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christensenda

Fri Feb 17 2012, 10:37AM

Registered Member #3062 Joined: Wed Aug 04 2010, 02:07AM
Location:
Posts: 22

Hello All,

I have started to research more seriously into a reliable means of switching very large amounts of stored energy relatively cheaply and efficiently for an Electrothermal Gun I want to build later on. I think however the switching of stored energy will be the most challenging part.

Since I have come to the conclusion that a paralleling of SCRs would work but would cost me a small fortune and I still run the risk of blowing them all. Since getting an SCR that has peak surge currents that are sufficiently high seem prohibitively expensive. I was wondering if anyone here has had success with building a DIY Trigatron for say about 10kJ worth of capacitance. If so if you could share some insight into trigatrons it would be much appreciated.

Cheers,
Christensenda

Forty

Fri Feb 17 2012, 08:48PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

A mercury Ignitron might be a way to go too.

klugesmith

Sun Feb 19 2012, 04:25PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

The stored energy value (e.g. 10 kJ) isn't directly useful to specify the switch.
Need to know the voltage and the discharge time (thus, the peak I and the I^2T integral). Doesn't take much to switch 10 kJ if the discharge time is an hour.

If you have enough voltage for a trigatron (1 kV is plenty, 400 V is a bit low)
then a mechanically closed spark gap can be robust & flexible.
Make the minimum gap adjustable doen to zero, for lower voltage experiments.
I claim experience, as do coin-shrinkers like Bert Hickman, or Hackerbot Labs (google them).

christensenda

Sun Feb 19 2012, 09:18PM

Registered Member #3062 Joined: Wed Aug 04 2010, 02:07AM
Location:
Posts: 22

So, the charging voltage will be 500V. In order to determine the peak current I would assume I would have to know the ESR of the capacitor and model this as an series RC circuit. where the current is given by I=(V/R)*e^(-t/RC).

Unfortunately I do not know the ESR ratings of the capacitor the datasheet being this :

but since I know the series type I know the ESR is between 12mohms ~ 31mohms. So highest current would be at t=0; Apeak ~ 42,000 amps.

Is this the correct way to determine the peak current ? Thanks again

klugesmith

Thu Feb 23 2012, 06:54PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

1. I think 500 V is still pretty low for repeatable firing of a spark in air, especially with the large smooth electrodes you will want. This may lean you back toward solid-state switches or a mechanical contactor. Or an ignitron -- but I think the most common ones have pulse current ratings on the order of 1 kA, so you would still be driving off-road.

2. As for electrical model to predict peak current, don't neglect the inductance of capacitor and your interconnections. Have you tried Barry's LCR simulator? (search recent threads here). You could accidently build in as much as a microhenry if you ignore low-inductance routing considerations.

3. Your RC model, with no resistance other than the capacitor's ESR, has some problems.
If the interconnect and load resistance are practically zero, then no power is delivered. The entire stored energy would be dissipated in the ESR, heating (and possibly damaging) the capacitor.

Energy storage caps aren't designed for the current of a true short circuit from maximum rated voltage. Fortunately, this forum is a good place to get practical advice about abusing electronic components.

If you want most of the energy to heat the load, the load must have substantially higher resistance than the C + switch + wires.

christensenda

Sat Feb 25 2012, 07:47PM

Registered Member #3062 Joined: Wed Aug 04 2010, 02:07AM
Location:
Posts: 22

Looks like I still have a lot of research to do, and Barry's site looks really helpful I will be sure to check that out thoroughly.

and the capacitor modeling lacking parasitic inductances' makes sense however I do have one more question, is there a reliable way to determine a wire's inductance based on its AWG or mm thickness. I have seen inductance calculators but most I have seen are simply for loops of wire with some N turns, and not just a length of X amount of wire.

Gomez

Wed May 08 2013, 10:59PM

Registered Member #2328 Joined: Tue Sept 01 2009, 03:44AM
Location: Denver, Colorado, USA, Sol-3
Posts: 7

A home-brew ETC gun is going to be fairly slow - unless you're already an experienced gunsmith who has built a few magnum class rifles... so use a mechanical switch, try to keep enclosed current loops reasonably small and don't lose too much sleep over it.

Steve Conner

Thu May 09 2013, 11:24AM

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

christensenda wrote ...

I do have one more question, is there a reliable way to determine a wire's inductance based on its AWG or mm thickness. I have seen inductance calculators but most I have seen are simply for loops of wire with some N turns, and not just a length of X amount of wire.

Inductance isn't really a property of the wire. It has more to do with the spaces between wires: the loop area enclosed by the circuit. If you have two wires going from your cap bank to your gun, the inductance of the circuit will be much lower if you strap or twist them together, than if you just laid them on the floor a foot apart.

Sometimes you'll see a figure of between 15 and 25 nanohenries per inch length of wire. This is for a single wire in free space with the return circuit at infinity. The inductance of that circuit would be infinite, but calculus makes it easy to figure out how much the infinite inductance changes per inch of wire added or removed.

It's not really relevant for finite-sized circuits, because the inductance is proportional to the current loop area, so it's actually nanohenries per square inch. The figure that sticks in my mind is that a 10 inch diameter loop (of fairly large badass conductor) has an inductance of 600nH.

Inductance isn't necessarily bad. If using SCRs as the switch, you need a certain amount of inductance to limit the rate of rise of current, so that they don't exceed their critical di/dt and explode.

Yandersen

Thu Jan 09 2014, 02:35AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Hey, guys, I've just developed a cheap and efficient way to trigger the air-operating ETG!
The first half of a video is a discussion of different ETG designs and explanation of my triggering mechanism, while the second half of the video is an actual demonstration of the trigger at work:

ETG rules!!! Hehey!

Ash Small

Thu Jan 09 2014, 04:29AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Looking good, Yan, can't wait to see an ETG using this technology

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