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lewylln

Mon Jul 09 2012, 06:55AM

Registered Member #3358 Joined: Mon Oct 25 2010, 09:47AM
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Posts: 20

Been doin a bunch of home electronics lately and im largely self taught. however having no official teaching in all this and going off online guides and such has left a fair few holes in my knowledge on the matter. My main question right now is wat on earth are the circled symbols in the pic. Am i to assume that all of the "ground" wires, as google tells me, are the negative terminal or something? And yes, yes i do know how idiotic this question is, i would rather make sure i get being young and stupid out of the way before i try being old and wise.

ps im not actually building that circuits, it just annoys me as i cant understand those circuits because of that one forsaken symbol which really does bug me. and if they are just a negative terminal why not put it as that symbol?

1341816826 3358 FT0 Cfpr Capacitor Charger

Dr. Slack

Mon Jul 09 2012, 07:18AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The simple answer is - assume all of those symbols are connected together at 0v. That will do for getting to understand how the ideal circuit should be working.

There are several layers of subtlety that you will have to get to grips with in future years, but all at 0v will do for now. The subtleties are to do with the fact that not all of those ground symbols will be at *exactly* the same voltage in a real operating circuit. For most circuits this won't matter too much, but when very high currents, or very high frequencies, or very low level signals are involved, it can make the difference between the circuit working as expected, and not, like an audio modulated Tesla coil for instance.

lewylln

Mon Jul 09 2012, 09:19AM

Registered Member #3358 Joined: Mon Oct 25 2010, 09:47AM
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Posts: 20

so then in the circuit above for the power supply where would the negative terminal connect?

lewylln

Mon Jul 09 2012, 09:24AM

Registered Member #3358 Joined: Mon Oct 25 2010, 09:47AM
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Posts: 20

also by connected together do you mean that they are all connected by wires???? Like i said in the op i have quite a few gaps in knowledge in this area of science

Dr. Slack

Mon Jul 09 2012, 12:58PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes, it's not a very well-drawn circuit. It doesn't show the power supply and assumes you already know enough to work it out. Use a 12v power supply with the +ve to all the 12V symbols, and the -ve to all the ground symbols. All the ground symbols are assumed to be at the same 0V.

On this circuit, the power supply negative does connect to ground. However, a different circuit could use a positive ground. Ground just means 0v. Most circuits we see, because of the peculiarities of solid-state physics that mean that NPN transistors and N-ch FETs are better and cheaper than the P-type, are drawn with a positive rail, which means the more negative supply terminal goes to ground. If the circuit used +ve and -ve rails, as an audio circuit with dual rail op-amps might, then the supply 0v terminal would go to ground.

The ground terminal does not have to be connected to any other "ground", at least not for a circuit like this, whether that's the house mains ground, or a spike driven into wet soil outside. Linking up with any other ground is strictly something that's done for other reasons, like safety regulations. The ground symbol when used like this really means the outer metal case of the bit of equipment that you're building. Unfortunately, ground has several different meanings, and it may or may not be obvious from the context what is meant.

"Are they connected by wire?" Good question, which goes right to the heart of my earlier warnings about subtleties you might eventually have to master.

Ideally, they are all connected to the same identical point, the so-called star ground. This can be awkward to use in practice, and while it's ideal, it's rarely necessary. But it is a very good starting point. Nobody ever messed up using a star point ground.

Next best is they are all connected to a very low resistance and low inductance conductor, like a plane of metal. This would typically be the ground plane on a board, or the metalwork of an instrument case.

A solid ground plane can be approximated well by a lattice of connected conductors or a lattice of traces on a board.

Remember you are always striving to get zero resistance and zero inductance between all of those ground symbols. OK, once you know a bit more, you can relax that restriction, but for the moment, that way will work.

A really good way to get nasty problems in your circuit is to forget that a length of thin wire does *not* approximate a star point or a ground plane well, that is its resistance and inductance are not negligible. If one part of your circuit is pushing a current through the wire, and another part is sensing a small voltage with respect to the wrong end of that wire, it will pick up an error voltage due to the current, or the changes of current, and may not do what you want.

If you have several systems, each with their own ground, then clearly a star point is not going to work, even theoretically. Believe me, even very experienced engineers can flounder with how to connect their grounds under those circumstances.

lewylln

Mon Jul 09 2012, 01:16PM

Registered Member #3358 Joined: Mon Oct 25 2010, 09:47AM
Location:
Posts: 20

hmmm alright, could i ask one more question then? im working on this circuit here atm

i've connected the negative terminal of my power supply to the "ground" symbol it has as it seems the only possible option really. If i understood wat you said correctly then im fine with having it connected as such right? if not where should the negative terminal attach to?

Dr. Slack

Mon Jul 09 2012, 03:46PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes, that's fine

lewylln

Tue Jul 10 2012, 05:32AM

Registered Member #3358 Joined: Mon Oct 25 2010, 09:47AM
Location:
Posts: 20

this circuit (

is still really annoying me. does the watt value on the resistors impact the performance of the inductor discharging into the capacitor bank by any chance?

Tetris

Tue Jul 10 2012, 05:52AM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Ah, I remember when I was a noob... I'm not a noob but I'm still novice at it. :P those circled symbols are one of the first ones I learned. It symbolizes "ground" or the point of zero volts, lower potential (voltage) than the source, or the negative terminal of a battery (though technically, it would be the positive, however, that's another story). It simplifies schematics, when its large, otherwise there'd be so many lines that over-complicates a circuit. :3 hope that was simple enough.

Mattski

Tue Jul 10 2012, 06:40AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

lewylln wrote ...

this circuit (

is still really annoying me. does the watt value on the resistors impact the performance of the inductor discharging into the capacitor bank by any chance?

The watt rating is a measure of how much power the resistor can safely dissipate before overheating. Power dissipating in a resistor is equal to the voltage across it times the current through it, and if you remember that V=I*R this means P=V*I=V^2/R=I^2*R. There's another rating to consider which is the voltage rating - at a high enough voltage there is some risk of failure even if the power rating is met.

The 1M resistor in the voltage sense section will have 500V across it which is pretty large, and it will be dissipating about 1/4W when the capacitor is charged to 500V. To calculate the exact voltage you also technically need to account for the 10k resistor and 12k resistor in series with it, but because the 1M resistor is so much larger it will be dropping most of the 500V and most of the power. Typical rated voltage for 1/4W resistors is about 250V, so to be on the safe side if using 1/4W resistors you could split the 1M resistor into two 500k resistors in series which would each be running at 250V and 125mW. You could also use a bigger resistor, maybe 1W or so.

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