Electronics > Beginners
On a 250V capacitor or a CRT, is the negative terminal -250V?
Starglider:
Thanks Arjunan. I don't really want to get off track of the point of this which is explaining how discharging a CRT or capacitor Works in terms a 10 year old child could understand. They wouldn't need to ask what the reference point is.
So let's rewind and reset.
Really the key is I want to explain WHY short circuiting a CRT does not require any connection to ground.
I had thought that I understood that when you short circuit the positive 250 and negative 250 of the cap it equals 0 volts. But apparently that is not what is happening. So if the positive terminal of the CRT is storing (for the sake of argument) +250 volts and the negative is 0 volts, why does connecting them together still equal 0 volts?
Mathematically that doesn't make sense to a ten year old because 250 plus 0 still equals 250. And 250 volts is dangerous.
Do you kind of get the point I'm trying to illustrate? The ten-year-old would ask where does the 250 volts go to if there is no ground wire connected? "It can't escape." That really is all I need to create a really basic analogy of. Where does that 250 volts disappear to?
Starglider:
--- Quote from: Brumby on May 31, 2019, 05:20:11 am ---OK. Now things start to get "interesting".....
Building on from the bucket example, a capacitor is like two buckets sitting on a bench, each of them half filled with water. When you charge up a capacitor, it is like taking water out of one of these buckets and putting it into the other. Connecting a wire is like making a path between them for the water to flow. A hose filled with water is like a wire and if you put both each end into one of the buckets (and keep them below the water line), then water will flow from the one with a higher level into the one with the lower level.
Notes:
* A hose without water inside it will be like the insulation around a wire. It won't let anything flow.
* When the buckets are at the same level (half full) there is no energy stored - as the energy can only come from a difference in their levels.
* When considering this water analogy, please understand that all analogies will have difficulties explaining everything and they usually come with "conditions". This bucket one requires that each capacitor has its own two buckets that must stay together and you can't have a single connection to another pair of "capacitor" buckets and have any flow.
--- End quote ---
This is great.
So let me just put this in my own words and you can tell me if it is still a correct understanding.
When I buy a brand new 250V capacitor from digi-key and open the package, could it be said that it is equivalent to 2 buckets, each containing 125 liters of water? We can call on bucket Mister negative can the other bucket Mister positive.
Then when I solder the capacitor to a circuit and turn it on, it is like pouring the water out of one bucket into the other bucket. Now the negative bucket is empty and the positive bucket has 250 liters of water?
When I turn the power off the water gets trapped in the full bucket.
But if I connect a metal hose the water shoots out of the full bucket until the negative bucket is half full and both buckets again have 125 liters inside?
Brumby:
Yes, you've got it.
... and the thicker the "metal hose" as you put it is, the more current can flow and the quicker the discharge will happen.
Starglider:
Many thanks!
Brumby:
I can see more installments in this series in the not-too-distant future.... 8)
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