Home-made AC power pack ?

I would not use a dimmer at all. The waveform would get too chopped up. A conventional postwar Lionel pulmore motor will run, and maybe a Williams, but I really doubt any MTH PS1 or PS2 engine would. Ditto for Lionel TMCC/legacy equipped engine.

I don't know what else would work, but the dimmer won't.

A quick google search finds a mth 750 (75 watts rms) new for $36 . At that price it seems like you could get a great unit, safe and reliable and no science fair project. A eBay search for used likely less. I say this because by the time you get a true Rheostat it will be approx $20, so 16 more and you have a new unit.

If you have a Powerstat handy I would use that to vary the 120 VAC primary to the transformer.
This will give you true sine wave output from your secondary.
Or use the Powerstat to vary the secondary output. You will have to experiment with the Powerstat settings depending on your transformer specs.
Powerstats come in 5 amp and 10 amp versions, and can be found on ebay.
If you have to buy a Powerstat however, you would be better off to locate cheap transformers as stated above.
There are always loads of them at train shows for 10 bucks or so each.

Rod

I'd forget this idea and just get a real train transformer. One look at the waveform that comes out of the typical dimmer will scare you. Also, the dimmers are NOT rated for inductive loads, and the dimmer would likely enjoy a very short life in that environment.

A powerstat is the same as a variac, right? I keep running across people who use this as the "homemade" option. Provided they came across the powerstat/variac cheap or free. The powerstat goes on the secondary of the step-down transformer, right? Or is it the other way around? I imagine it provides a cleaner sine wave than, say, a Lionel CW-80, right?

Sounds like the original poster may want to pursue the project not just for economical reasons but out of curiosity and inventiveness. If so, I am in the same camp. Are there other avenues toward a home-made ac power pack?

Jonathan

quote:

Originally posted by Jonathan Bowman:
A powerstat is the same as a variac, right? I keep running across people who use this as the "homemade" option. Provided they came across the powerstat/variac cheap or free. The powerstat goes on the secondary of the step-down transformer, right? Or is it the other way around? I imagine it provides a cleaner sine wave than, say, a Lionel CW-80, right?

Sounds like the original poster may want to pursue the project not just for economical reasons but out of curiosity and inventiveness. If so, I am in the same camp. Are there other avenues toward a home-made ac power pack?

Jonathan

Yes Variacs are the same...actually it's an auto transformer. And in the case of model trains should be used with a transformer for isolation from the ac line in my opinion. A 120vac one is designed to be used on a 120vac line, so I think, use it first. Also note the max current you are using, etc. Yes sine wave. They come in many sizes, even three phase, 240vac etc.

I have used a Variac/Powerstat to drive a fixed stepdown transformer for the trains. The stepdown transformer provides the necessary isolation for safety.

I would suggest including a Reverse button if you are running conventional so that you can limit the wear on the Variac's wiper tip.

Using a dimmer to drive a transformer sounds good in theory, but there are several hazards:
The chopped waveform (shaped more like a shark fin than a sinewave with the top chopped off) may cause extra heating in the transformer, and
As John mentioned, many dimmers don't like inductive loads. The Triacs can't shut off properly if there is too much phase shift between the voltage and current.

If you are running conventional mode, you will notice that a supply that ramps from zero volts doesn't immediately pull in the E-unit for direction change - a minor problem.

But if you are into experimenting, give it a try! You just might have the right combination of components. Please report back with your results.

I recently assembled a DC power supply for my experimental loop of G-gauge in the back yard. The supply is huge and heavy, and I had to add several additional components to make it a variable supply, but it was fun and it turns a doorstop into a useful 0-24V 6 amp supply. (I originally built the power supply while I was in graduate school in 1970 to power a 3M multitrack professional tape recorder - 42 years and counting.)

I have tried a household lamp dimmer on the output of a 18v transformer and it didn't work. Household dimmer switches only work in the 25-120 volt range. I tried it on the input to the transformer and that didn't work either.

I would really like to take apart a MTH Z Controller and figure out how it works. But then by the time I got it, took it apart, reverse engineered it, built a replica...

Well you get the picture. May as well just stick with what's already commercially available.

quote:

Originally posted by Flash:
I have tried a household lamp dimmer on the output of a 18v transformer and it didn't work. Household dimmer switches only work in the 25-120 volt range. I tried it on the input to the transformer and that didn't work either.

I would really like to take apart a MTH Z Controller and figure out how it works. But then by the time I got it, took it apart, reverse engineered it, built a replica...

Well you get the picture. May as well just stick with what's already commercially available.

My vote is with who ever said get a train transformer . It's hard to beat something with UL Listed on the side of it.
Even with the shipping it wouldn't be a bank breaker.

David

quote:

I had to add several additional components to make it a variable supply

Dale, I am curious about what components you used. Are you open to sharing?

Sometimes it looks like a DC power pack would be cheaper (and maybe easier) to make than an AC one. Is that true? If that is the case, would it make any sense to make a DC power pack, then put an oscillator on it to make it AC? I don't know much about converting from DC back to AC, but the thought occurred to me.

I am just learning this electronics stuff. If my questions are too basic, feel free to ignore them. I will not be offended.

Regards,
Jonathan

Many Thanks for all the input. I should have done more homework on this before I asked the question.

"Normal light dimmers are designed to only dim non-lunductive loads like light bulbs and electric heaters. Normal light dimmers are not suitable to dim inductive loads like transformers, fluorescent lamps, neon lamps, halogen lamps with transformers and electric motors."


Although it might work with a resistive 'ballast load' in parallel with a transformer primary winding, but I don't want to be blamed if someone tries it with bad results.

I have used a Variac/autotransformer on the 120 vac feed to a fixed-output step-down transformer, and it works fine. I used that before I figured out how to repair my RW and LW transformers, which was mainly a question of cleaning oxidized whistle and direction contacts.

I've also made a good cheap variable DC power supply with a 5-amp LM317 voltage regulator, and it works fine for DC motors or universal motors, or a Scout engine with mechanical reversing.

I live off-grid with alternate energy, so my household 120VAC is "modified sine wave" inverter (more accurately, modified square wave) which runs assorted transformers without issues.

Dale is correct, a dimmer switch output is actually a shark-fin waveform:

http://www.electricalknowledge...VE=true&TOPIC_ID=455

quote:

I am just learning this electronics stuff. If my questions are too basic, feel free to ignore them. I will not be offended.

We learn by asking.

The supply was originally a fixed 28V 8 amp regulated DC supply. Since I didn't have the proper transformer originally - only 24V output - I had added a second smaller transformer in series.

For the G-gauge application I wanted 0-24V adjustable. The original setup could be made adjustable, but the output transistors couldn't handle the power drop required for low voltage outputs. The output transistors must "burn off" the excess power that is being dumped when the output voltage is reduced. In this case, I might have up to 6 amps with a drop across the regulator transistors of 30+ volts - close to 200 watts!

My first step was to cut down the output voltage by removing that extra transformer. This meant I had a lower starting voltage, and hence less drop.

My regulator (a uA723) driving the output transistors wasn't very sophisticated - a marginal overcurrent protection without any temperature sensing. I decided to take an easy way out by adding a couple of high-power regulator chips to form a cascade of two regulators. The original regulator would provide a constant voltage source to the second regulator so that I didn't need to worry about things like voltage ripple. The new variable regulator would control the output voltage and burn off the power drop from 26V down to the final variable voltage. The power dissipation would be shared between the two stages of the cascade.

The regulator chips are a pair of very nice Maxim LT1083 adjustable regulators with internal overcurrent and temperature sensing. I tied two of these in parallel, mounting each on its own finned heatsink. These chips will shut down if they overheat, as they might if the output is shorted due to a derailment.

For the variable control device I chose one of the linear faders that I sell as the exclusive distributor for Penny & Giles. This gives me a smooth control with a stroke of three inches.

I wanted to include a current meter on the output, but I didn't have anything with a 10 amp range. I did have a nice 100 mA meter in an enclosure, and I was able to add a shunt to the terminals to recalibrate it to 10 amps. The shunt is just a short piece of bus wire. I determined the appropriate value by using my volt/ohm meter to read the output current to a fixed load while I adjusted the shunt to give the same reading on my power supply's meter.

I added a double pole, double throw center off switch for reversing polarity and killing the track voltage in the center position. I also included an incandescent pilot lamp across the output as a simple output voltage indicator.

When I got done, I had a kludge, but it did what I needed. I can run my four locomotives without the supply ever cracking a sweat. Sure, I could have bought something smaller and fancier, but what's the fun in that? Is it overkill? You bet! But it's mine, and I love it dearly....

I posted a more complete version of this reply with photos on my trainfacts.com website.

Thanks, Dale. Fascinating.

That animation is backwards. The Triac delays firing for part of the cycle, and when it fires, the output voltage jumps from zero to the sinewave's value. The "shark fin" should start from zero at the right side of the cycle, and work back toward the start of the cycle as the control is advanced toware ON.

You can control the turn-on of a Triac, but not the turn-off. It turns itself off when the voltage and current drop to zero. If the voltage and current don't both drop to zero at the same time due to inductive phase shift, the Triac will not turn off.

quote:

I live off-grid with alternate energy, so my household 120VAC is "modified sine wave" inverter (more accurately, modified square wave) which runs assorted transformers without issues.

Ace,
I would love to talk more about your alternative energy system. I have my layout in a newly built two car garage in the back yard. Right now it has no electricity other than an extension cord from the house. I would like to run almost everything in this garage off solar power. I have done some experimenting so far but I only have about 60 watts of solar panels.
Can you offer any advice for my application?

Currently I only have fluorescent tube lighting. I have found that these lights don't like running off a non-pure sine inverter. What kinds of lights do you use? Any suggestions?

Thanks

Dave

I have done exactly as the OP has proposed and it does work. The $7 dimmers work a little better than the $1 units.

I went on to build larger train power supplies with industrial 22 volt transformers and used the slide type ceiling fan controls instead of a dimmer and this works quite well.



One I built for my nephews over 15 years ago with whistle, bell, and direction buttons, and a set of Shur-Rite meters(amps & volts), is still being used today.

I use Re-Cirk-It panel mount resettable breakers for protection.

Dimmers rated for (house) track lighting are intended to control a transformer.

Very interesting! I guess I shot myself down too fast. Hard to keep up with all the new technology.

One again, Dale is correct about the dimmer waveform animation mentioned previously : it's backwards, and the website creator replied:

"You are correct on both accounts. It is backwards and the triac does rely on the zero crossing to turn off. A mistake I was unaware of when I built the graphic 7 years ago. if you just picture it the other way you should get the idea. Thanks for the comment."

Regarding alternate energy, some electronics may not like "modified sine wave" inverter power.