Capacitive Dropper PSU's  - Page 1
Forum section: electronics / general

author: kolbep
author_rank: Frequent Contributor
timestamp: December 21, 2011, 09:34:47 pm
content: Hi Dave.Loved your blogs about the LAB powersupply, but I have another question, :What about doing a blog on Capacitive Dropper Powersupplies.I have looked at various info, from the EPE magazine years ago, as well as various sites on the net, but just cannot get the hang of it.I do use the Correct X Rating Caps (which I strip from things such as mains controlled daynight switches, ceiling fan controllers, etc)Take for example my last attempt, which was an NTC Thermister Controlling an OPAMP to drive a relay that feeds the 230v element on a bottle warmer. There is not much room in the bottle warmer, so I decided cap dropper PSU would be perfect.But when I switched on, I blew one of the 1w resistors (that was part of my attempted Cap PSU), as well as sending the top of the opamp flying across the room, and destroying my LED that was on the output of the OPAMP.So in short, please could you consider this useful Blog / Tutorial

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 21, 2011, 10:28:14 pm
content: This Microchip Appnote covers it pretty well.http://ww1.microchip.com/downloads/en/appnotes/00954A.pdfIf you need more power than is practical with a capacitive dropper, or don't have room for a big dropper cap, Power Integratiions do some nice chips for nonisolated PSUs :http://www.powerint.com/products/linkswitch-family/linkswitch-tn

author: Unknown
author_rank: Unknown
timestamp: December 21, 2011, 11:59:17 pm
content: One complication of doing a tutorial about this is that it's much harder to work on these power supplies safely because of the lack of galvanic separation from mains. Your whole circuit should be considered as dangerous mains potential. If someone like Dave would do a tutorial on this subject, it might prompt people not experienced enough to deal with mains voltage to try it.

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2011, 12:40:58 am
content: Yeah, i think Dave wanted to stay away from blogs involving mains voltages.Maybe post the circuit you tried that blew up, we can help you to fix itThere are a few different types of these capacitor powersupplies, some are more easy to blow up than others.I've used something like this before without any issues.Ya just have to be sure its all encased in something plastic so no one can touch it.From memory, there is always 24V available but with respect to earth it moves up/down with 240VAC mains.So at some point the 1000uf cap may have 100V on one side and 124V on the other for example..

author: ejeffrey
author_rank: Super Contributor
timestamp: December 22, 2011, 01:05:12 am
content: From what I remember Dave considers 12V batteries to be HVYet another option for non-isolated supplies is the fairchild FSAR001. It basically carves pulses out of the low voltage skirt of the rectified AC waveform to give you a 5V regulated supply.Anyway, yeah: you can kill yourself with this stuff, so be careful. It has its uses, particular circuits that need to be connected to the mains anyway like TRIAC controllers and line voltage monitors, but just watch out and don't use them if you aren't sure of what you are doing.

author: Zad
author_rank: Super Contributor
timestamp: December 22, 2011, 02:23:10 am
content: The important point to remember is this: The impedance presented by the capacitor is inversely proportional to dv/dt. Now, with a nice constant sinusoidal waveform, this is all fine.Now imagine what happens when you switch the device on when the mains just happens to be at the top of it's cycle (or, for that matter anywhere but zero). Suddenly we have a huge voltage delta, presented over a tiny amount of time. Result: the entire mains voltage is present at the far side of the capacitor. The circuit which is expecting 5V suddenly gets >100V+, with predictable consequences.Capacitors are notably good at transmitting high voltage dv/dt spikes from thermostats, motors etc. MOVs can of course help to a lesser extent, but you are asking them to snub the over-volts on the entire phase, so there is only so much they can do, and they will only react to voltages greater than mains peak. They will have no effect on turn-on dv/dt, or 100V spikes happening at lower voltage parts of the phase.

author: gxti
author_rank: Frequent Contributor
timestamp: December 22, 2011, 04:00:27 am
content: What's a reasonable upper limit on how much current one can get from a dropper supply on 120VAC? I'm currently working on a device that's already attached to the mains to measure the voltage so this would be perfect, it just needs to run a dsPIC30 running full tilt and two op-amps. The competing solution is to buy a plug pack from digikey or ebay, smash the case open, and glue the board inside the project box. It has tremendous appeal to my lazy side, but I love making things more complicated than they need to be.

author: kolbep
author_rank: Frequent Contributor
timestamp: December 22, 2011, 06:17:25 am
content: Thanks all for the replies.I will check the appnote.I am aware of the safety aspects (I am an Electrician, so safety is my no1 priority), so no problem there.Dunno where I got the circuit I tried, but it was :--------------------L ----------- | | ---------------------| || | | BRIDGE |----------------------- CCT +-/\/\/\/\/\- | RECTIFIER | || | ---| | ---N --------/\/\/\/\/\-------------------| | |------------------------------------------- CCT -ThanksPeter

author: westfw
author_rank: Super Contributor
timestamp: December 22, 2011, 07:27:36 am
content: I've seen (I think) a fair number of supplies where an RC dropper circuit is used before the primary of a small transformer (which then gets to be a lot smaller than it would have been otherwise.)Does anyone have any details on how these are design/built?

author: BravoV
author_rank: Super Contributor
timestamp: December 22, 2011, 08:00:14 am
content: Quote from: gxti on December 22, 2011, 04:00:27 amSorry, don't mean to thread jack, just want to comment about the plug pack, I believe you could go through cheaper route rather than buying them online, use a phone charger, they're pretty decent and quite small without the case.Heck, I believe almost every household nowdays must have those old unused cellphones and their charger scattered around.Just make sure you use the original charger, not those cheap made in china substitute/knock off.

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2011, 08:54:43 am
content: Quote from: kolbep on December 22, 2011, 06:17:25 amYou need a zener in there to limit the voltage (unless there's already two zeners in the bridge like my circuit a few posts ago).The output voltage you'd get above currently is V= I_limit * R_load, so with no load at all (R=inf) the output voltage isn't limited.Having a zener effectively provides a continuous load that draws all the current available above it's zenering voltage.Simply put.. Using a 50mA current limiting capacitor 50mA flows through the zener @ noload, if you use 20mA for an led then 30mA is now flowing through the zener.You can either replace two of the bridge diodes with zeners (like my circuit above) or use one zener across the output.Its easier to read and understand with a single zener across the output but it uses more parts than just replacing two bridge diodes with zeners.It's quite easy to read a circuit diagram and miss seeing two zeners in the bridge because you've seen bridge rectifiers so often before your eye skips over them and assumes its a normal bridge.

author: kolbep
author_rank: Frequent Contributor
timestamp: December 22, 2011, 09:05:15 am
content: Thanks,Gives me some things to digest.I do really want to get this bottle warmer right before the little one grows up (only 5 days old today)P

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 22, 2011, 10:23:15 am
content: Quote from: gxti on December 22, 2011, 04:00:27 amThe limiting factor is generally the size and cost of the capacitor. Low tens of milliamps is a typical limit. Bear in mind it is a current, not power limit, so if you can efficiently use higher input voltages, either directly or with a buck regulator you can get more power for a given cap size.For devices containing relays (timeswitches, thermostats etc.) , it is not uncommon to see 24 or 48V relay coils for this reason.Don't forget that 5V phone chargers are pretty small these days, so for a 1-off, the insides of one of these may be an option, and be isolated.

author: peter_mcc
author_rank: Contributor
timestamp: December 22, 2011, 10:54:28 am
content: Quote from: gxti on December 22, 2011, 04:00:27 amWatch out if you do this - you'll need an isolated debug/program adaptor. If you use a standard one then the "ground" on your dsPIC will be at a different potential to the debug adaptor/PC and it will blow up something (or kill you!)peter

author: peter_mcc
author_rank: Contributor
timestamp: December 22, 2011, 10:37:22 pm
content: Quote from: ejeffrey on December 22, 2011, 01:05:12 amThat's perfect for something I'm doing - thanks for sharing it! Yes, it's in a proper box, etc, etc and will pass the appropriate electrical safety tests.peter

author: gxti
author_rank: Frequent Contributor
timestamp: December 22, 2011, 11:56:05 pm
content: Now that I think about it, the non-isolated switcher ICs look much more appealing than a cap dropper for anything more complicated than a Christmas light timer :-) Higher current, great regulation, more features, etc... not much reason to even look at the cap dropper except when mass-producing a simple device. And they don't need a transformer so I can just use off-the-shelf SMD inductors... I'm starting to sound like I'm marketing for these guys.Quote from: peter_mcc on December 22, 2011, 10:54:28 amYeah, I'm going to flash a serial bootloader on there and then talk to it through an isolated serial line, complete with reset circuit. That line will also be used for communicating with the application when it's up and running.

author: Psi
author_rank: Super Contributor
timestamp: December 23, 2011, 03:54:26 am
content: Not quite as much current though, at least not with that 35mA fairchild FSAR001 one.You can get 50mA pretty easy with a capacitive dropper and up to 100mA without too much trouble. It starts to get a bit hot/bulky after that though.However there's probably other ICs which can do more than 35mA.I wonder if you can parallel them up.Though, 35mA is enough for low power micro controller projects and a few leds.

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2011, 10:04:52 pm
content: The maximum current for a capacitive power supply is the point at which a small transformer becomes a more compact and cost effective than a large capacitor. For example, a 5V 160mA capacitive power supply run off 230V 50Hz would need a 2.2uF capacitor but it's probably more cost effective to use a little 7V 1.2VA transformer.

author: peter_mcc
author_rank: Contributor
timestamp: January 05, 2012, 07:54:04 am
content: Quote from: Psi on December 23, 2011, 03:54:26 amDoes anyone know of anything equivalent to the FSAR001B that will support 100mA or more? I want to drive a micro plus a relay and can't do it on 50mA... so am currently looking to change the relay to a TRIAC instead.peter

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2012, 08:06:09 am
content: Quote from: peter_mcc on January 05, 2012, 07:54:04 amOr you could try and find a version with a higher output than 5V. 5V relays need quite a bit of current.A 24V relay will need much less current, but you will need a 7805 to get your 5V from the 24VIf you go the "triac" way, have a look at opto-triacs.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 05, 2012, 09:16:54 am
content: Quote from: peter_mcc on January 05, 2012, 07:54:04 amhttp://www.powerint.com/products/linkswitch-family/linkswitch-tn