AC Output Power Supply

[PsychMan]

This may be a silly question (I sense it is), but bear with me.

If I wish to construct a power supply that simply gives out 18V AC, do I need any other component other than a suitable mains transformer?

I've seen many designs for DC power supplies, switch mode, transformer based with full wave rectifiers, chokes etc. But haven't seen one for AC, probably because it would just have a transformer ?

I need an 18V AC source and thought it best to ask before I lash something up with just a transformer

[Biggles]

I think you are right. Just a transformer of the appropriate rating and a fuse for protection. Possibly an indicator to show that it's on, and an on off switch if required.
Alan.

[PsychMan]

Ah yes thanks Alan, a neon across the mains would be a nice touch, and a small fuse holder with a glass fuse, thanks

[Ed_Dinning]

Or you could really go over the top and add a voltmeter (the supply will droop with load) and an ammeter.
Possibly even a low resistance var resistor to fine tune the output current


Ed

[Panrock]

This is the sort of requirement familiar for those who want to say, power the SSTran AMT3000 pantry transmitter.

When specifying a transformer, be aware of how dramatically the output voltage can vary between maximum rated load and open-circuit. I believe a change of 25% is typical for small transformers (though Ed will be more knowledgeable here).

For my AMT3000, I ended up by using a 15v AC transformer in an empty wall-wart case. I couldn't easily buy the correct wall-wart. The full 18v was unnecessary and would just make the regulator chip run hotter.

Steve

[PsychMan]

It is indeed for an AMT3000.

SSTran currently advise the following, but I believe their US kit ships with an 18V AC adaptor so I was trying to match that:

"15 - 20 VAC. DC output voltage should be rated between 20 - 24 VDC"

The SSTRANs have a good (should be copied) power input system, a bridge rectifier, a smoothing cap and a 15V regulator. So anything above (not too far above) 18V DC or peak AC will work. I have just measured the input to mine, 20V DC and the regulator is hardly warm. The whole thing is a good design, no penny pinching and no extravagance, it just works very well indeed. I knocked mine up in my lunch hour, worked first time and was a joy to build.

[G8HQP Dave]

A "glass fuse" might be OK in the secondary circuit, but in the primary circuit you might want something which won't arc.

[Ed_Dinning]

Hi Folks, a transformer with the windings side by side on the core will generally have a poorer regulation (voltage droop with load) than one that has the 2 windings on top of each other. This latter transformer does require more internal insulation to meet safety standards. The side by side unit has a built in safety barrier and is cheaper to produce. It is very often used in wall warts.

Ed

[QQVO6/40]

Thank you Ed for that little tidbit of information about the side by side construction having poorer regulation than the more conventional construction.
This forum never disappoints me as every time I come here I learn something.
Thank you again Ed.

Cheers,
Robert.

[bikerhifinut]

Originally Posted by Ed_Dinning

Hi Folks, a transformer with the windings side by side on the core will generally have a poorer regulation (voltage droop with load) than one that has the 2 windings on top of each other. This latter transformer does require more internal insulation to meet safety standards. The side by side unit has a built in safety barrier and is cheaper to produce. It is very often used in wall warts.

And in some commercial transformers marketed for the enthusiast DIY scene.
You've seen the ones I am thinking of Ed.

Andy.