My First OP-AMP Circuit

3 09 2011

All photos.

My Glassman high voltage power supply can be controlled with a 0-10VDC signal.

But most computer interfaces (like the arduino ) output 0-5VDC.

So today I built my first OP-AMP circuit to double a DC voltage:

I found a LM358M operation amplifier laying around.

Now I have a 2X voltage doubler! Sweet:

 





Coil Inductance

27 08 2011

Let’s take a look at the coil inductance.

A fun and easy place to start is look at some AC going through the coil (suggested in comments).

I used the oscilloscope’s test wave generator. I’m using two 10:1 probes connected thusly:

Channel 1 is shown above channel 2 on these oscilloscope screens:

Not much distortion at  500Hz:

At 50kHz we start to see some distortion:

At 500kHz we have obvious distortion:

Not sure what this tells us, but I’m sure it tell us something!

Now lets try a more abstract approach to the problem. We have a 6cm diameter air core coil with 60 turns. I found this handy inductance calculator. I looked up the thickness of 18 AWG wire and the relative permeability of air.

Theoretically the coil has 0.56mH of inductance. Sound right?
Given inductance and frequency, you can calculate impedance…which is what we’re after. But what’s the frequency of a single pulse?

UPDATE: The rectangular function may be useful for calculating frequency.





Ground Resistance

27 08 2011

All photos.

Now that I have a milliohm meter, I want to measure the resistance of everything.

Ground quality is important for safety and signal quality.  So I measured the resistance of my grounding network. I made two 5 meter long Kelvin probes to do so:

I have two main ground connections. One is the ground pin on a standard wall outlet. The second is a braided wire to a water pipe:

Normally these ground exits are star connected. As a test I disconnected them from one another and measured the resistance between them: 700mΩ.

For various test points on the machine and rack we see from 10 to 100mΩ

I realized that this potentiometer is not grounded at all!

I will ground this forthwith!





DIY Milliohm Meter

25 08 2011

I asked Joe Khachan this question:

Did you go to great lengths to reduce the resistance of the coil path?

He responded:

Yes we try and use a thick enough wire so that the resistance is small but it cannot be too thick in order to be able to have many turns. More importantly is the contact resistance when connecting to the wires. We try to keep this low. From this perspective, a milliOhm meter is helpful.

Milliohm meters start around $200 on ebay. Too much.

Then I found this excellent tutorial on Kelvin (4-wire) resistance measurement.

Luckily I had a current sensing resistor on hand for the current reading:

I used the resistor on the right.

So current sense resistor plus two voltmeters and BINGO: milliohm meter. Results:

The coil path is 0.35Ω.

The dummy coil is 0.185Ω

As a calibration I tested the 0.001Ω resistor. Result: 0.00088888

Not bad.

Now I have excellent tool to troubleshoot resistance in the coil path. AWESOME.





BIG STEP: Fired the Magrid Coil Inside Vacuum Chamber

1 08 2011

All photos.

Today I did all the wiring to connect the coil power supply to the coils inside the chamber.

I started by rack mounting the coil power supply:

Grounded the coil power supply:

I bought these rather expensive ($8 a piece) but totally necessary connectors for the feedthroughs:

I re-wound the coils with fresh magnet wire:

Test install:

Connect the coil to the feedthrough:

Make the cable from the feedthrough to the coil power supply:

Strain relief for the cable:

Finally ready to do a test shot:

I took the power supply capacitors up to 400V and did a sucessful test fire:

You can see the magrid flinch in the chamber when the caps fire. SUCCESS!

Very close to actually running the sydney experiment.





Reality Check

14 02 2011

All photos.

This project has been an ongoing lesson in electrical engineering. Now with an oscilloscope I can finally see a circuit’s behavior.

I’ve spent the last week reviewing my assumptions about the most basic circuits and components. Sometime my hunches are correct, but just as often I am confounded by what I see. Here is my test setup:

The most interesting behavior happens with an AC signal. Conveniently the oscilloscope has a built in square wave generator intended for calibration. I am passing this square wave through test circuits to see how the wave changes.

The oscilloscope generates a square wave that goes from ground to +0.4 V. The frequency ranges from 50 Hz to 5 MHz depending on the time setting. I start with 50 KHz. I use two probes. The first probe is connected to the signal source, the second probe is connected to various other points in the test circuit.

The first thing to note is the signal generator doesn’t provide much power. If you overload the signal generator, you will see it’s voltage sag.

Both probes ground to the oscilloscope chassis, so choosing appropriate ground points is crucial … incorrectly grounding a probe can drastically change the circuits behavior.

I started by looking at a single capacitor. I tested this circuit:

In this capacitive coupling configuration a capacitor removes the DC component from an AC signal. Probe 2 shows the same signal as Probe 1, except shifted down. Probe 1 goes from 0 V to + 0.4V whereas probe 2 goes from -0.2 V to +0.2 V. So that’s what it looks like to block the DC component.

Next I passed the square wave through a transformer. I’m using a variable resistor to limit the current into the transformer.

This is what I see:

signal generator at top. transformer output bottom

It doubles the voltage as expected, and adds quite a bit of color to the waveform. It also draws enough current to make the signal generator’s voltage sag… even the ground line as seen in this video:

From 2011-02-12

At some frequencies, the transformer really changes with waveform:

signal generator at top. transformer output bottom





Oscilloscope

25 01 2011

All photos.

I recently purchased a used Tektronix 2445 oscilloscope. It’s a 4 channel 150 MHz analog scope.

I also purchased a Tektronix 2430 digital scope which is en route. The digital scope has the key advantage that it can capture and display a single frame from a trigger. The digital scope will show readings from the Langmuir probe in the Sydney experiment.

It has taken me a few days of reading the manual and playing around with the scope to get a grasp. But I’m getting the hang of it, and wow… it’s a new way to explore the world!

My first discovery is that one of my bench DC power supplies is rather noisy:

The other bench power supply looks much cleaner:

But neither DC power supply is as clean as the perfectly flat signal you get from a battery.

I also used my iphone to display some sine waves:





Superconducting Magnet Test

16 10 2010

All photos and videos.

Yesterday I tried the the superconducting magnet‘s persistent switch again.

I failed to make a persistent superconductor, but all the circuits and LabView worked properly. More WIN than FAIL.

 

Superconducting magnet submerged in liquid nitrogen.

 

Conceptually this is the circuit we are testing. The heater functions as a variable resistor. The IGBT functions as the switch. Both are computer controlled.

This is the procedure:

I built a LabView VI to trigger the SC coil a variable number of millisecond after the heater:

We can measure the magnetic field produced with the DC magnetometer:

When I ran the experiment with 5A through the SC coil,  I only saw a tiny magnetic field:

6 Gausse from the SC magnet

Furthermore, use of the heater seemed to make no difference at all.

As a control I ran the magnet in this configuration to see what magnetic field strength we should expect:

The produced a much stronger field:

15 Gauss When connected directly.

So the full current is not going through the main coil, but through the heater. I suspect either the heater resistor is not working (I can hear and see it boil the liquid nitrogen) OR the splice in the coil has more resistance than the coil heater:

The last time I ran this experiment the YBCO corroded from condensation:

This time I ran 2A of current through the coil for several hours to warm and evaporate any moisture.





IGBT

8 10 2010

 

All photos.

Yesterday I got the IGBT working under computer control. I switched my desk lamp on and off from my computer. The IGBT will be used to switch up to 90 Amps going into the superconducting magnet.

IGBT

Schematic:

Here is a video of the win:

From 2010-10-07




EMI Next Steps

27 07 2010

Got some great feedback from the community about fixing our EMI problem. In summary:

  1. Try Lifting the USB’s ground on one side or the other.
  2. The last faraday cage we use was likely steel, which can have enough resistance to pass radio frequency. Try aluminum faraday cage.
  3. Use USB cable with ferrite bead like this one Dave purchased:

I have an electronics chassis I’ve recycled to try as the next faraday cage:








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