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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
I will try to make this one concise and to the point LOL!
I am throwing around the idea of building a voltage doubler to help me charge my cap bank up to its target voltage. If I do, I would also place a small variac on the 120v input of my Neon Sign transformers to allow precise adjustment up to my target voltage, and then by turning it back I could allow the voltage equalizing resistors across each seriesed cap to slowly bleed back down voltage, allowing me to adjust up and down.
If I choose to make a Full Wave Voltage doubler (and I will include a picture below of a circuit I found online that I would like to model after) for between my 90mA 4kV AC NST source and my 244uF 6,400 volt capacity Electrolytic cap bank (16 seriesed 400 volt 3,900uF caps), how would I go about choosing the F value of my 2 caps in the V-Doubler? I am assuming they will be much lower than my 3900uF caps in my cap bank and a much higher voltage...maybe nonpolar film caps or something higher quality than my actual electrolytics in my bank...
Especially if they are supposed to charge fully on each swing of the AC sine wave....
And also, I am having a hard time understanding how you can connect the 2 caps in the V-doubler right to my capacitor bank as its load.....I would think the bank would be sucking the V-Doubler's caps dry way before they have a chance to charge.....at least untill they too pick up voltage...
Thanks a ton guys...you guys always give me good ideas!!!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The caps are not completly discarged on each cycle (if that were you case you would have a huge ripple on the output), and in an ideal circuit would always be charged to Vin*1.4. As to to the size of the caps necessary, the bigger the better.
Actually, your best bet would be to just split your bank in half, and use the bank itself as the caps.
Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
WHOLLY CRAP!!!! THATS A GREAT IDEA!!! I can't believe this, if I just split the bank in half and used each half as one of the two capacitors in the voltage Doubler, then used the seriesed "pair" just as I was going to to run my flashlamp, then I would be set!!! I could then use the variac to make sure I don't overvolt, and since I am working with just separate halves of the bank at 3200 volt capacity, and my NSTs are 4kV i shoudl have NO problem pulling up to full charge on the bank as a total!
I never would have thought of that rock on man!! thanks!!!
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The best solution has already been mentioned. It should be noted that this solution is only feasible if you dont allow the current to go too high to trip the circuit breaker. In the case of NSTs you are fine.
I just wanted to mention that you can figure out the average voltage drop across the doubler capacitors by figuring out their capacitive reactance (in ohms) and using ohms law (because you know the current, more or less). This shows why high frequency input to a multiplier is much more effective than 60hz when using small capacitors (Xc goes down with increasing frequency).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You do know that most NSTs have a grounded centre tap? If you use a NST like this with the voltage doubler circuit, you may end up having to float everything else as a consequence of that. You will *not* be able to ground the negative rail of your DC supply, as you showed in the schematic above.
If you're lucky enough to have a NST with a totally isolated secondary then there's no problem.
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How to choose values of Voltage Doubler's Caps?
