17 07 2012

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All photos
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During the first trial, our langmuir probe told us that our electron beam intensity was fluctuating at 60 Hz.

This is a problem, because one of the main things we are trying to study is the way changes in beam intensity affect potential well depth, so we want a steady intensity. The frequency of the fluctuations suggest that the AC-powered hot cathode is to blame.  I don’t totally understand the details of how a hot cathode running on AC 120v 60Hz translates to this waveform:

data from the langmuir probe displayed on the oscilloscop

The important thing is to prevent it. To do that, I put a full wave bridge rectifier in the power supply. It converts the AC coming from the wall to DC

It has three essential components.

1) The bridge rectifier

This change the AC sine wave into a waveform expressed by the function abs(sin(x)):

Better, but still not steady DC.

2) The filter capacitor

This gets rid of the ripple. you could compare the capacitor to a bucket with a hole in the bottom. Even if I vary the rate at which putting water into the bucket, the rate at which it come out is always going to be more or less the same, provided that it is sufficiently large compared to the volume of water going in.

However, its impossible to get an absolutely perfect DC output with this setup, because the ammount of charge on the capacitor does affect the voltage at which the current comes out.

This 680 uF capacitor takes away enough of the ripple for our purposes:

the output of the power supply when hooked up to a light bulb

3) An isolation transformer

Usually, the diode bridge and the capacitor would be enough, but our AC isn’t coming from the wall, its coming from a grounded auto transformer. this is a problem because the rectifier only works if the AC input is floating. A transformer with an equal number of primary and secondary wingdings accomplishes this without stepping the voltage up or down.

Nest step is to test it in the chamber.

Domenick Bauer

## Electron Gun Power Supply

5 06 2012

The light bulb electron gun test had to be put on hold because we didn’t have suitable power supply, so we built one. Here’s the schematic:

The schematic of the power supply which will drive both the hot cathode and the accelerator anode.

Here’s the real thing:

It’s really two power supplies in one. Put simply, it will convert the 120v AC current coming out of the wall into a source of lower voltage, higher current, AC power for the hot cathode, as well as a source of positive potential for the accelerator anode.

This is how the hot cathode power supply will work:

1) AC 120 volts from the wall to a switch.

2) From the switch to a 0.5 amp fuse. This, as suggested by Rehan, will prevent a current overload should the cathode arc to the wall of the vacuum chamber.

3) The the current will go to a variac auto-transformer which will allow us to regulate the voltage and current of the hot cathode.

4) Out to the hot cathode.

This is how the accelerator anode supply will work how it will work:

1) Current from the wall will enter the box and go to the same switch as the cathode power supply.

(see above for photos)

2) The current enters a stand-alone DC power supply. It is essentially a transformer which steps the voltage down to 24v and then a rectifier which converts it to DC.

3) The current goes into a high voltage power supply which steps it up to 500 volts DC.

4) The negative HV output is capped, because we have no need for it. The positive high voltage output (which will ultimately create the large positive potential on the accelerator anode) goes into a potentiometer, which will allow for a variable potential on the anode, anywhere from zero to the maximum voltage output of the power supply.

5) A 2 MΩ resistor. This will act as a sort of safety-net resistor. If there is an arc from the hot cathode to the anode, this will prevent the cathode’s larger current from flowing into the HVDC power supply.

6) A voltmeter. Self explanatory.

7) Out to the accelerator anode.

And that’s all there is to it.

Domenick Bauer

## Arduino Controls 30,000 Volts

10 09 2011

Today I made arduino control 30,000 volts.

My arduino has 3 channels of analog output 0 to 5 volt.

For testing I used this sin wave generator sketch:

int pwmPin = 9; // output pin supporting PWM

void setup(){

pinMode(pwmPin, OUTPUT); // sets the pin as output

}

void loop(){

float something = millis() / 1000.0;

int value = 127.5 + 127.5 * sin( something * 2.0 * PI );

analogWrite(pwmPin,value);

}

This generates a lazy 2 Hz sin wave.

But the output is not really analog, it’s pulse width modulation(PWM):

This tutorial shows how to smooth out  PWM using a low pass filter. My low pass filter used 6kΩ resistor and 4.7 µF @ 45V capacitor.

Here we have the raw PWM output superimposed with the filtered output:

Looks good!

Now we just add the voltage doubling op-amp circuit I made previously, and BOOM:

This shows the source signal and the voltage doubled signal.

Sweet! Now we can control the 30,000 volt glassman power supply.

Here the arduino is sending a slow sin wave to the glassman’s voltage control:

 From 2011-09-10

The Glassman’s slew rate is really slow without a load.

Here is the setup:

## IGBT

8 10 2010

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

## Transient Voltage Suppression

4 05 2010

When the fusor’s plasma becomes unstable and sparks, it makes my DAQ crash. This is likely due to transient voltage spikes in the wires, or electromagnetic interference through the air.

I’ve been exchanging emails with Raymond R. about the details of voltage suppression. He suggest using TVS diodes to protect each channel of the DAQ from voltage spikes in the wire by clamping them just above  the channel’s operating range.

I’ve ordered these TVS diodes:

1.5KE7.5CA

Raymond suggests the following:

1) Buy the  NI USB-6000 Series Prototyping Accessory if you have the money and want to avoid messiness; otherwise you can kludge the wiring. (I’ve done this).

2) 16 (+ 4,5 spares) of  LCE10A-ND (I bought corresponding parts at mouser).

3) Go to differential mode on NI USB 6008 (good idea).

4) Connect the TVSs from each input/output line to  frame ground.

5) Take the cable shield and do not connect it at the USB board; connect back to the computer frame.  The TVS diodes go to the cable shield not to the USB box grounds.  Connect the USB grounds together and to the shield ground through a 1 meg resistor (alternately a .1 uf ceramic cap).

## Mass Flow Controller Online

21 10 2009

Big day. Big win. Big upgrade.

Got the mass flow controller online!

## Computer Controlled Sorensen Hack

7 10 2009

Sweet! Just got the high current (120 Amp) Sorensen power supply working with computer control (to power the superconducting magnet).

Here is a video:

Details after the jump.