High Current.

Things at the limits.

Re: High Current.

Postby chrismb » Fri May 20, 2011 12:34 pm

battery_var_high_current.jpg
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Re: High Current.

Postby johnf » Fri May 20, 2011 1:56 pm

Chris Doug is right
you need a non-saturating inductor in the drain (or source) of the fet to slow the inrush to the cap bank

my earlier post still stands I assumed that you wanted DC all it needed was a big bridge and caps on the output.
how many caps depends on the inductance of your Helmholtz coil. Those big torroidals are about three volts per turn (from memory)

If you guys have a need for big ferrite cores just latch on to a welder repair shop most of the modern ones are the inverter type and companies like Millar dont give out circuit info even to their agents. You get a nice core set of "U"s about 6"cubed thats good for about 5kW
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Re: High Current.

Postby chrismb » Fri May 20, 2011 5:21 pm

johnf wrote:my earlier post still stands I assumed that you wanted DC all it needed was a big bridge and caps on the output.
how many caps depends on the inductance of your Helmholtz coil. Those big torroidals are about three volts per turn (from memory)
Are you (/were you) talking about caps for smoothing 50Hz, then? I was wondering whether it'd be cheaper just to buy those multi-Farad suckers available for car audio, but still think the voltage ripple would be larger than a DC chopper.

OK, so if I were to attempt the battery chopper circuit, how much inductance should I stick in the line, and should I put it before the FET, or between the FET and the caps? Frankly, I'm still a bit confused, because I thought the idea was to get rid of as much inductance as possible and then there would not be severe voltage spikes? The maximum in-rush current is going to be a function of the internal resistance of the battery, which is comparable with the total current pull off the caps that I am trying to achieve anyway, so I'd have thought anything inductors could do at reducing in-rush would be comparatively minimal.
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Re: High Current.

Postby johnf » Sat May 21, 2011 1:34 am

Chris yes the car audio ones will help

you have to limit the dI/dT --simple maths ie you want to rise to max current at around the end of your on period
max current being below what your Fets can handle --check data sheets!!--average current that you need about half max

I have always used 4000uF per amp when designing linear supplies
but in your case you can ease off a bit due to the inductance of the load
so you have to know what L your coils have and solve accordingly.
multiple caps are much better to keep ESR in check and also dissipation

I would think a few ceramic caps then electrolytics in that order from the FET /s will be the order
note the ceramics need to be very close to the FETs and multiple uFs of them then the electros where ever you please

done in this order the ceramics take the high Freq and the electros do what they do well
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Re: High Current.

Postby chrismb » Sat May 21, 2011 2:37 am

IRL3713 is rated to over 1000 A surge. I don't see how a surge bigger than that could come from the battery direction (bigger than its cranking current), so does that mean it should be OK?

Please be gentle with me (!) - I've never properly understood inductive circuits so don't really understand how inductors feed out higher currents/voltages than are 'nominally' present.


EDIT: Oh, hang on a moment. I just noticed a 'note' next to the 1040A rating on the data sheet; " Calculated continuous current based on maximum allowable
junction temperature. Package limitation current is 75A." Not seen that on a data sheet before.
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Re: High Current.

Postby johnf » Sat May 21, 2011 3:01 am

Chris
now we are getting somewhere
And you do not want to go there
you do not want to go within 50% of what the data sheet says (My maxim)


now its not so easy!!!!!!!!

go linear and suck up to large bridges and many caps
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Re: High Current.

Postby chrismb » Sat May 21, 2011 3:31 am

Had a look-around and IRFS3004 with 240A package limit can be had for £22 for 10 pieces, off RS. So 3 in parallel should give me 720A, and if they blow up I'll do it again with six!?

...This is beginning to look like no cost-benefit over buying a proprietary 50A supply and putting on more wire turns!? The enthusiam begins to fade.....
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Re: High Current.

Postby johnf » Sat May 21, 2011 4:29 am

Chris

hence my comments on the transformers variac bridge and caps ETC

PS this is how we cancelled earth's magnetic field using Helmholtz coils before taking a superconducting magnetometer into its super conducting mode ie it remembers this field as the reference field--this used for measuring paleomagnetism of old rocks

Even I will bow down to Epay for a 1kW 30kV supply for US$250
I cannot make it for double that
FWIW
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Re: High Current.

Postby Doug Coulter » Sun May 22, 2011 9:53 am

Chris, John is telling you right in every respect. You can get many times the rated cranking amps from a battery -- only limit is series resistance. Without a series inductor (and with that you need a catch diode) you'd be trying to fully charge the capacitor to full voltage on the rising edge of the very first gate pulse == infinite current more or less required (ESR of the cap and fet the only limit). The inductor is needed to allow the fet to switch full voltage before much current is drawn, and then the duty cycle will control the output voltage. But there's just no need to make a fancy switcher when a few turns on a re-wound MOT will do it fine, be controllable, quick, cheap, easy, and reusable for other things once the experiment is over. I use that sort of thing to power my evaporator.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: High Current.

Postby chrismb » Sun May 22, 2011 12:57 pm

I have been persuaded to have lost interest in a high current self-build....

...but if I do ever choose to turn my own one-or-two turn transformer in the future.. well... funny that one should come along, in need of a burnt-out bobbin cutting off!! :)
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