Change of capacitor capacitance in a circuit - Page 1
Forum section: electronics / beginners

author: j57H8I4
author_rank: Contributor
timestamp: March 18, 2017, 11:41:14 pm
content: I am doing the attached experiment, but I do not have the capacitors of the indicated values. The experiment comes from this book:www.introni.it/pdf/101%20Esperimenti%20con%20oscilloscopio.pdf, experiment 51, page 82 of the pdf file, page 69 of the book.Can I replace C1 with a 22 nF capacitor and be safe doing the experiment?Moreover, can I replace C2 with a 10 microF capacitor and be safe doing the experiment? I have this 10 microF 450 VAC capacitor because I bought it to replace our dishwasher motor capacitor.

author: timb
author_rank: Super Contributor
timestamp: March 19, 2017, 12:06:56 am
content: Okay, you're working with 230VAC here, which is *real* voltage that can *kill* you. If you're at a point where you need to ask these questions, you really shouldn't be performing any experiments that involve line voltage.Stick to low voltage (<50V) DC until you're quite a bit more experienced.

author: pigrew
author_rank: Frequent Contributor
timestamp: March 19, 2017, 12:09:02 am
content: First of all, the posted schematic has different connections than the reference experiment 51. The miss-connection of the ground will cause a resistor to perhaps overheat (among other things).Secondly, this circuit is unsafe even if it is understood. I suggest skipping this experiment (and other experiments involving mains voltages). The concepts of this circuit could be (safely) learned by using a signal generator outputting 60Hz sine waves at lower voltages. I could suggest a few ways to make this safer, but you wouldn't learn much more than if you were to repeat the experiment with a signal generator.Perhaps in the early 70s, signal generators were too expensive? Though... the oscilloscope would have been a much higher cost than the signal generator. These days, you can use a computer's audio output jack as a signal generator (if you don't have a stand-alone generator).Thirdly, to answer your initial question, the capacitors have an impedance of 1/(j*2*pi*f*C). You should think of the three legs as three different voltage dividers. Changing the capacitors will change the output voltages. For it to be safe in the way imagined fifty years ago, you'd need make sure that each component has a power dissipation, current, and voltage less than the component's maximum rated value.[I remember in my undergrad, they had us wire things using AC transformers to the mains (with a fuse). You could use an off the shelf AC 12V power adapter to complete this experiment fairly safely.]

author: j57H8I4
author_rank: Contributor
timestamp: March 19, 2017, 12:20:22 am
content: I have already done part of the experiment with the switch at position B, because that uses only resistors and I can calculate the current flowing in that circuit, with the switch at position B, which is very low, around 0.5 mA, and everything was fine.I do not know how to calculate the current and the voltage when capacitors are involved in the circuit.

author: j57H8I4
author_rank: Contributor
timestamp: March 19, 2017, 12:26:39 am
content: Quote from: pigrew on March 19, 2017, 12:09:02 amYes, I made a mistake in my drawing, coping the circuit from that book, but I have corrected it. Thank you.

author: StillTrying
author_rank: Super Contributor
timestamp: March 19, 2017, 02:20:32 am
content: It's lethal.Depending on how unlucky you are, a bad connection on the L or G ? will allow 300V to 600V to get through to the scope and anything else.

author: j57H8I4
author_rank: Contributor
timestamp: March 19, 2017, 03:24:58 am
content: You could say the same thing of an hairdryer or many other appliances or mains powered instruments that you use every day. If that circuit is well done it is safe just as other mains powered objects are safe.

author: StillTrying
author_rank: Super Contributor
timestamp: March 19, 2017, 03:55:56 am
content: The reactance of C1 only works for a smooth 50Hz signal, any fast spikes such as switch on or off, and 100s of volts will go straight through 2 - C - Y.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 19, 2017, 07:29:10 am
content: Quote from: j57H8I4 on March 19, 2017, 03:24:58 amIf you can't see the difference between a circuit designed by someone qualified and tested before sale with something you cobbled together on your desk, you really, really shouldn't be messing with mains. This circuit can kill you, some of the individual components can kill you even after it's turned off, and you don't have a deep enough understanding to fully understand what you're doing. Do you know the voltage rating of all your components? Do you know the input voltage rating of your scope? Are you accounting for the real peak voltage and any potential spikes of 230VRMS?What's the point of even building the circuit if you don't understand what it is demonstrating? If you don't know what to expect to see you can't know what the things you are seeing mean.

author: tggzzz
author_rank: Super Contributor
timestamp: March 19, 2017, 08:37:59 am
content: Quote from: j57H8I4 on March 18, 2017, 11:41:14 pmIf that 230V is the mains, then you (and/or your equipment) cannot be safe with either that capacitor or the original capacitor. There are just too many failure modes.That experiment looks like it was designed (I use that word loosely) by an academic that has little practical experience. You know, just like the PhD that tries to use a multimeter to measure the impedance of the mains!

author: j57H8I4
author_rank: Contributor
timestamp: March 19, 2017, 09:54:07 am
content: I have not received the answer to my questions in this forum, so I build my mains - analog oscilloscope interface.WARNING: My pictures attached to this post show a very dangerous circuit, potentially lethal. If you reproduce it you do that at your own risk.

author: timb
author_rank: Super Contributor
timestamp: March 19, 2017, 10:29:57 am
content: Quote from: j57H8I4 on March 19, 2017, 09:54:07 amJesus Christ. You did it on a protoboard? Those are designed for low voltage circuits only! There's nowhere near enough creepage or clearance with those boards for mains levels!To put it simply, it's very easy for that line cord input to short across to one of the plated through holes, then to the next and the next and the next and finally through you, your scope or whoever is unlucky enough to get near anything connected to it.You seriously have no idea what you're doing and it's *very* dangerous.

author: j57H8I4
author_rank: Contributor
timestamp: March 19, 2017, 01:49:53 pm
content: Picture 1) Switch position B, probe x1, channel 1 AC, 0.2 V VOLTS/DIV, 2 ms SWEEP TIME/DIVPicture 2) Switch position A, probe x1, channel 1 AC, 50 mV VOLTS/DIV, 2 ms SWEEP TIME/DIVPicture 3) Switch position C, probe x1, channel 1 AC, 1 V VOLTS/DIV, 2 ms SWEEP TIME/DIV

author: Zero999
author_rank: Super Contributor
timestamp: March 19, 2017, 01:50:36 pm
content: Quote from: j57H8I4 on March 19, 2017, 09:54:07 amI think you'll find you have already received an answer to your question:Quote from: tggzzz on March 19, 2017, 08:37:59 amQuoteWhy did you ask if it was safe if you already knew the answer?Safety aside, the main technical problem with this experiment, is the non-polarised connector, meaning there's a 50-50 chance of it working.You can do this experiment less dangerously using an isolation transformer.Quote from: timb on March 19, 2017, 10:29:57 amGoing from his previous post, I believe he's well aware of the risks.Also being pedantic, mains is often considered to be low voltage.Irrespective of what the safety standards say, mains will not arc across the through plate hole, under normal conditions. The voltage needs to be much higher than 250VAC for that to happen. Of course a safely designed appliance needs to be designed with insulation capable of withstanding much higher voltages, to cater for spikes caused by lightning and electrical faults, but this is just a quick and dirty experiment, not something to be incorporated into a piece of equipment.

author: timb
author_rank: Super Contributor
timestamp: March 19, 2017, 08:44:52 pm
content: Quote from: Hero999 on March 19, 2017, 01:50:36 pmIt's not about arcing so much as it is about little gobs of solder (or anything metal in the vicinity) bridging adjacent pads. That said, if there's some sort of a surge on the line, it could very well still arc if there was a surge or lightning, like you mentioned. It's not even fused FFS!Even if this is an experiment, it's still dangerous, period. If the OP doesn't know enough about the circuit to understand how it works, he has no business messing with mains.Being even more pedantic, mains may not be "high voltage" but it *is* lethal voltage. That's why most electrical safety standards consider anything over 50V to be dangerous.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 19, 2017, 09:16:21 pm
content: Quote from: Hero999 on March 19, 2017, 01:50:36 pmIf that 50/50 chance is wrong it will also put 230V on the oscilloscope switches you're operating with your hands, assuming it doesn't just blow up the ground lead - but that assumes the scope was properly grounded.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 19, 2017, 09:35:51 pm
content: So, now that you build it, what did you learn? Since you didn't use the provided values, what the experiment was trying to demonstrate didn't work. Since you don't understand the circuit, you don't know why not.In the experimental circuit, the voltage at point 1 and 3 should be equal but out of phase, the voltage at point 2 should be almost the same but not quite, likely due a not-quite-right standard value. Does this seem to be the case to you? Do you know why not?This circuit is supposed to show you different ways to achieve the same impedance, but since you changed the values your impedances are wildly different and you didn't demonstrate anything. It's also not very useful if you don't know what impedance is or how to calculate it.Something is also seriously wrong with one of your connections in position A.

author: ebclr
author_rank: Super Contributor
timestamp: March 20, 2017, 01:04:49 am
content: If 220V is lethal I'm calling from another world.220 Can be lethal, is some very strict circumstances, But isn't lethal by definition.I have already taken several shocks of 220V and I am alive to write here.It is a good precaution to be careful, but it cannt be said that it is lethal'Voltage does not kill nobody, only current can kill

author: Nerull
author_rank: Frequent Contributor
timestamp: March 20, 2017, 02:26:00 am
content: And current can't kill without voltage, it is the combination that is dangerous.And a bare PCB built by someone who don't know what they're doing plugged directly into the wall outlet can provide more than enough of both to kill someone, so yes, it is lethal.Would you tell someone it's okay to look down the barrel of a gun because hey, not everyone who gets shot dies. Bullets aren't lethal!If you want to be patted on the head for being overly pedantic, telling someone tinkering with electronics to take risks with dangerous voltages isn't the place to do it.

author: tggzzz
author_rank: Super Contributor
timestamp: March 20, 2017, 07:44:57 am
content: Quote from: ebclr on March 20, 2017, 01:04:49 amThe problem with that is that it can be recast....Ifwalking across a road without lookingis lethal I'm calling from another world.Walking across a road without lookingCan be lethal, is some very strict circumstances, But isn't lethal by definition.I have alreadybeen hit by a vehicle several timesand I am alive to write here.It is a good precaution to be careful, but it cannt be said that it is lethal'Being hit by a vehicledoes not kill nobody, onlybeing hit hard by a vehiclecan kill

author: j57H8I4
author_rank: Contributor
timestamp: March 20, 2017, 08:33:53 pm
content: Quote from: Nerull on March 19, 2017, 09:35:51 pmI trust what Albert Einstein said: “If a person studies a subject for 15 minutes a day in a year he will be an expert. In 5 years he will be a national expert”.Quote from: Nerull on March 19, 2017, 09:35:51 pmIndeed I started this thread to learn that.Quote from: Nerull on March 19, 2017, 09:35:51 pmI am sorry, I made a mistake indicating the switch position for each of my pictures, infact I have corrected that now.In my attached picture there is the trace shown by my DSO150 with the switch at position C. My analog oscilloscope showed a double trace with the switch at position C because its trigger function started malfunctioning a couple of days ago.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 20, 2017, 09:28:59 pm
content: And Albert Einstein spent very little time in a lab - he learned by reading and understanding. This is a circuit (Well, I'd certainly do a safer version - impedance is impedance, there's no reason this circuit needs to run at 220V) you should be building while learning what impedence is and how to calculate it. Your statement that you don't know how to calculate the current through the circuits with capacitors shows you aren't doing this. Throwing parts on a breadboard without understanding what they do or how they work teaches very little. If you now wanted to design a circuit using capacitors to attenuate an AC signal, could you determine the right values to achieve the desire result? If all you did was built a single circuit, all you know is that you get a squiggly line on the scope screen.The first thing they tell you in a scientific lab - you need a hypothesis. You need to have an idea what your experiment should do, before doing it teaches you anything. The idea doesn't need to be correct, but you shouldn't start from a blank slate.

author: StillTrying
author_rank: Super Contributor
timestamp: March 20, 2017, 09:29:54 pm
content: I have a small halogen desk lamp with 2 metal bars going up from the base to the bulb, clipping on to the metal bars provides a useful 12 VAC 50Hz for testing purposes, it's a bit safer than using live mains as a signal generator!

author: j57H8I4
author_rank: Contributor
timestamp: March 20, 2017, 09:50:08 pm
content: Quote from: tggzzz on March 19, 2017, 08:37:59 amThat book was published by Philips, likely the largest electronics manufacturer in Europe. You can immagine how many engineers it had at the time of the publishing, so that book likely was throughly checked before and after publishing.

author: j57H8I4
author_rank: Contributor
timestamp: March 20, 2017, 09:57:04 pm
content: Quote from: StillTrying on March 20, 2017, 09:29:54 pmThanks for your suggestion, but I think that the voltage divider indicated in the experiment, made just with one 470k resistor and one 1k resistor, is a very easy, cheap and safe way to reduce mains electricity to a very safe level.

author: StillTrying
author_rank: Super Contributor
timestamp: March 20, 2017, 10:25:55 pm
content: LOLWhy don't you simulate the circuit, that's bit safer way as well.

author: timb
author_rank: Super Contributor
timestamp: March 20, 2017, 11:47:39 pm
content: Quote from: j57H8I4 on March 20, 2017, 09:57:04 pmUgh... You have no clue what we're trying to tell you. The way the circuit is constructed is inherently unsafe. You could learn the same thing, a lot safer, by using the output of a 12 or 24VAC transformer. You don't need to use raw 240VAC mains! Those protoboards are unsafe at that voltage for a variety of reasons.That book was written 40+ years ago, before inexpensive function generators or simulation were available. There's absolutely no reason not to just use a function gen or transformer or LTspice to learn about impedance and AC dividers today.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2017, 12:08:24 am
content: Quote from: j57H8I4 on March 20, 2017, 09:50:08 pmAh. You must be young. I have more than one book describing fun things to do with your X-Ray machine; aimed at kids, and with a warning that you probably shouldn't let your skin get too red.I've no intention of wasting my life trying to work out the quality of an arbitrary book nor of the context in which that circuit was discussed - but my comment stands unchanged.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2017, 12:12:44 am
content: Quote from: j57H8I4 on March 20, 2017, 09:57:04 pmGiven the number of times you have been warned and actively refused to understand/listen, that's definitely a good example of the Dunning-Kruger effect in action.I really don't care if you endanger yourself; that's your business. I do care if other people might be encouraged to follow your example.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 21, 2017, 04:56:04 am
content: Quote from: j57H8I4 on March 20, 2017, 09:57:04 pmBut it's not doing that. You're working with directly exposed mains, there is nothing protecting you from that. It doesn't matter what's going on partway through the circuit, it's the most dangerous exposed part that will get you - and you've got plug wires coming directly out of the wall outlet exposed.Each of the circuits also has failure modes that will pass 325V directly to your scope. (Did you bother to learn what RMS means?) This is above the maximum voltage rating of your analog scope and waaay above the maximum voltage rating of your digital scope, so you were risking destroying both.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2017, 07:54:00 am
content: Quote from: Nerull on March 21, 2017, 04:56:04 amJust so, but I fear your points will fall on deaf ears.The OP has repeatedly been introduced to the concept of working out how things fail, and so far as can see has chosen not to apply the concept to his experiment. He has even failed to consider how he could achieve the same results and understanding much more safely simply by using a lower voltage.

author: StillTrying
author_rank: Super Contributor
timestamp: March 21, 2017, 05:04:17 pm
content: Anything blown up yet?

author: j57H8I4
author_rank: Contributor
timestamp: March 21, 2017, 08:09:21 pm
content: Quote from: Nerull on March 21, 2017, 04:56:04 amOf course the circuit will go into an enclosure, although I do not know yet what type of PCB to use to rebuild it and I still have to calculate what wattage the resistor should have.Quote from: Nerull on March 21, 2017, 04:56:04 amAnyway I do not keep the probe in my hand when I do this experiment, I hook it and then i put the plug in the socket.

author: j57H8I4
author_rank: Contributor
timestamp: March 21, 2017, 08:11:38 pm
content: Quote from: StillTrying on March 21, 2017, 05:04:17 pmI do not have much time for doing experiments during weekdays.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2017, 11:34:51 pm
content: Quote from: j57H8I4 on March 21, 2017, 08:09:21 pmAnd with those two statements you demonstrate (again) that you do not understand what anybody has been telling you.Is there anyreasonwhy you don't just use a safe voltage, but persist in using dangerous voltages? Whatbenefitdoes using dangerous voltages give you?

author: Neilm
author_rank: Super Contributor
timestamp: March 22, 2017, 07:53:21 pm
content: Can your components take the surge you will get on a mains supply? Normal transients are spikes that are several thousand volts

author: j57H8I4
author_rank: Contributor
timestamp: March 22, 2017, 09:51:04 pm
content: Quote from: tggzzz on March 21, 2017, 11:34:51 pmI am interested in seeing how is the mains wave.

author: j57H8I4
author_rank: Contributor
timestamp: March 22, 2017, 09:52:55 pm
content: Quote from: Neilm on March 22, 2017, 07:53:21 pmCan an hairdryer stand spikes that are several thousand volts?

author: tggzzz
author_rank: Super Contributor
timestamp: March 22, 2017, 10:47:27 pm
content: Quote from: j57H8I4 on March 22, 2017, 09:51:04 pmSo, since you can do that with a transformer at safe voltages, the answer is that there is no benefit. Hence fiddling with high voltages and failure mechanisms that you don't understand is being wilfully and perversely dangerous.But I thought your circuit has capacitors and resistors, so your statement confuses me.

author: tggzzz
author_rank: Super Contributor
timestamp: March 22, 2017, 10:48:55 pm
content: Quote from: j57H8I4 on March 22, 2017, 09:52:55 pmIrrelevant.Stop avoiding the important points and issues.

author: Unknown
author_rank: Unknown
timestamp: March 22, 2017, 11:06:36 pm
content: Quote from: j57H8I4 on March 20, 2017, 08:33:53 pmI'd trust my own common sense, calibrated against the advice of reasonably knowledgeable others, more than an alleged Einstein quote that appears to have been made up years after he died..."I never said half the crap the Internet says I did" - Albert Einstein."Dear Albert, on Facebook it's more like 3/4" - Mark Twain.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2017, 11:35:38 pm
content: Quote from: j57H8I4 on March 22, 2017, 09:52:55 pmYes, a correctly designed hair drier can withstand short voltage spikes over several thousand volts and even if the voltage spike goes on for long enough to destroy the hair dryer, it will still not pose a safety hazard to the user.Quote from: j57H8I4 on March 21, 2017, 08:09:21 pmYou don't understand. As long as the circuit involves exposed parts which are directly connected to the mains, then it isn't safe. The correct way to assemble a circuit like this is to use an isolation transformer. A 24VAC transformer is perfect, since all the voltages will be divided by 10.

author: StillTrying
author_rank: Super Contributor
timestamp: March 22, 2017, 11:46:04 pm
content: Quote from: j57H8I4 on March 22, 2017, 09:52:55 pmThey try to direct any mains or switch on spikes back to neutral or earth, not out to the user!

author: Nerull
author_rank: Frequent Contributor
timestamp: March 23, 2017, 06:43:36 am
content: Quote from: j57H8I4 on March 22, 2017, 09:52:55 pmYes, it can, because it was designed by someone who understands what they are doing. Anything connected to mains needs to be designed to handle voltage spikes.An electronic device that blew up every few weeks wouldn't be very popular.

author: j57H8I4
author_rank: Contributor
timestamp: March 24, 2017, 06:31:06 am
content: Quote from: Nerull on March 23, 2017, 06:43:36 amWell, then this circuit can be improved to handle voltage spikes too.

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2017, 09:59:40 am
content: Quote from: j57H8I4 on March 24, 2017, 06:31:06 amGive up, guys; we're wasting our breath.There's an English proverb: "You can lead a horse to water, but you can't make it drink". For why that is relevant, I refer you to most of the posts in this thread!

author: beenosam
author_rank: Regular Contributor
timestamp: March 24, 2017, 10:37:22 am
content: I think we should really implement a new rule here. If someone is doing something inherently dangerous and refuses to listen to advice of experienced professionals, then that thread ought to be locked. We really don't need more beginners reading these kinds of threads and think that level of ignorance is acceptable. Nor do we need to continue advising these people further.

author: Gyro
author_rank: Super Contributor
timestamp: March 24, 2017, 10:49:33 am
content: ...and while you're at it, delete those stupid photo's too in case they encourage copying.

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2017, 10:52:48 am
content: Quote from: beenosam on March 24, 2017, 10:37:22 amIt is difficult, but I am biassed against locking threads.Beginners need to be exposed to practices that have poorly-understood dangers, then to realise that the dangersaresubtle, and that some people will persist in dangerous practices. Learning to recognise such people and behaviour is a valuable part of learning about life and the intertubesIn this case the OP will only harm himself and/or his equipment. I'm not too worried about that, it is his problem.OTOH, sometimes people advocate practices that could harm them and/or random strangers in the vicinity. Classic examples involve buying 400mW/1W lasers for cutting things and imagining they are safe, or misusing chemicals. That does get me angry, and such threads should probably be locked with a statement as to why they are locked.

author: Zbig
author_rank: Frequent Contributor
timestamp: March 24, 2017, 11:05:52 am
content: Quote from: tggzzz on March 24, 2017, 09:59:40 amYes, you're all wasting your time. Just click the guy's username and take a peek at his other past threads. That's the troll who argued to death that isolation transformers are useless (apparently because there are fewer of them than the scopes on the market) and that DSO's don't stand any chance to analog scope (because he's just learned what A/D conversion is). He first admits he doesn't know shit in his first post and then proceeds to fight everyone who actually does. Plus, he exhibits some unhealthy fascination with learning electronics by sticking probes to mains stuff. He's a lost cause.