# voltage divider

Discussion in 'The Projects Forum' started by prashantbhurat, Aug 3, 2014.

1. ### prashantbhurat Thread Starter New Member

Dec 12, 2011
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Kindly help!!
In design of 400kv impulse voltage dividers what are the crieterion for selection of resistors i.e. wattage and capacitors i.e value of capacitance, voltage rating.

2. ### to3metalcan Member

Jul 20, 2014
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400kV?! What's the application?

3. ### prashantbhurat Thread Starter New Member

Dec 12, 2011
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design of 400kv impulse generator

4. ### to3metalcan Member

Jul 20, 2014
228
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This depends entirely on the number of stages you're using. In most designs, the resistors don't dissipate the pulse...they only need to be rated for the charge voltage/current. Likewise, you're much more likely to be using lots of capacitors that are individually handling relatively small voltages. Theoretically, you can use smallish components...you just need LOTS of them.

Is this a personal project?

5. ### prashantbhurat Thread Starter New Member

Dec 12, 2011
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Theoritically i designed to use 80stages with 20kohm resistor, bur what should be the wattage of these resistors??and capacitors 400kv/80stages =5kV rating, but whar are the other design criterias??

Apr 5, 2008
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7. ### prashantbhurat Thread Starter New Member

Dec 12, 2011
8
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Thnx for the concern...but this is just theoritically to be done, for which i need the deisgning criterias

8. ### to3metalcan Member

Jul 20, 2014
228
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The resistors need to be able to handle P = charge voltage * current, since the caps will start out looking like short circuits when they're un-charged. Current will be 5000V / 20000 ohms = .25A. .25A * 5000V = 1250W (!). Theoretically, only the first stage's resistors need to be that big, since the successive stages are in series, and will see less current...on the other hand, taking chances at 5000V (let alone 400000) is a good way to end up with fire, explosion, death, etc.

Jul 27, 2014
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to3metalcan likes this.
10. ### to3metalcan Member

Jul 20, 2014
228
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(Bertus's concern is because you posted this in the projects forum, which is for things you actually plan to build. I would've said the same thing, but I'd already glanced at the prices of commercial units--tens of thousands of dollars--and decided you probably weren't really building one.)

11. ### to3metalcan Member

Jul 20, 2014
228
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...I mean, unless you're part of a terrorist cell...I think they use these to set off nukes! In that case, 5W ceramic resistors will do GREAT.

Apr 5, 2008
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13. ### prashantbhurat Thread Starter New Member

Dec 12, 2011
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Thnx...Since all the stages are same, the voltage shall be equally divided across all the components. so each stage has to handle a max of 5000v. and this being an impulse voltage divider, do the resistors be of that higher rating??... This is case of design of divider n rectifier for Impuse generator(personal theoritical project)

14. ### to3metalcan Member

Jul 20, 2014
228
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An uncharged capacitor is a short circuit. So on startup, your first 20K resistor is dropping the entire 5000V...only for a split second, then the capacitor begins to charge and offer more resistance. But it has to be able to withstand that turn-on pulse without incinerating or going open, and if the first cap goes leaky, it needs to be able to hold up indefinitely. Theoretically, the other resistors don't ever see that much voltage, but that's assuming absolutely everything goes according to plan...if that first cap reduces in value due to operating stress, or fails open, the next resistor is going to receive much more stress. This is why special liquid-filled resistors are used in practical designs, sometimes.

15. ### to3metalcan Member

Jul 20, 2014
228
23
Also, can you explain why you're worried about wattage if this project is purely theoretical?