NYT is covering Leeds Radio. My favorite spot for electronics in NYC!
Richard is an awesome dude… very knowledgeable and great stories of EE insanity.
I’ve learned much about the history of electronics just from his inventory. The vacuum tube era was a whole nother way of doing things.
NYT: Leeds Radio.
I received the new SCR. It can handle 3000 amps peak current. It is much larger than the SCR I recently destroyed:
Here it is installed. Everything works:
I also added leads to the current sense resistor to avoid touching the back of the power supply:
The pressure necessary for plasma is still too high. I tried another tweak to the electron gun:
No joy. It requires about 24 millitorr for a stable plasma. UG.
Electric double-layer capacitors are a newish energy storage technology. I find them fascinating and I want to play with them.
Like a capacitor they can charge and discharge very quickly due to their low equivalent series resistance.
But supercapacitors store roughly two orders of magnitude more energy than normal capacitors!
I bought five 3000 farad, 2.7V Boostcaps:
3000 farad!
I machined aluminum strips to form connecting bars. A 13.5 V series stack:
Check out this video of a full-charge short circuit:
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!
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:
With the focus adjustment I can broaden the beam:
The CRT power supply:
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.
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:
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| From 2011-09-10 |
The Glassman’s slew rate is really slow without a load.
Here is the setup:
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:
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.
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!
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