Cheap Low Voltage AC Power Supplies

[techie1234]

Here’s a suggestion from one of my other hobbies: if you are looking for a simple 8-20V variable voltage AC power supply that’s not regulated, rather than rigging something up yourself you might want to consider just buying a used Lionel model train transformer.  Although most are now 50+ years old, they were built very solidly and most work just fine today.  Most (although not all) have a circuit breaker in them and some are even U/L listed.  And if something is wrong, they’re easy to repair with specs and parts readily available.

You can pick up a single control 25-75W unit for +/- $10 on Ebay.  I keep one on my workbench for testing train equipment but also use it from time to time in my electronics as well.

[David Hess]

That is a great idea.  Further, the parts are available to refurbish things like the carbon wipers.

[techie1234]

That is a great idea.  Further, the parts are available to refurbish things like the carbon wipers.

Hehe...I actually have the Lionel "Type V" transformer that my father got in the late 1940's on my bench right now to clean and replace the carbon rollers.  It's going through its twice-a-century maintenance...

[techie1234]

Here's an example 75W one - it's the one I have on my workbench and does have a circuit breaker:

http://www.ebay.com/itm/LIONEL-4150-BLUE-TOY-TRANSFORMER-75-VA-LOOKS-AND-WORKS-GREAT-/112080177122

[bitseeker]

Good idea. I've got a red Lionel one somewhere...

[SeanB]

I have an old 12V downlight toroid and a small variac in a box as an AC source.  Works well as an adjustable voltage source, giving up to 50VA of power. Variac is a 115VAC 400Hz one, so derating to 12VAC 50Hz is perfect, and it is rated at 5A as well.

[David Hess]

That is the way I do it.  I have several variacs and hook up an appropriate 120 volt power transformer as needed to make a variable AC power supply.

[retrolefty]

That is the way I do it.  I have several variacs and hook up an appropriate 120 volt power transformer as needed to make a variable AC power supply.

 And with the added advantage of (if desired) isolation from the input neutral/ground of the input source.

[Back2Volts]

Hehe...  It's going through its twice-a-century maintenance...

Whether it needs it or not!    Preventative maintenance is a best practice!   

[Back2Volts]

That is the way I do it.  I have several variacs and hook up an appropriate 120 volt power transformer as needed to make a variable AC power supply.

 And with the added advantage of (if desired) isolation from the input neutral/ground of the input source.

Is the transformer to provide isolation?   Are there variacs with separate primary/secondary windings ?

[techie1234]

That is the way I do it.  I have several variacs and hook up an appropriate 120 volt power transformer as needed to make a variable AC power supply.

And with the added advantage of (if desired) isolation from the input neutral/ground of the input source.

A classic Lionel "train transformer" is little more than a power transformer with a wiper/roller on the secondary to select voltage.  The output is isolated from the mains and they all have two-prong plugs.  While I wouldn't bet my life on their absolute isolation, we are talking about low voltages on the output side.  In fact, quite a few people with layouts that have mains current devices (e.g., overhead lights) in proximity to their track purposefully connect their output common to earth ground as a safety measure in case the track somehow accidently gets "lit up" by the mains.

[techie1234]

Hehe...  It's going through its twice-a-century maintenance...

Whether it needs it or not!    Preventative maintenance is a best practice!   

Technically, I guess it's in for its thrice-a-century maintenance.  My dad (a ham guy who should have known better...) let the original cord and plug get pretty dodgy.  When I was about 8 or 9, I got a 110V wakeup call that taught me respect for electricity at an early age.  A bit chagrined, he changed the cord that evening.

[David Hess]

That is the way I do it.  I have several variacs and hook up an appropriate 120 volt power transformer as needed to make a variable AC power supply.

 And with the added advantage of (if desired) isolation from the input neutral/ground of the input source.

Is the transformer to provide isolation?   Are there variacs with separate primary/secondary windings ?

Variacs with separate primary and secondary windings do exist but are both rare and expensive; most are autotransformers with one winding so for safety, an isolation transformer should be used and if you want a low voltage output anyway, then a standard power transformer can provide both and boost the output current.  The Lionel train transformers are one of the few examples of a variac with separate windings.

[bitseeker]

Most of the isolated variacs I've seen consist of a regular autotransformer (aka, variac) behind a regular isolation transformer -- nothing fancy, just more expensive when they're both in the same enclosure. So, if you already have a variac, just add an isolation transformer to it. (Then, put them both in a single enclosure to boost their value )