Dropping voltage with little heat?

What is the current draw of this accessory?

You will find several appropriate answers in discussions on this very subject HERE.

Oman posted:

What is the current draw of this accessory?

If you use diodes instead of resistors or a rheostat, you don't need to know the current draw.

ADCX Rob posted:

Oman posted:

What is the current draw of this accessory?

If you use diodes instead of resistors or a rheostat, you don't need to know the current draw.

True enough, but the OP seem to be concerned about heat. Resistor or diodes, same heat dissipated. If the current is significant, I would consider using a TRIAC or SCR.

Why is your Burning Switch Tower to run on 12 vac? I looked on a couple of the manuals and the voltage is specified to be 12 to 18 vac or in one manual, 15 to 18 vac.

I was first thinking that they probably just rectify the ac voltage to dc and you could probably use dc instead and use one of the ac to dc switching supplies from ebay. But according to the manuals, you don't really have to do anything to reduce the voltage from say 14 vac. Not true?

The very simplest solution is to power that one accessory with a dedicated small transformer like a 1010/1011/1012/1014 and adjust it for optimum performance as needed, w/o concern for voltage dropping heat beyond the normal transformer operation.

If you want to drop the voltage from a higher voltage, the back-to-back diodes are a more predictable method.  The problem with resistors is the voltage drop varies with the current in the circuit, with diodes there will be very little change in the voltage for a wide range of loads.

Of course, Rob has come up with the best solution, just use a transformer that you can set at the correct voltage.

The manual to the tower says 10-12 volts, it is 6-12768 Burning Switch Tower.  I hooked it up and initially thought that it was 14-18 volts and then the seuth smoke unit stopped working and I had to check the manual and sure enough it says 10-12 volts.  I have no idea what kind of amps this draws but I imagine no more that all the other PW and modern accessories that I have hooked up.

 I did think of using another accessory line but this is the only accessory I have that requires low voltage. I am installing a fan driven smoke unit but it's the lights that blink on and off too quickly when more than 12 volts is run.

I have no idea how to use back to back diodes.

cjack posted:

Why is your Burning Switch Tower to run on 12 vac? I looked on a couple of the manuals and the voltage is specified to be 12 to 18 vac or in one manual, 15 to 18 vac.

I was first thinking that they probably just rectify the ac voltage to dc and you could probably use dc instead and use one of the ac to dc switching supplies from ebay. But according to the manuals, you don't really have to do anything to reduce the voltage from say 14 vac. Not true?

My questions too.  Why 12V and why not DC?  From what I can tell you have some lamps, a smoke unit, and a mechanism that moves the men.  Is it one of those vibrator motors that requires AC?

One of the links Rob shows above will describe the diode method - $1 on eBay free shipping from Asia.

Edit: the use of the adjustable regulator method ($1 DC eBay module) would allow you to adjust to 10, 11, 12, whatever.  It appears the lamps behave differently at different voltages, maybe adjusting the voltage would allow you to fine-tune the behavior of the men.  If your smoke unit applies incoming voltage directly to the resistor (without regulation), going from 10 to 12V is a 40% change in heat which can substantially change smoke volume; so adjustment capability might be useful.

What if you are wanting to run something inside a rolling stock car at 1 amp and 5volts DC and are at the mercy of the AC track voltage?  At 18 volt track voltage you could end up with 25volts DC and have 20 volts at 1 amp to get rid of.   Any cheap solutions other than a VRM?

What do you define as "cheap"?  And what about the various VRM approaches described in different OGR threads does not work for your application?

There are a number of switching down voltage regulators on eBay. You would have to add a bridge rectifier and a capacitor in front of it. The total cost would be something in the 5 dollar area.

Thanks for the help.  My "cheap" was in reference to the power dissipation.  I have built some VRMs at home, both fixed and variable, but I was hoping to avoid the potential 20W power dissipation issue if there was some alternative.

I haven't kept up with electronics in the past several years so I didn't know if there was a new handy device I could use.

There have been many threads showing various eBay switchmode voltage regulator modules.  It appears yours are linear voltage regulator modules.  For example, here's one thread showing some of the options:

https://ogrforum.com/t...age-ac-dc-conversion

Or, just enter "LM2596" in OGR search and you will get many more.

To your point, if starting from 25V DC and dropping to 5V DC at 1 Amp using a linear VRM, that's 25 Watts going in, 5 Watts going out, and 20 Watts wasted as heat which is really hard to get rid of in a piece of rolling stock.  20% efficiency.

With a switchmode VRM, the efficiency will be around 85%.  So for that same 5 Watts out you only need about 6 Watts going in, and only 1 Watt wasted as heat.  Much better!  The other benefit is lower input current since if starting from 25V DC, 6 Watts only means about 1/4 Amp (rather than the full 1 Amp).

The DC-DC switchmode VRM on eBay are about $1 (free shipping from Asia).  They are bigger than your designs.  You provide the AC-to-DC conversion with a bridge rectifier or 4 discrete diodes as you obviously know how to do.  There are switchmode VRMs that include the bridge rectifier.  There are other variants that are bigger/smaller, with different connection options, that handle different voltage ranges, and so on.

aussteve posted:

Thanks for the help.  My "cheap" was in reference to the power dissipation.  I have built some VRMs at home, both fixed and variable, but I was hoping to avoid the potential 20W power dissipation issue if there was some alternative.

I haven't kept up with electronics in the past several years so I didn't know if there was a new handy device I could use.

There are 2 kinds of voltage regulators. Linear, the type that your refer to, can and usually do dissipate a lot of power (heat) and switching that can operate over a wide operating voltage with about constant power (heat) loss regardless of input voltage.

For other than really low current applications, I don't use linear regulators nowadays.  The switching models are so low cost that it makes far more sense to go that way.  I buy the little eBay regulator modules and then just glue a bridge "dead bug style" on top and add an input cap.  If needed, I also add a 22uh choke for DCS compatibility.  That gives me a compact DC supply that will deliver plenty of power with very little generated heat.

For my projects that need 5V supplies, I've taken to using the Recom supplies, they directly replace the TO220 linear regulators. 

Here's the one that's on my Super-Chuffer. Recom R-78E5.0-0.5

GRJ brings up another option - the 3-terminal switchmode modules that insert into 0.1" spaced holes of a classic 78xx regulator design.  Since you already have circuit-boards made up to take a 78xx chip, this could be a winner.   Just be sure to check the specs on these devices.  The 78xx generic regulator is generally spec'd for 35V DC max input voltage.  Some of these 3-pin modules can be loose with the language when claiming "plug-in" or "drop-in" capability to a 78xx.  They aren't spec'd to operate up to 35V DC.  This is particularly relevant for O-gauge since using a bridge rectifier plus capacitor on a train transformer output can produce DC voltages approaching 30V or more.  Anyway, here's one that I would classify as a true plug-in replacement for a 7805....$4.30 at Digikey.

Now if you have a fixed-output transformer brick that "only" puts out 18V AC, and you do the 1.4x multiplier, and take the bridge rectifier voltage drop, etc. you indeed come out under 25V DC.  But most throttle transformers put out 20V AC or more.

So blabbering a bit more about minding the input voltage spec, with the eBay DC-to-DC modules you also need to watch the input voltage capability.  This is why I suggest modules based on the LM2596 chip which intrinsically handles the 35V DC like the 78xx chip.  There are smaller modules which use phrases like "LM2596 alternative" or "LM2596 replacement" in the eBay listings.  But for O-gauge applications watch out!  Make sure the module can take the DC voltage you present to it.  For example here's a 99 cent module on eBay:

It only takes 23V DC.  It's hard to read some of the eBay photos but if you know what you're looking for you can see the voltage regulator chip used on the module is the MP2307.  Nowhere on the eBay listing does it tell you the chip type used on the module.  Not that the internal workings matter to an end-user, but my experience with some of these Asia eBay sellers is they liberally use "cut-and-paste" of photos, specs, data, etc. so it's in your interest to read-between-the-lines to make sure you are getting something that will work!

And here's another similar 99 cent module based on the MP1584 that goes to 28V DC.  Again, no mention in the eBay listing of the internal chip type used...other than it is a LM2596 replacement.

If your application is converting, say, 14V AC accessory voltage down to 5V DC then these modules can be just what the doctor ordered!

For contrast, I found this photo from another thread showing a side-by-side of a typical LM2596 module vs. one of the smaller "LM2596 alternative" modules.  

There are other consideration (capacitance, adjustability, etc.) between the two module types but this post was simply focusing on the DC input voltage spec of whatever DC-to-DC converter you might be considering.

I now step down from the soap-box.