Change of capacitor capacitance in a circuit

[j57H8I4]

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.

[timb]

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.

[pigrew]

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.]

[j57H8I4]

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.

[j57H8I4]

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).

Yes, I made a mistake in my drawing, coping the circuit from that book, but I have corrected it. Thank you.

[StillTrying]

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.

[j57H8I4]

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.

[StillTrying]

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.

[Nerull]

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.

If 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 230V RMS?

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.

[tggzzz]

Can I replace C1 with a 22 nF capacitor and be safe doing the experiment?

If 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!

[j57H8I4]

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.

[timb]

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.

Jesus 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.

[j57H8I4]

Picture 1) Switch position B, probe x1, channel 1 AC, 0.2 V VOLTS/DIV, 2 ms SWEEP TIME/DIV

Picture 2) Switch position A, probe x1, channel 1 AC,  50 mV VOLTS/DIV, 2 ms SWEEP TIME/DIV

Picture 3) Switch position C, probe x1, channel 1 AC, 1 V VOLTS/DIV, 2 ms SWEEP TIME/DIV

[Zero999]

I have not received the answer to my questions in this forum, so I build my mains - analog oscilloscope interface.

I think you'll find you have already received an answer to your question:

Can I replace C1 with a 22 nF capacitor and be safe doing the experiment?

If 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.

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.

Why 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.

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.

Jesus 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.

Going 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.

[timb]

I have not received the answer to my questions in this forum, so I build my mains - analog oscilloscope interface.

I think you'll find you have already received an answer to your question:

Can I replace C1 with a 22 nF capacitor and be safe doing the experiment?

If 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.

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.

Why 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.

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.

Jesus 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.

Going 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.

It'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.

[Nerull]

Safety aside, the main technical problem with this experiment, is the non-polarised connector, meaning there's a 50-50 chance of it working.

If 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.

[Nerull]

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.

[ebclr]

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

[Nerull]

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.

[tggzzz]

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

The problem with that is that it can be recast....

If walking across a road without looking is lethal I'm calling from another world.

Walking across a road without looking Can be lethal, is some very strict circumstances, But isn't lethal by definition.

I have already been hit by a vehicle several times and 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 vehicle does not kill nobody, only being hit hard by a vehicle can kill

[j57H8I4]

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?

I 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”.

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.

Indeed I started this thread to learn that.

Something is also seriously wrong with one of your connections in position A.

I 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.

[Nerull]

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.

[StillTrying]

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!

[j57H8I4]

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!

That 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.

[j57H8I4]

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!

Thanks 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.