Capacitor Charging vs. Resistors Drawing dilemma. / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Capacitor Charging vs. Resistors Drawing dilemma.

Move Thread

LAN_403

MikeT1982

Fri Jul 27 2007, 12:05AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Ok, as I tend to do all of the time I was sitting here and pondering my Cap charging circuit. I am using 3 paralleled NST's each rated at 4kV@30ma, for 4kV@90ma.

Now what I used to think was that a transformer, like one of my NST's rated at 4kV@30mA would be able to provide 30mA of current at 4,000 volts. However I have heard people say that 30mA is only what the transformer would produce into a short circuit.

So that talk puzzles me and leads to my dilemma. My cap bank will have one 25,000 Ohm voltage equalizing resistor across each capacitor. I have 16 capacitors and therefore 16 resistors. Each capacitor is rated at 400 volts, for 6,400 volt capacity. However, with rectifying my 4kV NST's I multiply by 1.4 to get about 5657 volts where the caps theoretically would level off.

The way I see it is the 3 paralleled NST's will have a "goal" to charge my cap bank to that 5657 volts at which it will level off (I'm hoping it gets to at least 5kV for my flash tube!). When I first energize and the capacitors are discharged, the NST's will see a dead short and push the full 90mA right? But as the voltage climbs if the talk I read about is right, the current capability of the NST's will drop off as the capacitors charge.

So that I am thinking must be WHY capacitors charge slower the higher to their charge goal voltage they approach!

So to my parasitic resistor dilemma....all 16 of those 25kOhm resistors give me 400,000 ohms which when plugged into Ohms law equation with the 5657 voltage goal, gives me 14mA of current draw by the resistors.....and that when crunched through Ohms Law equation again gives about 80 watts of parasitc loss to the darn voltage equalizing (and power off safety draining!) resistors.

My big question is, will this 14mA of draw by my resistors actually prevent my capacitors from reaching that 5657 volt goal? Since according to what I have read, the NST's won't be able to supply 90mA at even 4kV....wouldn't in theory there be the point in the voltage climb at which the NST's hit a plateau when the "current delivery at a given voltage" meets my resistors parasitc draw......I just can't wait to try this real world and see where the caps charge to!!

Thank you ahead of time for reading my long winded post!

Simon

Fri Jul 27 2007, 12:31AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

MikeT1982 wrote ...

So that I am thinking must be WHY capacitors charge slower the higher to their charge goal voltage they approach!

Kind of. The reasons why the cap slows in charging is the same kind of reason the NST drops in current output. In both cases it's a resistance (or an effective one).

Your basic, raw cap charging circuit is a cap, a perfect voltage source and a resistance all in a loop. (The resistance is the equivalent of any charging resistors and internal resistances of the real voltage source.) Any voltage from the source is dropped either across the cap or the resistor. If the source is, say, 500V and the cap is only charged to 10V, that means there's 490V across the resistor. Basic ohm's law tells you there is a large current flowing to charge the capacitor, so it charges rapidly.

When the cap is charged to 490V, there's now only 10V across the resistor, so the current is a lot less, so the cap charges much more slowly.

You can get a feel for why the NST voltage drops with the same kind of idea. At high currents, a large voltage is dropped across the output resistance and hence only a small amount is left for the output.

wrote ...

My big question is, will this 14mA of draw by my resistors actually prevent my capacitors from reaching that 5657 volt goal?

Yes. The charging will fall far short of that 5.7kV.

A high voltage supply with a low current capability can be treated like a constant current source, rather than a constant voltage source. You don't need limiting resistors if you're okay with the peak output of the supply.

Bert

Fri Jul 27 2007, 01:24PM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

MikeT1982 wrote:
But as the voltage climbs if the talk I read about is right, the current capability of the NST's will drop off as the capacitors charge.

So that I am thinking must be WHY capacitors charge slower the higher to their charge goal voltage they approach!

Actually, that's only part of the reason. As your capacitor bank charges, the "on time" of your HV rectifiers becomes less and less. The HV rectifiers will pass charging current to the bank only during the peak portions of the AC waveform (i.e., when the transformer output voltage is greater than the bank voltage). This becomes a smaller and smaller portion of the incoming AC waveform as your bank charges. Adding insult to injury, your NST's will also deliver less charging current at higher voltages. Finally, your equilizing resistors are drawing significant load current from the capacitor bank ALL the time. All of these factors conspire to significantly reduce the maximum voltage reached by the capacitor bank. Using a X2 or X3 voltage multiplier in your charging circuit will help increase charging efficiency particularly at higher bank voltages, addressing the first two problems. However, you'll also need to closely monitor the bank voltage to make sure you don't accidently overvolt the bank. Increasing the equilizing resistor values will help adress the third problem.

MikeT1982

Fri Jul 27 2007, 08:45PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Ok cool guys thank you for all the input! If I get significantly lower than 5kV, I may definitely need to encorperate a voltage multiplier...but from what I read they only double, triple ect.... And really since I have no cutoff, I should have went with maybe a 5 or 6 kV NST as a good median compromise. My target voltage is between 5 and 6kV. The cap bank is 6,400 volt max and 7,200 volt surge. Looks like my 4kV NST's might not get me there

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob