As a preface , this is somewhat motivated by the “new hotness” that is the LokStore Digital LoDi decoders (operator 4wd-ac) which allow you to feed AC switching power to the Märklin turnout motors , to supposedly solve the issue of the motors failing to switch after some time.

So the idea is rather than DC power to the coil driven motors, feed them with real AC analog power to move the coil core.

The usual comment is to use the existing railroad transformers such as the Märklin 6646, which has the yellow 17 v AC 32va output.

The thing is , if you don’t have one, those cost $115 each. You get that variable AC track power output with knob that will never be used in this use case.

So why not just buy a 16v AC door bell transformer (note: assuming you will build this into a box to keep the 120v side safe and blocked from contact ) that can be had for $24 for a 30va unit.

Those that have the Märklin units or know about their insides , does the yellow fixed AC output stage have any additional protection on it that I’d not get by using a door bell transformer with an appropriately sized fuse, and a proper plastic box around the line voltage stuff ?

I have never read that DC power would be the reason for Märklin turnout motor (microswitch) failure, only that the digital (square DC) power could be a source for that?

However, I’m feeding my LDT turnout decoders with 20 V pure DC from an ”old” laptop charger, by recommendation from Peter Littfinski, the LDT founder. Never had a problem, so far.

The 6647 quickly switches off in case of a short - much quicker than my older blue Märklin transformers. I think that is important when driving trains.
For turnouts, there will be no derailments causing shorts.
A door bell transformer, even with an extra fuse, is not a toy. But you're an adult and IMHO it will be safe for use by an adult.

Don't go to extremes. I read about a chap using a transformer for halogen lamps with his model rail road: 105 W or more more and switching off at 90°C or so. When he had a short, the thin 0.14mm² vanished in thin air instantly, so the thin cable worked as overload protection.
Staying at smaller sizes like 30 VA or using fuses for 2 A or 3 A makes sense IMHO.

An overheating turnout motor that also damages the turnout will cost some money, so better play it sufficiently safe.

Super. Thanks. That makes it easy enough to build one’s own out of a doorbell transformer and some extra electronics since I only would need the fixed vac output.

Now to find the modern equivalent of the older bimetallic temperature sensitive fuses.

ESB 707 3CH - opens at 105c

Can find some 100c ones easily enough.
The Sekicon ST22 on Amazon series looks promising.