Best way to efficiently power a ultra-low power circuit from mains? - Page 1
Forum section: electronics / beginners

author: Gecko
author_rank: Contributor
timestamp: March 28, 2014, 01:08:26 pm
content: At the moment I am working on some wireless sensor/actuator stuff. They run from battery great for a long time.But next step I would like to do is include the sensors into wall outlets and/or switches.What is the most power *efficient* and smallest footprint to achieve this?Isolation from mains is not necessary, but nice to have (As I've mentioned, the circuit will work in a wall outlet, fully isolated by the housing)Efficiency is really my main concern. The sensor board itself only needs a couple of microwatts in average, with some milliwatts peak (wireless transmission). So even wasting just 10mW constantly in the power supply would be a shame.Footprint has to be damn small too (think of the space in a wall outlet). So I'd say thumbnail size.Any ideas? My first ideas were some transformerless design with capacitive voltage divider and zener diodes. But that would already blow away some hundred milliwatts.

author: gxti
author_rank: Frequent Contributor
timestamp: March 28, 2014, 01:40:00 pm
content: Why are you so concerned about the efficiency of a milliwatt-scale load on the mains? In any case, you are not going to waste a "hundred milliwatts" if you size your supply to match your load. The zener only burns off the unused current, so if you keep the unused current to a minimum then loss will be low.If you are obsessed with efficiency you can get a buck converter designed for mains use, or there are even some specialty parts that get power by trimming the "skirt" of the waveform, switching on at the zero crossing and off when the mains voltage rises above the output voltage.

author: Gecko
author_rank: Contributor
timestamp: March 28, 2014, 02:59:04 pm
content: Quote from: gxti on March 28, 2014, 01:40:00 pmBecause one unit wasting some hundred milliwatts is not much, but hundreds, or thousands or millions units, each wasting this amount adds up and keeps a power plant running.Quote from: gxti on March 28, 2014, 01:40:00 pmThe sensormodule consumes nearly no power all the time. So the zener has to burn the current nearly all the time.Quote from: gxti on March 28, 2014, 01:40:00 pmI also thought about a buck converter. But I could not find any which run directly of 240Vrms for such a low load (typically they are built for some hundred of milliamps up to some amps). Efficiency for such a low load (nearly no-load) would be very bad. I also looked at other solutions which use a buck with transformer. But then you have the magnetization and core losses all the time.The "skirt trimming" solution sounds interesting. Do you mean a thyristor/triac, or are there actually some integrated regulators which do that? Do you have some name for these types of regulators or maybe an example part number which I can google for?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2014, 03:45:16 pm
content: A series cap dropping circuit draws at least full load current, but most of that is reactive; real power is still real power. Not really any good way to drop the idle power unless you want to make some weird voltage mode capacitive buck or something. Which actually needn't be too bad, you could probably make a hysteretic phase controlled looking thing that does well enough.A blocking oscillator might be suitable. Example:http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/RegBO.pngStartup/bias comes from a 1M resistor and the transistor kicks periodically; every flyback pulse charges the 0.1 at the bottom, which can be used to charge the 0.01 faster via the opto. Faster charge = faster rep rate. So, idle current is extremely low, it only draws what it needs. Note that it won't start up into a short circuit, or maybe even into the rated resistive load.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 28, 2014, 04:03:08 pm
content: Check out Power Integrations' Linkswitch devices (LNK30x I think)

author: gxti
author_rank: Frequent Contributor
timestamp: March 28, 2014, 05:02:54 pm
content: Quote from: Gecko on March 28, 2014, 02:59:04 pmYou don't size it for the peak current, you rely on the fact that the average current is extremely low. For example you could drop the mains to 10 volts, store it in a capacitor, and use a linear regulator or a low-cost buck converter to deliver the target voltage as the capacitor drains. Now your current is averaged over a long time period and you can use an even smaller dropper capacitor.QuoteThere are plenty of non-isolated mains power supply chips like the PI ones mike mentions. They're essentially just high-voltage buck converters.QuoteUnfortunately I don't have a part number and I dug through my post history but couldn't find it. I don't know if it it would even be more efficient. But think a standard non-isolated converter is probably your best bet. Either way I would start by looking at Digi-Key's AC-DC converter IC section, and look at chips with low output current.

author: Gecko
author_rank: Contributor
timestamp: March 28, 2014, 09:42:00 pm
content: Okay, thanks everyone for the help!I just ordered some LNK302 samples. Seems to be the solution which fulfills the requirements "small footprint" and "not super bad efficiency" the best.The LNK302 will drop the voltage down to something in the range of 10 to 30 V. A linear regulator will then drop this down to the MCU supply voltage (~3V).P.S.: @gxti: I think I found the thread you mentionedhttps://www.eevblog.com/forum/suggestions/capacitive-dropper-psu%27s/

author: miceuz
author_rank: Frequent Contributor
timestamp: March 29, 2014, 12:57:08 am
content: I've seen this project on hackaday last week, noticed a weird chip connected directly to mains.http://lowpowerlab.com/switchmote/#schematicTurns out it's SR086 "Inductorless Switching Off-linePower Supply"http://www.supertex.com/pdf/misc/SR086DB1.pdfMight be relevant..

author: LukeW
author_rank: Frequent Contributor
timestamp: March 29, 2014, 11:43:24 am
content: Do you need galvanic isolation from the mains?If so then you'll obviously need a conventional flyback SMPS with a transformer, and you can forget about transformerless options, either cap-dropper style or off-line buck converter type chips.

author: Bobson
author_rank: Regular Contributor
timestamp: March 29, 2014, 03:38:47 pm
content: I don't know the voltage you require, but you can try inductive coupling with live wire. My quick test provided me with 1mv per cm of length at 1 cm distance at 220V live wire with NO LOAD. Voltage would be higher at closer distance, and you'll get full galvanic isolation as a free bonus.Sure, if there would be any load connected to the AC outlet, voltage would become much higher, so voltage conditioning is a must.Some facts on inductive coupling:http://www.smar.com/en/technicalarticles/article.asp?id=18