# 120VAC to ~240VDC?

Discussion in 'General Electronics Chat' started by polandbear193, Apr 8, 2009.

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1. ### polandbear193 Thread Starter New Member

Apr 8, 2009
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How would I create a voltage doubler w/ an input of 120VAC (from standard wall outlet) to ~220VDC (at the maximum, 300VDC)?

This is to charge 4400uF 300VDC caps.

Last edited: Apr 9, 2009
2. ### beenthere Retired Moderator

Apr 20, 2004
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That would be unwise. You should always leave some voltage headroom in the capacitor charge. Going right to the limit leaves no margin of safety. Those caps are expensive to replace.

The best way to proceed would be with a 1:1 isolation transformer to safely isolate the charger from the line. When rectified and filtered, the voltage will be on the close order of 175. If you can find a transformer with an 80 volt secondary, you can operate it in reverse (apply 120 VAC to the 80 volt winding), and get 170 VAC out the primary side. That will rectify to 250 VDC.

3. ### polandbear193 Thread Starter New Member

Apr 8, 2009
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Thanks! That makes sense (or at least my electrician friend said so =P).

Its either 2400uF/450VDC Cap or 4400uF/300VDC for the same price. Would it be worth it to risk 4400uF/300VDC w/ a 250VDC (basically a headroom of ~50V), or do a voltage doubler and get the 2400uF/450VDC?

P.S. We're trying to build a low-cost Rail Gun for a physics project.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
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Just by preference, I would use the lower voltage with larger conductors. At whatever voltage you decide to go with, each capacitor will carry a lethal charge. Make sure things are well insulated.

5. ### polandbear193 Thread Starter New Member

Apr 8, 2009
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My friend explained that when we rectify something, it creates oscillations, and if it oscillates too high, it will fry the caps, too low, it will only charge to the minimum voltage and that this is a problem. How do we reduce the oscillation, or is he totally wrong?

6. ### leftyretro Active Member

Nov 25, 2008
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He is totally wrong!

Lefty

7. ### polandbear193 Thread Starter New Member

Apr 8, 2009
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He says "I was using a resistor instead of a capacitor the WHOLE TIME!!!" (in a simulator)

hahaha =P.

What kind of diodes should we be using to rectify the AC current from the outlet to charge the cap.?

Also, is that correct?

P.S. By the way, our plan for the cap. bank is use 10 4400uF 300VDC caps (charging w/ the above circuit), and release this amount of power into 2 aluminum rails which should shoot out a nail (or other projectile in between the aluminum rails), creating a rail gun. Exciting!

Next challenge is to make this portable =P.

Last edited: Apr 8, 2009
8. ### thingmaker3 Retired Moderator

May 16, 2005
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Be sure you count your fingers before and after. (This assumes you don't put your eye out, of course.)

9. ### polandbear193 Thread Starter New Member

Apr 8, 2009
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We'll be fine. First test run will be done in a controlled place w/ us behind a steel frame and using cameras to see.

10. ### polandbear193 Thread Starter New Member

Apr 8, 2009
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Bump Up My Post

11. ### italo New Member

Nov 20, 2005
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If you need a short time discharge then use a pump to double the voltage or tripple if you can afford the caps. but why rectify it? then just fire an SCR into the load. and forget about 1:1 transformer safety at 28 you can feel it. these are dangerous levels lethal realy so just be carefull.

May 16, 2005
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