Is there such a thing as an AC-AC Buck Converter?

Here's a previous post from Stan2004 that explains how to use inexpensive diodes to get multiple lower voltages from your AC power source. Stan is very good at explaining things and I think this post pretty well covers it, with pictures and details too! This seems to be the preferred method around here for dropping AC voltage. 

Yes. An AC-AC buck converter is called a transformer. So is an AC-AC boost converter.

You can also use a number of diode pairs to get the reduction you want, or you can use Zener diode pairs of the proper voltage.

RoyBoy posted:

Yes. An AC-AC buck converter is called a transformer. So is an AC-AC boost converter.

Mike CT posted:

Buck/boost transformers are used to ….

There are interesting wiring diagrams. Some applications require two, or three, buck/boost transformers, (three phase).  That .75 KVA is 750 watts of accessory power. Wired for 12 volts, it can be wired for 24 volts.  Hardly even being used. 

 

all I can think of here is this

Get a separate transformer such as a Lionel 1033 to power up the accessories that need a different voltage. You can then adjust the voltage to your specific needs.

Larry

Thanks Gentlemen. Perhaps I didn't make my question or request  clear.

I am fully aware what a transformer is and does and have quite a few.

My desire is to have an AC to AC device similar in size to the AC-DC Buck Converter shown here (Size - 2-1/2" x 1").  Input AC voltage is adjusted by the pot to output DC voltage. (Nice for LED installations in passenger cars.)

This would allow the use of a multi-port terminal wiring block to get power from one transformer output but allow individual taps to be regulated independently if needed. MTH terminal blocks have 12 or 24 positions or so. Each one (if necessary but probably not all) could be independently set up to output any voltage required.

Would it be possible to have such a board or is there one already available? Thank,   Joe.

That probably doesn't exist, and if it did, it would be a lot more expensive than a AC-DC or DC-DC buck converter!  Since a transformer does the job so well, I don't see the reason for inventing such a device.

Actually, I lie a little, there is one fairly common one that will handle the kind of power you're talking about, it's the MTH Z-4000!  It's not exactly small...

Transformers aside, I'd say you have 3 options to reduce one AC voltage to another AC voltage in your prescribed form-factor (since size matters):  

1. As you initially proposed, a resistor would work.  Less than $10 for an adjustable resistor/rheostat.  Dial in a suitable voltage and you're done.  If you add additional lights/loads you'll need to re-adjust.  Very simple wiring - though probably requires soldering to attach wires to the rheostat.  You need to select a suitable resistance and power handling capability.  We can help you with that if you go this route.

2. As suggested by others, a string of diodes can drop AC voltage.  Just a couple dollars in components.  But assembly required which may not be in your comfort zone.  Example photo from another OGR thread shown above.  The idea is you would find a voltage tap that works for you.  If you add additional lights/loads you probably don't need to re-select the tap.  You need to select suitable diode/rectifier components.  We can help you with that if you go this route.

3. As you ponder, the closest modular AC-to-AC "buck" converter is a lamp-dimmer.  Just a couple dollars.  AC-in, AC-out with adjustable control using triac-technology.  But all the lamp-dimmer modules I'm aware of operate with an incoming AC voltage of 110/220 for obvious reasons.   These can be modified to work with "only" 18V AC coming in but this should only be attempted by a DIY'er with electronics background.

4. Not a module so won't fit your form factor requirement, but an MTH Z-controller is essentially a lamp-dimmer that operates with "only" 18V AC coming in.  You can probably pick one up used for, say, $10-20 (?).  

Thanks Stan. I'll go with the rheostat which is in my comfort zone of capability (not great in electronics unfortunately). Any suggestions as to what type or rating I should look for?

Appreciate the detailed response to a low savvy electronics guy.

And yes. John...I have a number of Z-4000's and, surprisingly enough, know what they do. Just thought that since low voltage AC to DC  converters were available in the form shown in the photos that maybe similar low voltage AC to AC regulators were also available. Z-4000 and other transformers are high voltage AC to low voltage AC regulators as you know. To my non-electronics mind, it didn't seem unreasonable to expect to find a board already available that would regulate about 22 VAC down to about 15 VAC. 

Joe

Rigatoni Express Railroad posted:

... I'll go with the rheostat which is in my comfort zone of capability (not great in electronics unfortunately). Any suggestions as to what type or rating I should look for?

Where do you shop for these kinds of parts?  Is eBay from China OK (several weeks wait)?  

Tell us a bit more about how many and exactly what kinds of widgets are on this circuit.   Do you have some representative MTH item numbers?  As others have pointed out earlier, one downside of using resistors is the voltage changes as you change the load (no regulation).  Hence if you have multiple blinking signs hooked up that blink at different times, I can imagine a scenario where one lights blinkings causes one or more other lights to dim/brighten. 

Rigatoni Express Railroad posted:

...To my non-electronics mind, it didn't seem unreasonable to expect to find a board already available that would regulate about 22 VAC down to about 15 VAC. 

Regulate is the buzzkill.  In basic terms semiconductor/solid-state electronic components are DC devices...and hence work great regulating-controlling-adjusting DC voltages.  AC voltages are another matter.  This is not at all obvious.  So to your point, it seems perfectly reasonable and rational to expect $1 AC-to-AC buck regulator modules. 

Stan,

ebay from China is fine with me. That's where I bought the other buck converters I showed.

I'll have about 10 MTH lighted buildings on the circuit, most with blinking signs. MTH numbers as follows:

30-90050, 30-90174, 30-90195, 30-90205, 30-90217, 30-90227, 30-90297, 30-90438, 30-90449 and 30-90476.

I thought I could use DC voltage to power these with the AC-DC converter I showed earlier. The building lights  were okay with it but, most of the blinking signs didn't like it, even though MTH states in their instructions that either AC or DC can be used. That would have been too easy a solution.

Hope that helps.

Thanks,  Joe

Joe, no insult was intended, I guess I was just pointing out what is obvious to me.  FWIW, there's really no difference between taking 120 volts AC down to a lower variable AC voltage than taking 20 volts AC down to a lower variable AC voltage.  The process and expense would be exactly the same for a given amount of power output.

Like Stan says, to a guy that isn't into electronics, it's probably perfectly reasonable to expect such things exist.  However, once you start actually seeing what it would take, you realize why they don't exist at any reasonable price.

A dumb question.  Why not find a few $5 20W-30W little transformers and use those?  They'll work better than rheostats I would imagine.

John,

No insult was taken. Just wanted to make sure you knew that I had a number of Z-4000's and knew what they do and wasn't a total neophyte, although, a neophyte I am.

If, as Stan says, the signs are affected by the "sequence" in which they blink, I will take your advice and look at the cheaper transformers.

I'm always glad to get an education from the guys who know what's going on with, to me, the magic of electricity/electronics.

Chemistry was my thing! Electricity/Electronics always baffled me!   Joe

Rigatoni Express Railroad posted:

...

I'll have about 10 MTH lighted buildings on the circuit, most with blinking signs. MTH numbers as follows:

30-90050, 30-90174, 30-90195, 30-90205, 30-90217, 30-90227, 30-90297, 30-90438, 30-90449 and 30-90476.

I thought I could use DC voltage to power these with the AC-DC converter I showed earlier. The building lights  were okay with it but, most of the blinking signs didn't like it, even though MTH states in their instructions that either AC or DC can be used. That would have been too easy a solution.

Hmm.  I looked at the MTH instructions for your first 3 buildings and they all have the same instructions:

I think we need to figure out why the buildings don't work on the DC output from your AC-to-DC converter. 

You apply 18V AC to your AC-to-DC converter.

You adjust the "blue" screwdriver control to read 14V DC on a DC meter.

You connect the converter DC output to the two building input clip terminals.

And what exactly happens (or doesn't happen)? 

Stan,

That's exactly what I did. I check the DC output with my meter before installing...15 volts DC out.

The buildings light up. One of the blinking signs works but none of the others. As soon as I go back to the AC voltage all is well with the signs.  Joe

Does changing polarity on the non working sign structures make a difference?

Rich may be on to something.  Try reversing the DC connections on a sign which does not blink (but blinks if AC applied).  Note that since AC alternates polarity, applying "backwards" DC does not present any risk of damage.

In techno-speak, perhaps MTH used only a diode (half-wave) rather than a bridge (full-wave) to internally convert AC to DC for the blinking sign circuit which undoubtedly requires DC.  This could explain why it only works with 1 polarity of DC.  

gunrunnerjohn posted:

Joe, no insult was intended, I guess I was just pointing out what is obvious to me.  FWIW, there's really no difference between taking 120 volts AC down to a lower variable AC voltage than taking 20 volts AC down to a lower variable AC voltage.  The process and expense would be exactly the same for a given amount of power output.

Like Stan says, to a guy that isn't into electronics, it's probably perfectly reasonable to expect such things exist.  However, once you start actually seeing what it would take, you realize why they don't exist at any reasonable price.

A dumb question.  Why not find a few $5 20W-30W little transformers and use those?  They'll work better than rheostats I would imagine.

Joe,     John is spot on I have seen people sell the smallest Marx transformers for $2 at train shows, and they have a throttle for setting the brightness where you like. I bought the small Lionel transformer in this pix at a meet in Atlanta for $3  and keep it attached to a TMCC Base on my workbench at 75W It is powerful enough to light a small city or any loco on the bench.    j

Stan, Will try the try reversing the connections and let you and Rich know what happens. Won't be able to do so for a few days but will do.

Johnacton  and JohnGRJ...if all else fails, cheap transformers will be next.

Thanks to everyone.   Joe

He is a cheap way to drop AC voltage. I use it to lower the voltage on a steep track decline and it works great. Since I am lowering the voltage for an engine that needs a few watts of power I use 5A 1 ohm wire wound power resistors. One in series will probably lower the track voltage about 5V but it depends on the power draw of the items being used.

Thanks Dennis. Will take a look at that possible solution also.   Joe

I went with the 50 Watt, 10 ohm variable potentiometer option, thinking it should be idiot proof. It turns out I'm a bigger electrical idiot than I thought.

Here's how I tested it:

The transformer red lead goes to the first tab as shown.

The transformer black lead is hooked to the voltmeter's black lead.

The voltmeter's red lead goes to the second tab.

Meter reads the 17 volts that the transformer is putting out.

But, rotating the knob on the pot does nothing to effect the voltage.

Tried it on another pot with same result. 

I'm missing something I'm sure.

Any help appreciated.   What is the third tab for?   Joe

You have no current flow, so basic Ohms Law says, no current, no voltage drop.  

In order to get significant voltage drop with a 10 ohm pot, you'll have to draw amps through it!  It'll also get quite toasty.  It may be rated for 50 watts, but that's with heat sinking I'd imagine.  That pot with 50 watts being dissipated in open air will be an excellent soldering station.

Again, go with the transformer, it's the right tool for this job!  For a single sign or two, they'll likely draw a very small amount of current, certainly under an amp.  You may need a higher resistance value to do what you're trying to do.

Rigatoni Express Railroad posted:

I went with the 50 Watt, 10 ohm variable potentiometer option, thinking it should be idiot proof. It turns out I'm a bigger electrical idiot than I thought.

Here's how I tested it:

The transformer red lead goes to the first tab as shown.

The transformer black lead is hooked to the voltmeter's black lead.

The voltmeter's red lead goes to the second tab.

Meter reads the 17 volts that the transformer is putting out.

But, rotating the knob on the pot does nothing to effect the voltage.

Tried it on another pot with same result. 

I'm missing something I'm sure.

Any help appreciated.   What is the third tab for?   Joe

 

The three tabs from left to right should be: Ground, Input, Output.  You have nothing connected to output and are measuring input. So, the pot is working so far.  Try measuring across ground and output to check the value of the pot setting. Then, hook that up to your buildings.

George

As GRJ says, you need a load attached to see the effect of the potentiometer.

So where you have the voltmeter attached, attach the "hot" wire of your 10 buildings; the "ground" side of your 10 buildings go to black as before.  Note that the potentiometer will never increase the voltage so not matter what you do at least you're not at risk of making the situation worse!

You'll then find that turning the dial to fully one direction will read about 17V and fully the other direction will be something less...maybe 10V or so and you'll see the lights dim along the way. 

Thank all you gentleman, John, George and Stan for the explanations.  Will hook it up and see if I can manage to not screw it up.

I should stick to chemistry.   Joe