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Need to make a cheap but safe current controller to test with mains (240v)

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jpoopdog

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Im going to start working on my coilgun again, however, i need a faster charger. to reach the 400v fast i want to use mains to charge the capacitor bank, via an AC doubler, works fine now on a 5W inverter, but takes forever to charge the back.
On the other hand mains scares the hell out of me, so i want to restrict the output to around 50W. I absolutely do not want to run a charger of mains straight, even if its with a fuse/breaker, im also concerned i might damage the capacitors because of the high currents. Anyway, this is mostly just irrational fear and i acknowledge that, but still ill never run something ive made off anything other than inverted 240v, not unless i wont directly touch it or have to leave it running unsupervised.

To accomplish this, what do i do? is there a testing circuit i can buy so i can screw around like i do with my short protected PSU, which also has settable maximum current, or do i simply use a heavy duty resistor and a fuse?
Or do options for 1:1 type transformers exist that output 240v but with limited output current capacity, I have other things here and there id like to test with current limited mains incase of a short or sudden drop in resistance for other reasons, or in certain motors to delay overheating.

Im not incompetent, or unknowledgeable, mains is just something im afraid of and would like to avoid. Im fine working off a 300W inverter, even though the electrocution risk is the same just about, but i simply cant do mains.

(the 300w inverter considers the bank a full short even at a 600W peak allowance, so it wont charge it, and its my good car inverter so i dont want to risk anything)
 

Tony Stewart

Well-Known Member
Most Helpful Member
big e-Caps have two important ratings. ESR and max ripple current. ...Often in the few amp range per cap. REport Cap PN or check specs.


Inverters don't like big currents like 300W light bulbs on a 300W inverter , which is why the 600W peak rating.

The tungsten filament changes R with temp over a 1:10 range from cold to normal voltage rating. however bulbs heat up in a few dozen cycles so the current will drop 10:1 worst case if started on peak voltage.

I would suggest the cheapest solution is use a 60W light bulb in series with your voltage doubler, so worst case appears as 600W load resistance when cap bank is discharged and light goes out when the bank is charged.

Now assuming 600W/240V= 2.5A as the peak ripple current and your caps can handle this, it should be fast and cheap. If you have 2 parallel caps, which can handle 3A ripple current , they wont heat up but make sure the diodes can handle >10A for margin and no heatsink,

it is possible to consider more like 200W bulbs if you have a zero crossing trigger Triac switch, then surge currents will be much lower.

*much more details on voltage doubler needed to be given from you to recommend this.

Charge time on average is RC=T for average R of bulb and C for caps to reach 60% full range.

or. rough estimate T(sec)= 240Vac* C(Farads) / Pbulb(W) * 10 to get >90% full charge. < guestimate>. assuming voltage doubler also doubles charge time.

How many Joules (Watts-seconds) do you want to store from online charger, determines C sizes and ripple current? what is your coil resistance??
, which should be >> ESR of Caps.

e.g. 1000 uF 600V caps are ~ $60 when used at 400Vdc without doubler off line with full bridge on 240Vac gives 330Vdc, doubler should give <630V from 240Vac sine grid, but from square wave inverter, you get 400Vdc , I believe.

summary:
your cap pn's?
bridge ?
coil R?
required charge time?
desired coil power? or energy? or essentially velocity and weight of slug, 1/2mV^2
 
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