Selection Guidelines - Convert DC Wall Wart to AC Wall Wart

[Vons Garage]

It seems I have no shortage of DC power adaptors - at all! Occasionally I need an AC version; for example, clocks requiring 60Hz for time base.

Given a needed AC voltage, which DC power adaptor voltage rating would you experts select to break open, pull out all the DC conversion components, retain just the transformer, and then glue the whole thing back together again?

Should you try to match the desired final AC voltage exactly? Or select one that has a higher or lower voltage than the target?

And finally, your opinions: what do you think these things should be officially called: external power supplies, power adapters, AC adapters, or power bricks? Has anyone come up with a more elegant name than “wall wart” that clearly identifies these type that actually have the guts at the point-of-connection to the mains?

 

[sagor1]

To calculate the AC voltage for a wall wart, measure the DC voltage with no load and divide by 1.414. That will give the no load AC voltage. Under the rated load (same load as DC rating) the AC voltage may drop a bit, but not as much as it does at DC mode.
Find a power device that gives the required AC voltage, plus or minus a volt or so. Most devices requiring AC rectify it to DC internally anyway, except devices that need 60Hz for things like motors.

 

[Nigel Goodwin]

You need to know what the requirements of the clock you're trying to feed are - although I've never seen (or heard of) a wall wart AC powered clock?.

Bear in mind as well, most wall warts are switch mode, you would need to find one that uses a conventional mains transformer, and quite probably one that's unregulated as well (most regulated ones are switch-mode).

 

[Vons Garage]

Sagor1: Noted, printed out, and added to my piles and piles of DC supplies for future reference. Cool.

Mr. Goodwin: per manual: Power consumption: About 5.7 watts, (0.6 amps @ 9.5 volts AC)
Try this link: https://transistorclock.com/
This cool kit, made by KABtronics, is no longer available. What a build it was back when I got it!
Someone has cloned it's design. Check eBay for Mega Transistor Clock.
And I will bear in mind about the different varieties of supplies. Probably by hefting them!

 

[Nigel Goodwin]

Well it's no MegaProcessor https://www.megaprocessor.com/ but it's pretty impressive, and a LONG build

Thinking on, you could program the MegaProcessor to be a similar clock

 

[For The Popcorn]

A switch mode supply won't work for this. There is no line frequency transformer.

You need an old-school linear supply. These will be heavy for their size, and roughly "regular" cube shaped – roughly equal size in all 3 dimensions, not thin and flat.

 

[sagor1]

I would suspect any DC wall wart that gives 12.5 to 14.5VDC with no load will do. Getting 0.6A will require a heavier transformer unit, always verify the current capacity.
The picture you show, shows a bridge rectifier (4 diodes) in the lower left corner, so most of the clock would run off about +12V. The AC voltage is just to get the 60Hz for the clock sync. There don't seem to be any voltage regulators or IC chips, just transistors, so it just runs off the approx. +12VDC

 

[For The Popcorn]

I would suspect any DC wall wart that gives 12.5 to 14.5VDC with no load will do.

No, I guarantee you a switch mode power supply won't work for this.

 

[ChrisP58]

Another option would be to use a 12 Volt SMPS wallwart, and add a 32KHz watch crystal driven time base.