High Current.

Things at the limits.

High Current.

Postby chrismb » Thu May 19, 2011 2:05 pm

I'm contemplating setting up a Helmholtz coil to provide a large magnetic field in an experiment. Not 'strong', just several 10's of Gauss needed in a big volume.

I've stacks of 4mm^2 single cable I bought some time ago when Farnell was selling of 100m reels of the stuff at 5 quid (~$7) a reel. The copper alone is around 3kg per reel! So with that I am planning just 30-40 turns or so at ~100 A.

Now, it is the 100A I am wondering how to go about applying in the cheapest way poss. I'm wondering if anyone has any bright ideas.

My 'best plan' to date, because it is cheap and I already have generally all the bits needed: I take a 12 V automotive battery (big diesel one, with high crank current - I hope it still 'lives' not used for a while) and parallel up a few high current IGBTs, and switch them with variable duty cycle, in the 20kHz range, into a big capacitance.

Any advice? Is it this easy, or more difficult than it looks and I should buy a proprietary power supply?
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Re: High Current.

Postby Joe Jarski » Thu May 19, 2011 5:58 pm

Chris, that's in the same ballpark as what I'm trying to use in my evaporation system. I'm modifying an old "buzz box" welder that was given to me for the low ~2V stuff and using my TIG welder for the higher ~20V stuff. It may be another option for you - the old ones tend to be pretty cheap because they're big and heavy compared to the newer inverter types.
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Re: High Current.

Postby johnf » Thu May 19, 2011 6:32 pm

Its real easy guys
you will need farnell 953-2536 500 watt toroidal mains transformer
you now put on 3 turns of 0 or 1 gauge wire for a low volt high current secondary
a simple variac on to the primary to set current a SSR between variac and transformer primary to switch on /off
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Re: High Current.

Postby Doug Coulter » Thu May 19, 2011 9:00 pm

I think Chris needs DC. The first thing that came to my mind was rewinding a microwave oven transformer -- a couple turn secondary plus enough to overcome the diode drops. It's going to take one heck of a capacitor to filter that at mains frequency. And you'll wind up dissipating more power in the low-drop schottky diodes than anything else. I'd begin with twice the turns and half the amps on the main magnet though. Huge capacitors can be found surplus, or at full price for tweaker auto stereo houses (farads).

An inductor good enough to pass 100 amps into a capacitor (which I assume is what you meant as you can't switch into a capacitor directly) is going to be a major project on its own. Even the pros in the solar business (or big battery eliminators for Winnebagos) don't even try it, but break the problem into about 25 amp chunks for switchers and parallel those.

Big electroplating houses use brush-type generators (like the one I use from a fighter plane) for this, run off a motor that runs on the mains. Most efficient way probably if you don't want to make a synchronous switcher (to get rid of all diode drops, and an IGBT has more drop, so fets-only for that one). IGBTs are much too lossy at low voltages due to their minimum drop being large.
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 » Fri May 20, 2011 1:00 am

Doug Coulter wrote:I think Chris needs DC. The first thing that came to my mind was rewinding a microwave oven transformer -- a couple turn secondary plus enough to overcome the diode drops. It's going to take one heck of a capacitor to filter that at mains frequency. And you'll wind up dissipating more power in the low-drop schottky diodes than anything else. I'd begin with twice the turns and half the amps on the main magnet though. Huge capacitors can be found surplus, or at full price for tweaker auto stereo houses (farads).

Absolutely. I don't have experience of TIG welders and I get the impression, from the specs, that AC fluctuations are not that important to the makers nor the welders, so I don't really think I will get the purity of DC I want.

So that's why I thought I'd start with a DC supply and chop it. A 20kHz 'chop' of 100A into 12 parallel 6800uF should give me only a few 1/100th's volt fluctuation. Probably more fluctuation from the chemical behaviour of the battery. Maybe it will be better, even, than proprietary power supplies - I don't really know what their voltage ripple is, hence my Q's.
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Re: High Current.

Postby Doug Coulter » Fri May 20, 2011 8:29 am

Well, building a 100amp low ripple switcher is for sure a project. It's one on my list for my solar panels (where I actually need about twice that). The inductor(s) required between the switching and the caps are pretty daunting however. It's no small thing to make a decent inductor that size that won't saturate or be super-lossy (skin resistance in the wire) so no one does that. As in good luck finding a big enough ferrite core (kilograms) and making your own litz (multi conductor for less skin effect loss) wire of the required size. I wrote a perl script to calculate the desired value for a buck switcher, based on that power supply book JohnF recommended (and I do too -- it's the straight skinny on these). That may or may not be enough C - at these kind of currents, it's more the ripple current rating of the caps than the uF -- the uF being a lot easier to get than to get it with low parasitic R and the resultant heating in the capacitors.

You obviously cannot hook a 20khz sq wave to a capacitor -- instant smoke someplace -- you know that, right?

Every example of that around here uses multiple sets of fets and inductors to get to those kind of current levels, not just one switch block/L/C -- you just can't get the parts to make that in a single anyplace. You can't use IGBTs reasonably with their very high saturation voltage drop -- well over a volt, which would be wasting 100w per device -- they are only used in high voltage designs where that drop isn't so significant as a fraction of total power. The much lower effective on resistance of FETs at low voltages make them the clear winner in a design where they fit.
It's only at mains voltages and above that the IGBTs show any advantage -- at 12v FETs are perhaps 10-100 times less lossy -- not a small factor. But at the high currents, it's going to be the required inductance that will be very very difficult to assemble.

Which is why a few people here, who all really know their stuff, say "Don't bother trying that". It's not that it can't be done. It's simply not a very good way, and will consume all too much time and resources when there are much easier ways -- so you can get on with whatever the actual experiment is quicker. Capacitors of multiple farads are now available for super car audio systems that draw kilowatts at 12v and exceed what the car electrical system can do on the peaks. I had one such system in a car, which required those, they work fine.

Code: Select all
#!/usr/bin/perl

#use Tk;
use strict;

#Compute L for a buck switcher, for min current 1/10 max, note no input error checks
# can't use warnings as we do something a little slungy to have default inputs.


my $frequency = 30000;
my $Vin = 120 * 1.414; # set some defaults
my $Iout = 10;
my $T = 1/$frequency;
my $Vout = 24;
my $L;
my $temp;
my $Mh;

print ("Input Voltage?  ($Vin)  >");
chomp ($temp = <STDIN>);
if ($temp + 0 > 0) {$Vin = $temp;}

print ("Output Voltage?  ($Vout)  >");
chomp ($temp = <STDIN>);
if ($temp + 0 > 0) {$Vout = $temp;}


print ("Current?  ($Iout)  >");
chomp ($temp = <STDIN>);
if ($temp + 0 > 0) {$Iout = $temp;}

print ("Frequency? ($frequency)  >");
chomp ($temp = <STDIN>);
if ($temp + 0 > 0) {
   $frequency = $temp;
   $T = 1/$frequency;
   }


print ("Vin is now $Vin\n");
print ("Vout is now $Vout\n");
print ("Iout is now $Iout\n");
print ("Frequency is now $frequency\n");
print ("Time is now $T\n");

$L = (5 * ($Vin - $Vout) * $Vout * $T) / ($Vin * $Iout);

print ("\nL is $L Henry.\n");
$Mh = $L * 1000;
print ("L is $Mh millihenry\n");

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 » Fri May 20, 2011 8:55 am

Doug Coulter wrote:You obviously cannot hook a 20khz sq wave to a capacitor -- instant smoke someplace -- you know that, right?

Not really? Why is that? [I'm no electronics buff, hence me asking!!]

I was thinking to use IRL3713 - 30V and 260A each. I think I may have some already, but probably buried and easier just to buy some more. But what I was thinking was that the inductances of just a few inches of line may become significant and exceed 30V.

Not sure why it is a problem to a capacitor. Is it the inductances you think will cause repetetive over-voltage on the caps?
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Re: High Current.

Postby Starfire » Fri May 20, 2011 11:40 am

Chris
A older stick welder with a high current bridge and a car battery is the best bet and cheapest - you will need the welder to keep the battery charged if you are going to draw continuous - the battery will give you a good quite DC because of the impedance. If AC I would use a induction heating system which only needs a cap and thyristor circuit.
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Re: High Current.

Postby Doug Coulter » Fri May 20, 2011 12:12 pm

A capacitor is a dead short to AC, that's why we use them as filters (or, coupling capacitors that pass the AC between the terminals but allow DC across them). You need an impedance between your 20khz and it or something has to give, as at 20khz (not counting the higher harmonics in the sq wave) the caps will be a tiny fraction of an ohm to ground -- having to fully charge and then fully discharge (howevermany joules) on each swing to follow the waveform. At the capacitor sizes you're talking about, it's maybe millions of amps during the rise/fall times and of course, no filtering happens. So there's a series inductor in 100% of switching supply designs that allows for AC on one end and the DC on the other, with little DC loss --any standard buck supply has one, no exceptions. You could use a resistor if you didn't mind volt drop, wasted power etc.
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 » Fri May 20, 2011 12:20 pm

??!

No ground planned. Maybe what you are describing is something different to my plan. I'll do a diagram.
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