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A few circuits questions...

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A few circuits questions...

Unread postAuthor: saefroch » Tue Oct 11, 2011 12:16 am

Is it possible to discharge one side of a capacitor to ground? I'm fairly sure it is... so assuming that it IS possible, will it discharge fully given an infinite amount of time (I'm a bit familiar with RC circuits, so I know capacitors never discharge fully).

Is it possible to charge a photoflash capacitor (or two or three) faster by using more batteries in parallel to power it? I'm not sure if this will make a difference or just oversaturate the transformer.

Does the dielectric strength of a material specify at which voltage for a thickness it "breaks down" and just conducts? I read the wikipedia article, so I'm just looking for a "yes" or "no" here, since I'm also wondering if most dielectrics (like polycarbonate) have appreciable conductivity <i>before</i> the breakdown field strength is reached (I suspect 10<sup>13</sup>Ohm*m is pretty darn high, but I have no perspective on this).
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Unread postAuthor: Technician1002 » Tue Oct 11, 2011 12:28 am

A capacitor has to have a flow path. If for example you take a camera flash cap and charge it to +300 volts, and disconnect the ground then connect the + to ground, you won't get a spark. The plus will simply be at ground refrence potential. The negative lead of the cap will be at -300 volts. The relation between the + and - won't change.

Using transistor commutation to do the switching is commonly used to generate a - voltage for op amp circuits. for example +12 can supply + and - 12 volts for a 24 volt circuit. This is sometimes used in car stereos for the output amp. It can provide a speaker swing from +12 to -12 instead of from 0 to +12. The weight advantage is it does not need a transformer.

The inverter in a camera flash is designed to draw a limited current to prevent overload on a battery. Adding more batteries will not change the charge time except when the battery is low. Adding more inverters with the extra batteries can cut the charge time. For example, hook 3 flash circuits up to one cap (remove 2) will reduce the charge time to 1/3.

Dialectric strength and resistance is two different items. Dialectric strength is related to the max voltage a material will take before breaking down.

Resistance is related to the amount of current a material will carry. This is what causes capacitors to self discharge over time. The higher the resistance the longer the charge will hold.

For real examples. Lightning will break down the dialectrc strengh of air and create an ionized path. We use this for spudgun ignition. Damp or dry, the arc path is about the same for a given voltage.

Moisure in the air does change the resistance. Very dry air causes all kinds of items to pick up a static charge. Walking on a carpet and grabbing a door handle will often result in an impressive jolt.

In damp locations, it is hard to get a static charge on anything as the lower resistance bleeds it off as fast as it is generated.
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Unread postAuthor: saefroch » Tue Oct 11, 2011 12:52 am

Technician1002 wrote:A capacitor has to have a flow path. If for example you take a camera flash cap and charge it to +300 volts, and disconnect the ground then connect the + to ground, you won't get a spark. The plus will simply be at ground refrence potential. The negative lead of the cap will be at -300 volts. The relation between the + and - won't change.
I think I see what you're getting at here, but I'm still confused by the fact that there is a charged object, but you're saying it cannot discharge to ground...

Technician1002 wrote:Using transistor commutation to do the switching is commonly used to generate a - voltage for op amp circuits. for example +12 can supply + and - 12 volts for a 24 volt circuit. This is sometimes used in car stereos for the output amp. It can provide a speaker swing from +12 to -12 instead of from 0 to +12. The weight advantage is it does not need a transformer.
This doesn't make any sense to me at all. Are you saying I can produce a voltage with something called an OP AMP circuit from a DC source, and arc to ground from that?

I've got Chemistry at 9:00 in the morning, I'll have a look here in the afternoon. :)
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Unread postAuthor: Zeus » Tue Oct 11, 2011 3:33 am

You can make AC from an op(erational) amp(lifier), but it won't be high voltage. It will only be the sum of the +ve and -ve. To give an example, I'll do some ASCII drawing.

Code: Select all

+ve
|              -
|            -  -
|           -    -       
Ground -------------
|                  -   -
|                   - -
|                    -
-ve



You see? You have a positive rail, a ground rail and a negative rail.

Polycarbonate should break down immediately, but don't hold me to it.

Can I ask the application for this information?

Edit: Goddamn code tags
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Unread postAuthor: saefroch » Tue Oct 11, 2011 2:51 pm

So either rail can arc to ground?

It's just an idea I was tossing around in my head, a sort of taser truncheon.
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Unread postAuthor: ramses » Tue Oct 11, 2011 3:30 pm

I cannot explain why a capacitor cannot arc to ground (I would love to hear the answer), but emperically they cannot. The Marx Generator works by charging a bunch of capacitors in parallel, then, through spark gaps, discharging them in series. The first spark gap is either triggered by overvoltage or some external force, and the rest see the sum of the voltage of the lower stages (so the 5th stage would see 5*vin). This makes them overvolt.
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Unread postAuthor: Zeus » Tue Oct 11, 2011 3:58 pm

Ramses, nor can I, I've been asked a lot of odd questions, but this befuddled me. In electrical theory, some things just work and you don't question them.

Saefroch, spot on. Assuming the rails are at +10vdc and -10vdc, between the rail and any rail there will be 10 volts, and between the +10 rail and -10 rail there will be 20 volts.

If you keep in mind that electricity always has to return, you'll have no problems building a stun baton.

(TASER is an acronym: Thomas A. Swifts' Electric Rifle.)
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Unread postAuthor: saefroch » Tue Oct 11, 2011 4:48 pm

I've no doubt that I can build a stun baton, but I'm wondering if it's possible to build it such that it doesn't matter where it's touched (I've a few ideas where it would matter where one touches it, but that's not desirable) Ideally, it'd have a very large capacitance so as to get lots of kick without too much danger of arcing to the user.

Never knew TASER was an acronym. Thanks, I'll use it correctly now :P.

What would happen if one were to ground one side of a capacitor and charge it?
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Unread postAuthor: Zeus » Tue Oct 11, 2011 7:05 pm

What do you consider "ground"? I see it as the circuit return, but I think you're seeing it as earth?

If you earth one side of a capacitor, and the target is in contact with earth, then they will be shocked. If I were you though, I'd just have electrodes running parallel to the baton, alternating between each lead of the capacitor.

And remember that electroshock devices intended for less-lethal use should use high (10kV plus) voltage. Higher volts hurt more, current disables and kill more. Try to keep average output to less than 20mA. I wouldn't use more than 50nF at 10kV regarding capacitors. Otherwise you start removing flesh, tunneling burns etc.
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Unread postAuthor: saefroch » Tue Oct 11, 2011 9:37 pm

Yup, I'm aware of the effects of voltage as opposed to current on flesh.

I've thought of the alternating electrodes idea, but that would be difficult to build, carry a rather significant issue of arcing, and have a fair probability of not triggering in some situations.

What if I were to earth one side of the capicator to me and the other side to the baton? Assuming I have fair electrical contact with the earth, will I electrocute myself when the baton discharges?
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Unread postAuthor: POLAND_SPUD » Tue Oct 11, 2011 9:47 pm

What if I were to earth one side of the capicator to me and the other side to the baton? Assuming I have fair electrical contact with the earth, will I electrocute myself when the baton discharges?
TAZER FAIL!!

:D

I am not very good at this stuff but even I can tell it's a stupid idea
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Unread postAuthor: saefroch » Tue Oct 11, 2011 11:00 pm

I'm not so sure, but I've little education here, so this post will probably seem smart but be dumb.

When charged, electrons migrate from one side of a capacitor to another, according to a logarithmic function (IIRC), which tapers off dramatically. So they charge almost to a limited voltage.

The earth is normally considered an infinite source of positive and negative charges. So I conclude that I can use electrons from the earth and cram them onto one side of a capacitor, leaving zero charge on the other side, since it's connected to the earth, and as soon as electrons are taken off, they're replaced.

Assuming the capacitor doesn't make current flow backwards through the charging circuit, it should be stable in this configuration. I should then have a net charge on one side, and nothing on another. Thus, the baton is charged.

No? I'm guessing not... but where's my reasoning flaw?
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Unread postAuthor: Zeus » Tue Oct 11, 2011 11:04 pm

What PS said, alternating electrodes arcing would be a good thing, intimidation.

Remember chances are, the thing you intend to shock (I read it as human) will be wearing shoes, that'll stop just about anything. The closer together the electrodes, the more pain and less risk to the heart there is. That's why most cattle prods and shock batons have two electrodes as opposed to a monopolar device.

Feel free to PM me if you believe anything would contravene the rules.

Edit: I can't explain why, but you need a return attached to the capacitor for your purposes. I understand your logic, I don't know why it doesn't work, but I know what does.
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Unread postAuthor: saefroch » Tue Oct 11, 2011 11:10 pm

I don't intend this to get into any ambiguous areas, it's just theory right now.

I think there <i>is</i> a return, it's the earth. The charge came from there, and it's headed back... right?
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Unread postAuthor: Zeus » Tue Oct 11, 2011 11:32 pm

saefroch wrote:I don't intend this to get into any ambiguous areas, it's just theory right now.

I think there <i>is</i> a return, it's the earth. The charge came from there, and it's headed back... right?


No. The charge came from the charging circuit (wait, don't rage yet) into the capacitor, they stay there until they can cross over to the other side of the capacitor. The capacitor <i>is</i> the ground. The electrons have nothing at all to do with earth, all they know and will ever know is the capacitor.

Am I starting to make sense? I can tell you it won't work, but if you want to try it, please do.
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