  It's just that USPS First-Class shipping from DigiKey starts at about $5.  So if that's all you're buying from DigiKey, a couple 78 cent trimpots balloons to $3 a piece!  On eBay you can do a bit better but the underlying problem is you are forced into USPS First Class package rates since the trimpot size means a package >1/4" thick.  OTOH you can mail 1/2W fixed resistors using regular USPS envelope rates since they are thinner than 1/4".  Probably too much information but I really don't feel comfortable spending other-people's-money! 

Well, I guess if I wanted a volume control. I'd probably want a continuously variable one so I could set the exact volume I wanted.

Thanks for the tip regarding the volume pot John! Hmmm, decisions decisions. Thanks for the 2 for $1.10 Stan. I might just go that route after all. Then, if my hearing goes over time I can increase the volume as needed LOL.

Its amazing what one can do with all of the help and info on this forum. I bought 2 of these boards and put them on a DPDT toggle out of the buck converter and hid them in the scenery. It adds another dimension to the layout.

Something I would have not even considered or known was possible if I was just "Joe basement O guager" with no clue about this forum.

@RickO posted:

Thanks for the tip regarding the volume pot John! Hmmm, decisions decisions. Thanks for the 2 for $1.10 Stan. I might just go that route after all. Then, if my hearing goes over time I can increase the volume as needed LOL.

Its amazing what one can do with all of the help and info on this forum. I bought 2 of these boards and put them on a DPDT toggle out of the buck converter and hid them in the scenery. It adds another dimension to the layout.

Something I would have not even considered or known was possible if I was just "Joe basement O guager" with no clue about this forum.

Hey, whadda  ya want?

Does anyone know when the term "buck converter" came in to use to describe a solid state switching regulator?

They're called that because they "buck" the voltage, i.e. drop it.  Same reason as the "boost" converter raises the voltage.  I don't know when they got the handle, that's another story.