Does anyone make an all-in-one 120VAC-3.3VDC converter?

January 7, 2014 12:13 PM

I'm looking for a discrete device that combines a 10:1 transformer, bridge rectifier, caps and linear voltage regulator. It's to convert 120VAC to 3.3VDC in a wireless dimmer switch. Does such an animal exist or am I just being lazy?
posted by ostranenie to Technology (11 answers total)

I'm not sure what you mean by 'all-in-one', but this POS is only one part.
posted by Confess, Fletch at 12:23 PM on January 7, 2014 [1 favorite]

There are plenty such products, perhaps one in the linked search will meet your form factor requirements. I constrained this search to relatively low power devices, but there are many higher power options as well.
posted by Behemoth at 12:24 PM on January 7, 2014

Or perhaps this is closer to what you're looking for.
posted by Behemoth at 12:29 PM on January 7, 2014

Response by poster: Did I mention my requirements were extremely cheap and extremely small? ;-)

Right after submitting the question (naturally) I found this circuit, which is interesting - it uses a cap, some diodes and a zener to step down, rectify and regulate voltage. Any opinions on this regarding safety, effectiveness, and so on?
posted by ostranenie at 12:30 PM on January 7, 2014

Best answer: There's some good (albeit somewhat theoretical) information about transformerless power supplies here (PDF)
posted by pipeski at 1:39 PM on January 7, 2014 [1 favorite]

Response by poster: AN954 was exactly the kind of thing I was looking for. Thanks!
posted by ostranenie at 2:09 PM on January 7, 2014

I'm gonna step up and say that in this case if you don't know if what you want exists and you're asking here instead of an electroncs forum, you definitely shouldn't be playing with something as potentially dangerous as an unisolated supply where all components are floating at full mains potential.
posted by Pinback at 2:17 PM on January 7, 2014 [2 favorites]

I would agree with Pinback. If you don't know what you are doing, Application Note AN954 can get you killed. You would only use this type of supply in a completely self-contained, electrically isolated device. Anything connected to this power supply should be considered high voltage.
posted by JackFlash at 4:42 PM on January 7, 2014

You would be better off starting with something small and cheap like this Kindle USB charger. It provides safe, isolated 5 volts which you can easily regulate down to 3.3 volts with a single chip and a couple of capacitors.
posted by JackFlash at 4:47 PM on January 7, 2014

Response by poster: I'll make sure to run whatever I come up with by Actual Engineers(tm) before I plug it in. Thanks!
posted by ostranenie at 7:32 PM on January 7, 2014

You should probably do that while designing it, not after.

For a start, half what little safety there is in such designs - and all the protection against not destroying the LV side of your circuit by subjecting it to excessive voltage - comes from accounting for the load requirements.

You can't, for example, just say "OK, my load is primarily resistive & draws 10mA, so I'll choose component values to guarantee 3.3v @ 50mA", because the load type & current dramatically affects the regulation of simple reactive droppers like that - what works for one static load won't be suitable for another.

And if you've got a non-static load - for example you're switching an indicator LED on/off, or your micro is going in and out of sleep - you've just made it a lot harder…

(I can almost hear people going "But there's a zener there which prevents the voltage from exceeding V_zener!". Yeah, go and read about voltage regulation vs zener current…)
posted by Pinback at 8:22 PM on January 7, 2014