Discrepancy Between Circuit Simulation and Reality

29 09 2011

Previously we modeled the polywell coils and power supply in SPICE.

Today I returned to that model.

All resistance values in the simulation are based on real world measurements with the exception of coil inductance (code).

Starting with an estimate for coil inductance of 0.1 mH the discharge current looks like this:

The simulation’s peak of 1.5 kA is nowhere near the 2.3 kA we are getting in the real world:

OK. Maybe the value for coil inductance is off?

I played around with the value for coil inductance but the simulation would not match reality.

As a control I replaced the simulated inductor with a 1 mΩ resistor (code). Looks like this:

The simulation predicts ~1.8 kA but in reality we see 2.3kA!

Where does this discrepancy come from?

UPDATE: Reader Andrew solved the mystery:

You could try changing the ON resistance of your switch/SCR to something a bit lower than 100mOhms

.model MySwitch SW(Ron=.1 Roff=1Meg Vt=3 Vh=0)

I can’t see the part number of your SCR but 2mOhms would seem reasonable.

I didn’t notice that rather high resistance lurking in the SCR model.

Now the simulation matches reality very closely with 0.06mH coil inductance (code):

Good work Andrew and the rest of the internet brain!

26 09 2011

I completed some nice upgrades to the coils power supply for safety and quality.

I added DIN rail terminal blocks and rearranged the parts to emphasize discharge path:

Added terminal blocks to the trigger board:

These pins on the SCR never soldered well because they are mini-fastons. Fixed:

For safety, I added an AC switch:

I replaced the trigger’s battery with a 9V wall wart:

And added a DIN rail 0.5 A circuit breaker:

CRT in Focus

19 09 2011

I finished the CRT power supply and got the CRT running with focus and grid:

As you can see we have an interference pattern. To the eye the beam looks much brighter in the center:

The CRT power supply:

DIN Rail Terminal Blocks

16 09 2011

My shop-mate Stuart is a master prototyper. He frequently uses Phoenix Contact DIN rail terminal blocks for wire-up.

Taking a page from his book, I got my own set of Phoenix Contact DIN rail terminal blocks (3044102):

Terminal blocks make for easy changes. The red bridge-bars create busses with as many connections as you need. You can easily probe any point in the circuit. Everything is bolted down to the chassis.

Connectors may seem like an insignificant part, but these will really help.

Welding 3D Printed Steel

16 09 2011

My shopmate had a TIG welder here the other day.

I took the opportunity to try welding the 3D printed metal parts I made last year:

These are intended to be coil holders, so I installed a 40 turn coil prior to welding.

Mike welding the halves together:

We used no filler rod on the theory that the infused bronze would melt and form a braze of sorts.

It worked very well:

The coil insulation didn’t survive: The coil is conductive to the casing.

I’m encouraged by the weldability. I am ordering more test parts to keep pushing this approach.

14 09 2011

I am testting out viton gaskets for the vacuum chamber.

The gaskets are rated down to 10e-8 torr, which is fine for now.

The smaller gaskets installed easily, but the large 8″ gaskets were impossible to install… they fell off the flange.

I called MDC and asked “what’s the trick?”

The trick is a little vacuum grease to retain the gasket!

I ordered APIEZON TYPE M vacuum grease from ebay, also rated down to 10e-8 torr.

I made 8 tiny dabs of vacuum grease:

And it worked!

After a few hours the chamber went as low as 5.5e-5 torr. Totally fine for now, but could be better.

I will continue to use them.

Oscilloscope Camera Mount

13 09 2011

Reader jsults turned my on to g3data… it’s a little open source program that helps extract data from graphs.

It looks like this in action:

I tried it with my oscilloscope photographs and it worked OK.  But it does not compensate for trapezoidal distortions.

If my camera were perfectly lined up with the oscilloscope g3data would work great.

This got me to thinking… maybe I could build a camera holder for the oscilloscope?

So I did.

I designed this mount using sketchup and had it printed at shapeways. It came in the mail today.

Works like a charm. Now all my oscilloscope photos will be perfectly centered and flat: