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Registered Member #2317
Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27
Would it be acceptable for me to operate this circuit without an isolation transformer if I install a fuse on the input?
If not, I'll probably have to ditch this design, because I don't think I'll be able to find a sufficiently powerful isolation transformer for a reasonable price. I already have a Variac and a microwave oven transformer along with a bridge rectifier, I might just go that route.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
With an isolation transformer, accidental electrocution would require you to touch 2 parts of the circuit at the same time, instead of just one place and ground.
If you -must- make coilguns with multi-kJ energy, can you defer the fast rate of fire requirement? Almost all of the power coming out of the charger goes into heating your coils. What is the coil temperature rise from a single shot?
Registered Member #2317
Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27
The cap bank is actually for an electrothermal system of accelerating a projectile. I'm not sure what you mean when you say that most of the charging power, in the case of a coilgun, would go into heating the coils. If I were making a coilgun, I would not involve the launching coil in the charging circuit.
But, yes, I could accept a less than "fast rate of fire," I just don't want to wait five minutes to charge the caps when I'm testing something.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Jordan aka Axiom wrote ... The cap bank is actually for an electrothermal system of accelerating a projectile. I'm not sure what you mean when you say that most of the charging power, in the case of a coilgun, would go into heating the coils. If I were making a coilgun, I would not involve the launching coil in the charging circuit. ...
Sorry -- a few weeks ago you were working on a coilgun, and I thought the new charger was for that.
My point about heat: Practially all the power output from cap charger is stored in the caps. Then (in coilgun application) 90% of the stored energy ends up as heat in the coil. (A little bit heats the cap, switch, interconnections, and projectile, and some actually goes into projectile motion). Suppose you discharge a 5000 J cap bank into a coil containing 1 lb of copper. After 3 shots the coil will be hot enough to boil water. Another view: if charger delivers average power of 100 watts, and bank is fired as soon as it's charged, the load is effectively a 90 watt electric heater.
I appreciate your point about charging time. My can crusher took a few minutes to charge with its original semiconductor-free voltage-doubling PS (with two thermionic diodes!). After switching to a new charger that did the job in 10 seconds, it was no longer tedious to do effects-oriented tests varying lots of parameters.
Registered Member #2317
Joined: Thu Aug 27 2009, 01:45AM
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Posts: 27
I'm sorry, perhaps I chase too many figments and delusions of grandeur. But, hopefully this system will be able to beat 10%.
In my attempt to understand the practical benefit obtained from an isolation transformer, would not a capacitor in series with the input also isolate the circuit from being physically connected to mains power?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I belive that with the capacitor isolation you would still only have to touch the output and ground to get electrocuted.
I think that it would be alright to go along with your original scheme if you protect yourself at all times. Make a nice enclosure for your charger so that there is no chance of you touching anything bad when it's operating. Sure, an isolation transformer would be safer, but really, how safe is discharging thousands of joules into strange devices !
As long as you treat line current with respect and take all precautions, you will be plenty safe without the limitation of an isolation transformer.
Registered Member #2317
Joined: Thu Aug 27 2009, 01:45AM
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Posts: 27
Found something interesting about it on Wikipedia
"Capacitors transmit information and energy by an electric field. As it is possible for some capacitors to fail short circuit, there are special ratings for capacitors used for isolation, such as "Class Y"."
Just another update, I found out that thermistors are commonly used for inrush current limiting. I'm planning on using a few of those on the input to the system to limit any large current surge through the capacitors. Now that I have four giant resistors, I guess I can use them to safely discharge the bank if I need to.
Registered Member #2317
Joined: Thu Aug 27 2009, 01:45AM
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Posts: 27
GhostNull wrote ... In voltage multipliers the current is limited and determined by the capacitor size...
Can I calculate the maximum output current of the multiplier using the equation that determines capacitive reactance using the capacitance of an individual capacitor in the multiplier?
Cx = 1/(2(pi)(Hz)(Farads))
Cx = 1/(2(3.1415)(60)(.001))
Cx = 2.653 Ohms
I = Volts / Resistance
I = 120 / 2.653
I = 45.23 Amps
Does this mean my voltage tripler, with 1000uF capacitors, 120V 60Hz input, would draw a maximum theoretical current of 45.23 amps?
If this is true, then I need to find some smaller capacitors to use. Thanks in advance for your help.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Jordan aka Axiom wrote ...
GhostNull wrote ... In voltage multipliers the current is limited and determined by the capacitor size...
Does this mean my voltage tripler, with 1000uF capacitors, 120V 60Hz input, would draw a maximum theoretical current of 45.23 amps? If this is true, then I need to find some smaller capacitors to use. Thanks in advance for your help.
Inrush current of 45 amperes would be no problem if it only lasts for a couple of cycles -- no worse than switching on a 500W halogen work light, or starting a MWO at a moment close to zero-crossing of line voltage.
Your inrush current problem has nothing to do with the value of the "tripler" capacitors. Your main capacitor bank (68000 uF?) is connected to the mains with 3 diodes in series, and would want over 1000 A trying to charge to 1x the line voltage in the first cycle or two. You should be able to discover ways to mitigate that.
[edit] As for designing the tripler ... start by specifying an output current requirement. ( How long would it take to charge your main bank at 1/2 amp? ) Then follow a link somebody gave in this thread, to an online calculator to get C values.
Here is a variant that gives you true isolation and current limiting, with no exotic transformers. Run a MOT primary on 12 to 24 VAC, and use a simple half-wave rectifier on the secondary (which can still deliver a lethal shock). Current limiting comes from the MOT leakage inductance.
Until you are a lot more proficient with electronics, how about choosing a charger whose no-load output voltage does not exceed your cap bank voltage rating.. Accidental overvolting from negligence, or failure of meter, or failure of regulator circuit could cause your weakest capacitor to literally explode.
Registered Member #2317
Joined: Thu Aug 27 2009, 01:45AM
Location:
Posts: 27
Klugesmith wrote ...
Jordan aka Axiom wrote ...
GhostNull wrote ... In voltage multipliers the current is limited and determined by the capacitor size...
Does this mean my voltage tripler, with 1000uF capacitors, 120V 60Hz input, would draw a maximum theoretical current of 45.23 amps? If this is true, then I need to find some smaller capacitors to use. Thanks in advance for your help.
Inrush current of 45 amperes would be no problem if it only lasts for a couple of cycles -- no worse than switching on a 500W halogen work light, or starting a MWO at a moment close to zero-crossing of line voltage.
Your inrush current problem has nothing to do with the value of the "tripler" capacitors. Your main capacitor bank (68000 uF?) is connected to the mains with 3 diodes in series, and would want over 1000 A trying to charge to 1x the line voltage in the first cycle or two. You should be able to discover ways to mitigate that.
As for designing the tripler ... start by specifying an output current requirement. ( How long would it take to charge your main bank at 1/2 amp? ) Then follow a link somebody gave in this thread, to an online calculator to get C values.
Here is a variant that gives you true isolation and current limiting, with no exotic transformers. Run a MOT primary on 12 to 24 VAC, and use a simple half-wave rectifier on the secondary.
I thoroughly examined the link to the calculator, however, the calculator uses a design very different from the simple voltage tripler shown above. The one in the link requires 6 capacitors for a three-stage multiplier. I'm assuming that the design shown above does in fact triple the voltage, ideally. I have thought through it and read descriptions of it, and it seems it would work fine, but it is different from the calculator so I'm attempting to calculate the current manually.
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Voltage Tripler - Cap Charger
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