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Current Limiting Resistors/Capacitor Charging

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MikeT1982

Tue Aug 21 2007, 09:51PM

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

Lets say you have a very "stiff" non-shunted step up transformer that you want to use to charge a capacitor bank with for say a laser flashlamp supply. A transformer that is "stiff" enough that if you didn't have any current limiting resistor it would blow fuses, overheat, ect. with the turn on "short" it would feel from the dead caps (unlike the NST's I am using needing no current limitng resistor at all).

Would you simply use ohms law formula to choose a current limiting resistor that if you conected it by itself to the transformer would just about "max it out"? Like if the tranformer could supply 2 amps, is it common to choose an ohm rating that would be just under 2 amps draw if you just connected it by itself?

I'm wondering what they typically do in the industry in commercial setups, like I saw flashlamp pumped lasers in the 5 or so hertz range....that makes me think they use a VERY stiff chargin supply...I didn't even know caps could stand that rapid charge scenario. I gues they aren't like charging batteries LOL. I'm assuming if they discharge that fast, they may also be able to charge just as fast....

Marko

Tue Aug 21 2007, 10:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

You can charge caps as fast as you like, as long as you can provide enough power (P = E/t).

NST's have magnetic shunts which assure leakage inductance always limits the current to safe level.

You can use an inductive ballast to limit the current for any transformer as well, and is probably best for quicky solution, but is bulky and causes poor power factor.

Industry seems to mostly use SMPS's.

Steve Conner

Tue Aug 21 2007, 10:25PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, and before SMPS I think they used DC resonant charging. What this means is that you charge the capacitor off a high voltage DC supply, using an inductor and diode. It was developed to power the first radar sets that had to charge a pulse-forming network maybe 400 times per second.

As soon as you empty the capacitor into your flashlamp (or radar magnetron or Tesla coil or whatever) a pulse of current comes through the inductor to recharge it. The diode then traps the voltage in the capacitor so it stays there until you are ready to fire again.

Unlike a current limiting resistor, which must burn 50% of your input power as heat, the inductor is nearly 100% efficient, and as a byproduct of that, it charges the cap to nearly twice the DC supply voltage.

The regular DC resonant charging setup tries to refill the capacitor as soon as you empty it. For situations where that caused a problem, you had a version called command resonant charging.

MikeT1982

Tue Aug 21 2007, 11:07PM

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

Nice nice. Thanks! What does SMPS stand for, is it Switch Mode Power Supply? I've heard about them I will look them up.

Myke

Tue Aug 21 2007, 11:27PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Yes, SMPS stands for Switch Mode Power Supply.

Dr. Slack

Wed Aug 22 2007, 07:09AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

One of my favourite ways to control charging current into capacitors is with the use of a filament lamp in series. It's almost never used commercially because of the 50% inefficienycy that Steve mentioned, and the fragility of the lamp. However, neither of those are real objections for home stuff.

As a power resistor, a filament lamp has a number of advantages.

a) cheap and readily obtainable (for the moment, until compact flourescents drive them off the shelves in a decade or two's time, though halogens should stay around even as commodity lamps vanish)
b) a rated power dissipation you can trust, and use
c) a strongly varying resistance as a function of current, in a charging scenario they behave as a quasi-constant current source, and are practically a dead short at low current
d) it gives you a visual indication of where you are in the charging process
e) there's so little to go wrong, even a diode and inductor are complex in comparison, and do you really want to lash up a SMPS?

for use with a step-up transformer, put it in series with the primary.

Obviously the filament starts from cold, so there is an initial high current inrush, but that is no worse than switching on the lamp itself on the primary side fuse and switch, just make sure that the secondary rectifier has sufficient I2t to cope with the inrush (most rectifiers have a pulse as well as a continuous rating to handle inrush)

Marko

Wed Aug 22 2007, 12:16PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

When rapid charging is needed as it appears in mike's case it may come to several kilowatts being pulled from mains in a short time, and using bulbs would be problematic in that case.

Otherwise I also think they make great ghetto cap charger.

Steve Conner

Wed Aug 22 2007, 01:07PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, there's always electric heaters, toasters, hair dryers etc... they all make good ghetto ballasts. Just don't try operating the switch on the hair dryer when you are running two in series off 600V DC >_<

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