AC-DC wall wart with no filtering!! - Page 1
Forum section: electronics / projects

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 06, 2022, 07:12:00 pm
content: I used this wall-wart as a replacement for another on something I designed years ago. Things didn't quite work right, so I connected my scope and the the output waveform looked terrible, like it had no filtering at all. I assumed the capacitor in the wall-wart was completely gone. So I opened it to fix it....and no capacitor! WTF?I can hardly believe it. I always thought "DC" meant smoothed DC. It makes sense because this is an old cellular phone charger and wouldn't need filtering, but this gives me a new perspective on these things: do not assume they aren't unfiltered DC.Another good reason to start designing gadgets with their own rectification/filtering and use AC output wall-warts. Not much to go wrong with those, and nothing much to cheap out on.

author: Monkeh
author_rank: Super Contributor
timestamp: November 06, 2022, 07:20:14 pm
content: Or just use a modern switching supply instead of that ancient brick - it belongs in the recycling box, and has done for about 15 years.

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2022, 07:28:05 pm
content: It's low current and probably from the times of Ni-Mh batteries. It could be on purpose, as a sort of way to reduce average current on batteries as they charge and keep the cooler.Nothing stopping you from adding a small capacitor there to make the output cleaner. There's footprints on the pcb.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 06, 2022, 07:32:24 pm
content: Quote from: Monkeh on November 06, 2022, 07:20:14 pmWhat is the advantage of that when I need about 20mA and already have this one? It's not like switchers don't have their problems, especially cheap commodity ones.It's beside the point anyway. I was only pointing out this peculiarity. Using a linear vs SMPS is an entirely different discussion and they each have their advantages.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 06, 2022, 07:33:19 pm
content: Quote from: mariush on November 06, 2022, 07:28:05 pmThat's exactly what I'm going to do since it's already open

author: Monkeh
author_rank: Super Contributor
timestamp: November 06, 2022, 10:01:40 pm
content: Quote from: jrmymllr on November 06, 2022, 07:32:24 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2022, 10:05:24 pm
content: 2.3 W is already quite low for a classical transformer. So it may be worth keeping that one. Many are worse.

author: Monkeh
author_rank: Super Contributor
timestamp: November 06, 2022, 10:08:54 pm
content: Quote from: Kleinstein on November 06, 2022, 10:05:24 pmIt's only a tiny wee one. A 15VA comparable to the switcher is about 3.2-3.5W.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 06, 2022, 10:18:51 pm
content: 2.3W is about 1.7kWh, thats $0.10/month over here... btw that sort of transformer that i love to repurpose, one or two of its bigger siblings i will put in my "Lab Bench" Split Rail Tracking PSU... cheers.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 06, 2022, 10:28:13 pm
content: Quote from: Monkeh on November 06, 2022, 10:01:40 pm2.3W all year would cost me $2.40/year if I did the calculation correctly. And the transformer I'm using is quite a bit smaller. I'll take the lower noise and higher reliability of a low power linear for that cost.

author: Monkeh
author_rank: Super Contributor
timestamp: November 06, 2022, 10:30:40 pm
content: And thus, new power plants get built..I've had an order of magnitude more linear warts with failed capacitors than small switchers fail for any reason. Something to do with one type sitting there roasting all day and the other barely exceeding room temperature..

author: Whales
author_rank: Super Contributor
timestamp: November 07, 2022, 03:56:26 am
content: I've had back luck with some little 12V SMPS. They too have "missing parts" far too often.WES Wagner sells some 12V 1A units that output massive 1MHz spikes to both the output (~1V IIRC) and the input (causing you to see the same interference on the power rails of other power supplies plugged into the same power strip!). In my situation they stopped a Raspberry Pi Zero W from being able to initialise it's HDMI output (but surprisingly everything else still worked) even though the pi was after a second 12V->5V buck.(I tried telling them they were bad in general and probably don't meet Australian standards, but they held their "sorry it doesn't meet your intended use case" line)For some giggles I decided to buy some 12V supplies off Aliexpress recently. Both types were missing any form of input filtering, one unit weighed less than its attached cable. I'm going to try buying some more expensive units now to see if it makes a difference or if they're the same. I did appreciate the fact the plastic cases were only snap-shut instead of welded or gluedEDIT: To answer the OP: yes I've seen these no-capacitor no-regulator transformers+diode supplies before. Plugging them into PC speakers yields particularly fun results. I had to go through my collection of non-SMPS 12V supplies a while back because I blew up one pair of 90's computer speakers (for a retro project) after trying to use a SMPS unit. The speakers expected an isolated power supply, my ground-referenced 12V SMPS shorted them to the TRS signal ground of my computer.

author: jrmymllr
author_rank: Regular Contributor
timestamp: November 07, 2022, 01:26:29 pm
content: Quote from: Monkeh on November 06, 2022, 10:30:40 pmThis wasn't the intent of my original post. There are clearly pros and cons to both types. But a one-for-one replacement drawing 1.5VA under load has to rank near the bottom of any household power consumption. I even put some things on power strips to cut the standby power, but this device has to stay powered

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: November 08, 2022, 05:37:16 am
content: Quote from: Monkeh on November 06, 2022, 10:30:40 pmI reckon that's exactly why that wall wart farmed off the filter-cap requirements to the device being powered

author: rdl
author_rank: Super Contributor
timestamp: November 08, 2022, 09:43:08 am
content: It's not as common these days. but in the past devices often had very specific requirements regarding their AC adapters. Some needed only transformers, because they converted the incoming AC to dual polarity DC.

author: thm_w
author_rank: Super Contributor
timestamp: November 09, 2022, 12:46:30 am
content: Quote from: jrmymllr on November 06, 2022, 07:12:00 pmPlease no.Either use 5V DC with a USB connector, or some other higher voltage if absolutely necessary (12V, 24V DC) with a 5.5mm jack.Quote from: rdl on November 08, 2022, 09:43:08 amYes but this is rectified and specifically says 5VDC