Where to get tiny 120VAC to 7+VAC tiny transformers for 5VDC PS?

Thread Starter

Holes Flow

Joined Oct 26, 2017
11
Experts,

All I need to do it provide an internal 5VDC to a project, and take AC in, fuse it, but internal as compared to an external PS.

Man was I surprised at how scarce cheap ones are! After all, $5 wall-warts come with them, so the xformer should be ~$3 neighborhood, or so I guessed. All I found on Google, Amazon, eBay, Mouser, Digikey, etc. were either north of $5 or huge! I'm only powering a uC and relay, and at most it pulls 190mA. I figured rolling my own PS was least costly.

I must have made a mistake, and will be happily humbled if someone has an idea which MFG can source these, or if I need to get a ton via slow boat...

Thanks!
 

Papabravo

Joined Feb 24, 2006
16,107
Experts,

All I need to do it provide an internal 5VDC to a project, and take AC in, fuse it, but internal as compared to an external PS.

Man was I surprised at how scarce cheap ones are! After all, $5 wall-warts come with them, so the xformer should be ~$3 neighborhood, or so I guessed. All I found on Google, Amazon, eBay, Mouser, Digikey, etc. were either north of $5 or huge! I'm only powering a uC and relay, and at most it pulls 190mA. I figured rolling my own PS was least costly.

I must have made a mistake, and will be happily humbled if someone has an idea which MFG can source these, or if I need to get a ton via slow boat...

Thanks!
The reason they are not available, and probably never were is because there is no market for them. As small a power transformer as I have ever seen was a 6.3 VAC filament transformer, good for an Ampere or so, in a vacuum tube TV set. It's probably been 50 years since they quit making vacuum tube televisions. They do things differently nowadays, with offline DC-DC converters going through a much smaller transformer at several hundred kilohertz. Buy some cheap phone chargers and do a teardown.
 

Papabravo

Joined Feb 24, 2006
16,107
Yup. That's what I was thinking of. How much do they weigh and what would shipping and taxes be over the $7.52 purchase price? I'm thinking the TS may find them unpalatable.
 

Ian0

Joined Aug 7, 2020
2,192
You can get 50Hz transformers down to 0.35VA.
https://www.block.eu/en_EN/productversion/vb-03516/
These are 230V primary, so I presume that somebody makes 120V versions.I wouldn't advise using them - they are designed to run in saturation and rely on the resistance of the primary winding to limit the current. The output waveform is horribly distorted, and they run really warm.

If you search "AC to DC converter" you'll find something like this
https://www.rapidonline.com/vigortronix-vtx-214-0005-105-0-5w-smps-ac-dc-converter-5v-88-0038
which will do the job for you.
 

MaxHeadRoom

Joined Jul 18, 2013
23,092
The early Wall-Warts were mains transformers, now SMPS, you may find a few of the former on the second hand stores or on-line markets, if lucky.
Also look for early scrapped electronics, you can often salvage the small power trans. from them.
I have collected a box of them over the years.
 
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