mains dropping capacitor for 6A - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 07:08:41 am
content: OK don't shout at me I only need to drop 16V although my calculation puts that at 1200uF. What voltage capacitor do I need ?

author: MF-jockey
author_rank: Regular Contributor
timestamp: July 13, 2015, 07:24:15 am
content: Nobody can give you a serious answer to such a poor question.More information about your application is needed!

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 07:35:38 am
content: I need to power 8 13V fans off the mains, this means a 600W power supply, or put them in series, put a diode in series and I get 120V pulsing at 50Hz, I need to be at 104V so need to drop a further 16V but this is with 5-6A going through

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 07:41:50 am
content: for me it works out around 1000uF I actually only need 5A so 16/5 = 3.2RThe capacitance for this is: 1/(2 x pi x 50 x3.2) = just under 1000uF so if I had a 50V 1000uF it would act as a lossless 3.2R resistor to take off my excess 16V. Or course I need a zener across it to make sure it does not go over voltage when the fans start up (it should see on average 32V).

author: bktemp
author_rank: Super Contributor
timestamp: July 13, 2015, 07:45:07 am
content: Why do you only use one half cycle? The fans will probably not like that. The next problem is, a capacitor won't work, because by adding a diode you have no longer ac but dc .Give us more information, there is probably a much easier and less dangerous solution. Maybe a simple transformer will be enough.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 07:50:47 am
content: Oh yes a simple chuncky 600W transformer will be fine, but they want compact. A toroidal may well do it if it's small enough. I have a 200VA transformer at home that looks beefy enough without considering something 3x the power.If I use one diode I will have pulsating DC, i suppose I need something to discharge it again. The other option is literally 80W of resistor, it will be force cooled anyway.I can't see DC fans having a problem with pulsating DC.Yes the one big drawback is that every fan is live. I'm yet to find out much detail about the installation so I'm just brainstorming at the moment.

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2015, 08:00:43 am
content: Psssssst-BANG is the most likely result.Non-polarised caps with a 6A ripple current rating are likely to be bulky and expensive.Connecting multiple DC fans in series without a bulk decoupling cap + clamping Zener across each one is *NOT* a good idea. During commutation their instantaneous load current is likely to vary considerably, and it will be very easy to exceed the max Vdd spec of the BLDC controller chips inside the other fan motors. You can usually get away with it for a while with only two fans, but more is asking for trouble.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 08:02:46 am
content: these are brushed fans, if we went to the expense of BLDC fans they would be straight 240V AC which I am also looking into.

author: bktemp
author_rank: Super Contributor
timestamp: July 13, 2015, 08:05:13 am
content: Quote from: Simon on July 13, 2015, 07:50:47 amDid you search for a AC rated 1200uF cap? It won't be much smaller than a 600W SMPS...You do not need a 600W transformer: It must only provide 16V at 6A = 100VA. Wire it in series with the mains voltage to remove 16V.QuoteThey will vibrate at 50Hz. It will make acoustic noises and the vibrations might damage the bearings.Why don't you use a bridge rectifier?

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 08:10:57 am
content: the idea of the single diode was to remove half the voltage and get down to 120V easily.I've not heard of using a transformer before, will it still take 16V out if it is unloaded ? that sort of implies that transformers use their rated current wether they are used or not or are you suggesting that it will just act as an inductor in which case that is what i need. If I use a transformer and bridge rectify i now have 136V and 680W to remove, not so small anymore.The next best thing is bridge rectify and PWM at say 1KHz at 43%

author: bktemp
author_rank: Super Contributor
timestamp: July 13, 2015, 08:15:39 am
content: Quote from: Simon on July 13, 2015, 08:10:57 amA diode does not half the voltage! It halfs the power. I assume you have 240V. A diode reduces the voltage to 240V*0.707=170V.Because of that, a transformer will be much bigger: ~420VA.QuoteThis is probably the best solution.

author: Zero999
author_rank: Super Contributor
timestamp: July 13, 2015, 08:18:34 am
content: I wouldn't worry about the extra voltage.The half wave AC waveform could be more of a problem though.How about using an autotransformer to get 120V (this will be half the size of a 600VA transformer) and run the fans off a bridge rectifier.

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2015, 08:22:04 am
content: For brushed fans, its not so bad, but I would expect a high risk of insulation breakdown on the armature as the brushes wear. The whole idea is a nasty hack and unless the fans are installed so it is IMPOSSIBLE to access them with power applied, IMHO is unsafe. The interlocks and additional enclosures required for safety are likely to cost more than a proper SMPSU or the difference in price for mains voltage AC fans.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 08:24:40 am
content: there is a 24V version of the same fan so that makes life much easier. A zenner and small filtering capacitor could be put across each fan.I'm not too concerned about 15V instead of 12V or 30 instead of 24V but we do have a history of problems with the supplier trying to wriggle out of warranty claims because they only rate an automotive fan to 13 or 26V which is mad - bloody iti's

author: grumpydoc
author_rank: Super Contributor
timestamp: July 13, 2015, 09:18:43 am
content: QuoteSurely it will just half-wave rectify and you will still have the peak voltage present - 240*1.414=340V, not 170.If you stick a capacitor after that what you get depends on the load and value.I'm afraid it all sounds a bit dodgy not least from the safety angle, is there scope for an SMPS or mains fans.If you absolutely must go with this idea it should work to put in a cap to drop the voltage, followed by a full wave bridge, smoothiing cap and then your fans in series. Don't forget the "top" one is going to have 100V WRT ground so safety is still going to be a consideration - especially as someone looking at a fan with a 12V label isn't going to be expecting that sort of voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2015, 09:29:53 am
content: I doubt low voltage fans are double insulated approved, either. (Though they might pass as such.)What's wrong with a bigass turbine and ducts?Tim

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 09:47:10 am
content: yes I will probably get more power than I need with one diode, hadn't considered it like that. I have just been finding out more information on the project and spoken to a supplier so I think a direct 240VAC fan will be used.

author: amyk
author_rank: Super Contributor
timestamp: July 13, 2015, 09:58:17 am
content: A 240V mains fan would be the best option here - a single one could probably replace several lower-voltage fans and not need as much current.Quote from: Ian.M on July 13, 2015, 08:00:43 amThey actually do make them that large... and you're right about "bulky and expensive":http://www.mouser.com/ProductDetail/WIMA/DCP6I07103EG00KS0F/?qs=sGAEpiMZZMv1cc3ydrPrFxHYOjNyEwr5YvxDbo6biTg%3d

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 10:00:28 am
content: sounds like it would need to be a smaller value anyway if I'd need to take as much voltage as people suggest. 24V fans do make it needless anyway and I'm hoping for a mains option to come up.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 13, 2015, 10:01:18 am
content: Quote from: Simon on July 13, 2015, 09:47:10 amIt's not going to work with one diode - the capacitor will simply charge-up.This idea has disaster written all over it - do yourself a favour and step away from the bodge.

author: Dave
author_rank: Super Contributor
timestamp: July 13, 2015, 10:31:02 am
content: Quote from: grumpydoc on July 13, 2015, 09:18:43 amEh, no.He was speaking of RMS voltage. The RMS voltage of half-wave rectified mains (which is 230V in UK) is 163V. Do the math, it checks out.AC dropper caps only work when you only have AC currents in your circuit. This is not a trait of a half-wave rectifier.Quote from: Simon on July 13, 2015, 07:41:50 amRepeat after me: A capacitor is not a resistor.There are phase angles involved, something which you clearly haven't taken into account.Wiring fans in series is abadidea. A fan is not a predictable load.Let's say one of the fans gets a bit of crap into the bearings and gets slightly more load. The speed of the fan will drop, reducing EMF, the overall voltage on that fan will drop, meaning every other fan will get a voltage boost. Do you see the problem?Simon, hate to break it to you, but you clearly don't have the experience to work on a circuit like this. Just get a pre-made SMPS (something like a 12V Meanwell with a trimpot) and do it safely, or switch the fans with ones rated for mains voltage.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 13, 2015, 10:42:43 am
content: Quote from: Dave on July 13, 2015, 10:31:02 amOk, true enough but the peak is still170325-357V.QuoteAgree, but you put the cap before the rect - this seems normal in consumer electronics with transformerless supplies, obviously at less current than 6A though.I don't think any one has suggested this was a good idea and it sounds like Simon has seen sense and found some mains fans to use.

author: Dave
author_rank: Super Contributor
timestamp: July 13, 2015, 10:48:46 am
content: Quote from: grumpydoc on July 13, 2015, 10:42:43 amNo, the peak is 325V.QuoteBefore a bridge rectifier, yes. Before a half-wave rectifier, absolutely not.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 10:49:50 am
content: DaveNo I don't have much experience with bodges, I tend to avoid them too hence as already stated a straight 240V option is under way. I just thought it worth thrashing out every concept.The air will be fairly clean as they are drawing air through what amounts to a guard but yes that still does not account for variations in life span and i have reiterated continually that a 240V option is best. We don't really have room for a power supply anyway.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 13, 2015, 11:00:47 am
content: Quote from: Dave on July 13, 2015, 10:48:46 amOops - typo; I did say 340 earlier (assuming 240V mains, in fact as mains can be 253V then the max peak would be 357V)QuoteAgain, that's what I suggested - seen a single diode in crappy low current consumer stuff though.

author: oldway
author_rank: Super Contributor
timestamp: July 13, 2015, 11:16:10 am
content: Low cost ac dimmer + bridge rectifier.Adjust dimmer for 12V on each fan.http://www.ebay.de/itm/2000W-SCR-Voltage-Regulator-Dimmer-Motor-Speed-Temperature-Controller-AC110-220V-/141263301801?pt=LH_DefaultDomain_77&hash=item20e3f2eca9

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 11:18:15 am
content: Quote from: oldway on July 13, 2015, 11:16:10 amI'm not nuts enough to put fill 240V pulses across a 12V fan

author: oldway
author_rank: Super Contributor
timestamp: July 13, 2015, 11:23:26 am
content: You have 8 fans in serie of 13V each, that's 104V.If they are brushed dc motors without electronics, there is no problem at all.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 11:24:47 am
content: and there are 24V options so brings it nicely to 208V with the recomended 26V but then the supplier is an asshole as it's an automotive fan so should work at the very least to 28V and ideally 30V

author: Dave
author_rank: Super Contributor
timestamp: July 13, 2015, 11:26:07 am
content: Quote from: grumpydoc on July 13, 2015, 11:00:47 amYes, single diode half-wave rectifiers can be seen in cheap circuits, however not with a series AC dropper capacitor. It can't work. Impossible.

author: oldway
author_rank: Super Contributor
timestamp: July 13, 2015, 11:52:56 am
content: Schematic...

author: vk6zgo
author_rank: Super Contributor
timestamp: July 13, 2015, 12:31:29 pm
content: The best way to make your customers hate you is to make an ancillary system so complex that it becomes the most likely failure point.You haven't given us any information on the size of these fans-----are they standard "muffin fan" size like you see on the back of Oscilloscopes,or smaller,or larger?The standard 240v ones are dirt cheap,are brushless,have good reliability,& are simply wired in parallel across the Mains.If one fails,you still have the others-------with a series string,if you lose one,they're all gone!If you absolutely have to use low voltage dc ones,buy a Meanwell power supply & wire them in parallel.If the Meanwell only needs to provide 6A,it will be quite small.OOPs!--sorry,it will be 48A.You could wire them in series--parallel,though,with a 36v supply or similar.Knowing Techs,the first time your fan system fails,they will throw them in the bin,hunt down suitable 240vac fans & retrofit them.The word will go around about the "dumbos that fitted those @##$!!!!! dc fans in the first place!"

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 12:42:44 pm
content: and we are infact looking for 240V fans (I have one contender but it may be too powerful). I was asked to power 8 fans, now that I know more about the project I'm able to make a better recomendation and it has been a 240V fan. They are using them in banks of 4 with 2 setups together making the 8 although I thin it may be 3 now so seperate 240V fans is the plan

author: grumpydoc
author_rank: Super Contributor
timestamp: July 13, 2015, 01:12:48 pm
content: Quote from: Dave on July 13, 2015, 11:26:07 amWell, not quiteActually I suspect that all the ones with a "half wave rectifier" have been this sort of configuration.I've looked at these, thought: "diode, zenner - OK" but thinking about it some more, the fact that the zenner is infrontof the rectifier is important since it will actually conduct on both positive and negative half cycles so the cap remains in an AC circuit.

author: tszaboo
author_rank: Super Contributor
timestamp: July 13, 2015, 01:26:30 pm
content: Quote from: Simon on July 13, 2015, 07:08:41 amOh no, someone hijacked a moderator account and posts questions which never should be asked. Seriously, 6A through a capacitor?Saying that, for my next work project I could use a 40000W 24V dummy load (seriously).

author: c4757p
author_rank: Super Contributor
timestamp: July 13, 2015, 01:41:25 pm
content: Who the hell allowed you to work around mains, and how haven't you burned anything down yet?

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 01:51:54 pm
content: Quote from: c4757p on July 13, 2015, 01:41:25 pmMaybe because i ask questions and get advice ? the best thing to know is "what you don't know" then you ask and find out. We have stopped using subby's who were too stupid to ask or take advice and did it wrong despite guidance given.

author: con-f-use
author_rank: Unknown
timestamp: July 13, 2015, 02:09:42 pm
content: Quote from: Simon on July 13, 2015, 01:51:54 pmBut if you don't know at least the fundamentals, even that becomes dangerous. Maybe one of your questions will be ill posed and misunderstood. Then someone gives you the wrong advice, and you'll make a mistake. Or maybe at one point you are sure, you understand the answer, only you got it wrong. The most dangerous thing in conjunction with anything over 30V and/or 2A is half-knowledge. You know enough to think it's possible and to think you can handle it. That's when it literally blows up in your face. There is a reason, they only let professionals handle that kind of stuff. There's a reason why mains circuits have warning labels and standard procedures. People could touch your contraption or something connected to it! You could burn a house down! PLEASE, be safe.

author: c4757p
author_rank: Super Contributor
timestamp: July 13, 2015, 02:10:54 pm
content: If you don't understand why a series capacitor is not an acceptable solution at 600W, you should be back on lower-risk circuits where you can ask these questions in an environment where the result of a bad answer can't kill someone. Commercial mains work isonlyfor experts who don't need to ask these simple questions anymore.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 02:12:21 pm
content: I have already advised that putting a bank of fans in series while theoretically a solution would have safety implications and that the customer must treat each one as a 240V device and i have pointed out that this may not pass safety. And I advised right from the start that we use a 240V fan. I was asked to use this particular fan just because it was in stock which means I will just disregard it.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 02:14:33 pm
content: Quote from: c4757p on July 13, 2015, 02:10:54 pmYou mean 6A 16V with a potential for 240, 600W on it's own is meaningless. It's not a solution I wanted to give serious consideration but i wanted help to rule it out. And we have. I'm getting more grief than people trying to make tasers

author: Dave
author_rank: Super Contributor
timestamp: July 13, 2015, 02:26:53 pm
content: Heh, you still don't understand why putting fans in series is a bad idea.

author: c4757p
author_rank: Super Contributor
timestamp: July 13, 2015, 02:29:23 pm
content: Quote from: Simon on July 13, 2015, 02:12:21 pm*snerk*Okay... so disregarding the usual 240V safety implications....What happens when one fan starts up before another?What happens when they aren't matched? One fan spins faster than another.What happens when one gets stuck or meets higher resistance?What happens when one fails open?What happens when one fails short?

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 02:30:07 pm
content: Quote from: Dave on July 13, 2015, 02:26:53 pmI do, and I said to my boss that while doable it was crude and not a good idea. If we were to consider it seriously I'd recomend a zenner across each fan. At the moment I have one possible 240V fan from a different supplier and they are looking into something less powerful for me.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 02:35:00 pm
content: Quote from: c4757p on July 13, 2015, 02:29:23 pm

author: c4757p
author_rank: Super Contributor
timestamp: July 13, 2015, 02:35:16 pm
content: No, it'snotdoable, for reasons that have been mentioned here, and any attempts to do it would bemassivesafety concerns no matter how many "zenners" you sprinkle in the circuit. You should not have even said it was doable, and you should not be working on mains if you don't know why.

author: c4757p
author_rank: Super Contributor
timestamp: July 13, 2015, 02:37:54 pm
content: Quote from: Simon on July 13, 2015, 02:35:00 pmMore like process variations... try it, start up two fans simultaneously and see if they actually start simultaneously.QuoteEven small variations tend to be exacerbated by positive feedback effects. They aren't "identical".QuoteIs it a laboratory cleanroom? Horseshit.QuoteThe fact that you think the Zener diode addresses the incorrect failure mode makes me seriously worry.

author: rolycat
author_rank: Super Contributor
timestamp: July 13, 2015, 03:03:04 pm
content: Quote from: NANDBlog on July 13, 2015, 01:26:30 pmWell, a cheap'n'cheerful method might be to parallel, say, 10 x 6.5m lengths of 1mm enamelled copper wire. Arrange that in a hundred litres or so of water and feed it 40 kW and my (crude) calculations suggest it will take about 7 minutes to heat from 20 to 60 degrees. If you need more time you could add a cold water supply and a drain.

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2015, 03:09:07 pm
content: Quote from: rolycat on July 13, 2015, 03:03:04 pmThat doesn't sound like a good idea. 166.7A down a 1mm wire is going to blow like a fuse unless the terminations are also fully immersed.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 03:09:18 pm
content: Quote from: rolycat on July 13, 2015, 03:03:04 pmI think we will stick with a 240V fan thank you

author: oldway
author_rank: Super Contributor
timestamp: July 13, 2015, 03:20:47 pm
content: What happens when one fan starts up before another? Not possible, they are dc brushed motors, with the same current. They have the same torque. (dc permanent magnet). Why should one start before the others ?What happens when they aren't matched? One fan spins faster than another. As resistive torque of a fan is proportional at the third power of rpm and the motor torque is proportional to the current (who is the same in all the motors), the difference stay very small. Nothing occurs.What happens when one gets stuck or meets higher resistance? Higher resistance ? Never see this occuring in a dc brushed motor. ...If it get stuck (if can't rotate anymore), it's more or less the same as failing short.What happens when one fails open? They all stopWhat happens when one fails short? Others fans have higher voltage (13,7V instead of 12V)The true problems:1) No redundancy and lower MTBF2) Isolation problems: 12Vdc fans are not made to withstand mains voltage between ground and windings.3) Safety risk for maintenance...

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 03:28:48 pm
content: Quote from: oldway on July 13, 2015, 03:20:47 pmWhich is pretty much what i told my boss.

author: rolycat
author_rank: Super Contributor
timestamp: July 13, 2015, 04:03:20 pm
content: Quote from: Ian.M on July 13, 2015, 03:09:07 pmOh, you would definitely want to immerse the terminations.

author: rolycat
author_rank: Super Contributor
timestamp: July 13, 2015, 04:05:03 pm
content: Quote from: Simon on July 13, 2015, 03:09:18 pmI wasn't attempting to address your problem, Simon. You seem to have enough people doing that.

author: Ian.M
author_rank: Super Contributor
timestamp: July 13, 2015, 04:18:21 pm
content: @rolycat: Your proposed load is operating at approx 0.2W/sqmm. I believe the commonly accepted maximum surface power density for water heating elements is around half that.

author: rolycat
author_rank: Super Contributor
timestamp: July 13, 2015, 04:50:26 pm
content: Quote from: Ian.M on July 13, 2015, 04:18:21 pmI can well believe it - I did say it was a cheap and cheerful idea. It was largely a thought experiment, and without knowing NANDBlog's circumstances I have no idea whether it would be appropriate.For a one-off test, though, I think it might work. Wires immersed in water can cheerfully carry five times their fusing current in free air.

author: Simon
author_rank: Unknown
timestamp: July 13, 2015, 04:56:16 pm
content: Quote from: rolycat on July 13, 2015, 04:05:03 pmReally ? most of them did not read past the first page if that.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 14, 2015, 05:14:55 am
content: QuoteQuoteReally ? most of them did not read past the first page if that.Simon,that's the problem---you didn't give us enough information in your first posting,like what size of fans,how they were being used,etc.Perhaps you could have made the point more strongly that you favoured a 240v ac solution,& were looking for information to help you rebut your Employers/customers idea of using 13v dc fans.

author: Simon
author_rank: Unknown
timestamp: July 14, 2015, 05:59:45 am
content: Putting fans in series was my idea given the 240V requirement and the 12V fans, that moved to 24V fans and no need for voltage dropping when I found the same version. Later I got to read the emails and they were already talking 240V.Lots of people seemed to have assumed that I was talking about putting brushless fans in series, which is a silly thing to do.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 14, 2015, 06:54:58 am
content: Either do it properly and use a 240v fan, or us a switchmode PSU - 600W isn't a huge deal, even less so if you can put it in the airflow for cooling.