9V AC wall wart - Page 1
Forum section: electronics / beginners

author: azrimola
author_rank: Contributor
timestamp: June 16, 2012, 11:38:02 am
content: Why would a consumer product need a 9V AC (alternating current) 1A power supply and not a DC one?The product in question is a Cable modem/router/hub combo.

author: Psi
author_rank: Super Contributor
timestamp: June 16, 2012, 11:56:29 am
content: Hm.. only possible reasons i can see are..Maybe they could get AC wall warts (and put a bridge rectifier+cap into the product) for cheaper than just buying DC wall warts.If they put a rectifier in the product it frees them up to supply either AC or DC packs depending on which is cheaper at the time.Or it might use the 50/60 cycles for syncing the time or to actively filter out mains hum from the line.Or it might have a diode/cap voltage multiplier inside to generate Power Over Ethernet voltage.I could think of a few more possible reasons, but they start to get crazyMost likely it's just a cost saving thing.

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2012, 12:02:09 pm
content: It is generating a set of +-15V rails internally to power the internal analogue interfaces, and uses a switch mode power supply to generate the 3.3v needed for the digital side from the 12V that it gets from full wave rectification of the input voltage. Cheaper than having a second switcher to provide a set of 18V rails via a transformer, you still need a bridge rectifier and capacitor in the wall wart instead of the box. 4 capacitors and 4 diodes are much cheaper.

author: rr100
author_rank: Frequent Contributor
timestamp: June 16, 2012, 12:09:31 pm
content: I have quite a few older small switches that work on AC 7-9V input, probably precisely for mentioned differential voltages needed.

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2012, 12:17:06 pm
content: Most ADSL modems, as well as almost all dial up modems, use an AC supply, as they need a power rail at low current with low noise for the interface to the phone line. This is generally +-12 or 15V, and is needed to drive the transformers that do the coupling to the phone side.The ones that run from USB have to have a small inverter to provide this supply, and those that are internal cards get it from the motherboard power supply rails.

author: azrimola
author_rank: Contributor
timestamp: June 16, 2012, 12:19:36 pm
content: Great answersI did not mention that it uses a "standard" 5.5 mm plug.So, by now you can guess that I have blown some unsuspecting device with it.Surprisingly the 3.3V regulator(AMS1117) survived and was happy to give out 3.3V DCThe 5V one died in a few seconds and was happy to give out 0V

author: Psi
author_rank: Super Contributor
timestamp: June 16, 2012, 12:23:37 pm
content: Yeah, AC adapters with general plugs and DC adapters with center negative are really annoying.

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2012, 12:26:35 pm
content: The sad thing is that the output voltage and current along with the fact it is AC is always printed indelibly on the case................

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2012, 12:31:21 pm
content: Those things used to be pretty common also in telephone answering machines for the same reasons. It lets you use a cheap 2 contact barrel connector and generate split supplies without mucking about with switch mode supplies or filtering the resulting noise. Back in the day, the most common DC wall warts were also simple line transformers with the rectifier and filter caps built in. They were not regulated since a linear regulator would generate too much heat.

author: azrimola
author_rank: Contributor
timestamp: June 16, 2012, 12:37:58 pm
content: It clearly says "AC Adaptor" which I read as "AC Input".I got the hint when I saw the 9V ~ 1A part. I love how they branded "OEM".(Added the modem/router guts.)

author: T4P
author_rank: Super Contributor
timestamp: June 16, 2012, 06:04:55 pm
content: Yep, it's clear by now.They either use AC plug packs or Switch Mode DC suppliesThose 1N400x's make it clear

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2012, 05:43:03 am
content: In this case it looks like they needed to reduce radiated RFI via the power leads, so supplying the 9VAC to the device saves money, as the bridge rectifier noise is suppressed as well, and the reservoir capacitor serves 2 purposes, as both the DC store and the SMPS supply. If it had a separate power supply giving 12VDC they would still need the components in the case, but here they save on a capacitor, diodes and the need for RFI suppressors in the power brick as well.