Testing and using the Codatron® shunt regulator
In testing any device, always draw a complete schematic of the test circuit and assume as little as possible.
Note that all resistors have a voltage rating and a power rating, are not linear and have a temperature coefficient that is also not linear. In most cases, these non-linearities can be ignored, and temperature coefficients are nominally low enough to also be ignored.
Just make sure that these non-ideal characteristics are not forgotten.
Refer to the schematic above and note that the supply voltage V can be adjusted and has its own internal resistance Rv which may depend on the voltage setting and current load.
For testing and operation of a shunt regulator, a series dropping resistor Rs and a load resistor Rl must be used. Note that the total series resistance is Rv + Rs.
It is good practice to use test values that are the same as in a final circuit.
Note that the use of a meter to measure V will not measure the exact value of V due to the voltage drop inside the supply due to Rv. Also note that the meter will draw current depending on its internal resistance. Most Digital Volt Meters (DVMs) have an internal resistance of 10 megohms and their rating makes them unsuitable for the direct measurement of voltages greater than 1000 volts.
Normally, a current monitoring resistor Ri is added in series to the shunt regulator VR.
This has the disadvantage of decreased regulation, so a low value of 10K (ten kilo ohms) is used which will develop 600 millivolts at a nominal regulating current of 60 microamperes.
Use of a DVM (Rm2) to measure that voltage will have very little effect on the actual voltage, as 10 megohms put in parallel with 10K will change the resistance only 0.1% which can be safely ignored.
Use of capacitor Cl is optional and should be used in a test fixture if the final circuit uses one. Again, values should be the same.
Measuring the load voltage with a meter Rm3 will increase the total load; again a DVM is unsuitable for direct measurement of voltages over 1000V.
For testing purposes, Rm1 and Rm3 should not be changed during any measurements such as line regulation (changes in V), load regulation (changes in Rl) or temperature regulation (changes in the temperature environment).
For temperature testing, it is a good practice to start with only the shut regulator in the temperature chamber; that way any temperature sensitivities of the resistors is not a part of what one measures.
Furthermore, it is good practice to change only one variable at a time.
For example, change only temperature, then change only the line voltage V, and then change only the load Rl.
Take as many data points as you need for each setting; say change temperature for a given voltage and load setting and then change the voltage setting and take measurements for the same temperatures, and so on. Even time can be a variable: say one subjects the regulator to a step in voltage or a step in temperature, making measurements of the output voltage at regular intervals.
Use a good spreadsheet program and enter the data, and use the graphing capabilities for two dimensional or three dimensional graphing.
Measuring high voltages means that one must use a special high voltage resistor in series with either a DVM or a VOM. That resistor must be at least as accurate as the measurement requires, and the meter inaccuracy will always add to the total inaccuracy.
One percent high voltage resistors rated to 20KV and 5,000 megohms are available via DigiKey or Mouser (Ohmite Mini-Mox, Maxi-Mox and Slim-Mox).
For better accuracy, one must make their own using 0.05 percent resistors, but hundreds may be needed as 470K is the maximum value available (DigiKey: Susumu 0805 SMD chip resistors rated at 100V each).
Alternately, one could use 0.1 percent resistors where Susumu 0805 SMD chip resistors are available to 1 megohm at 100V or use IRC RC55 0.1 percent leaded resistors from Mouser which go to 1 megohm and apparently are rated at 100V each.
Is There Oil There?
One needs to be knowledgeable in geology, chemistry and nuclear physics to determine if a given rock strata of formation contains prospect for oil.
GAS-FILLED NUCLEAR RADIATION DETECTORS
One method is to detect low energy, or thermal neutrons; either emitted naturally from isotopes of elements in the rocks, or by way of these isotopes having been previously excited by an external radioactive source,
For more than forty years, gas-filled nuclear radiation detectors have been used to detect thermal neutrons.
All designs employ coaxial electrodes, the inner electrode being the anode. When the DC supply voltage is sufficiently high, a charged particle or a gamma quantum entering the inter-electrode space ionizes the filling gas. The electrons created are collected by the anode and the ions by the cathode. The negative-charge pulse at the anode is detected in an RC (load) circuit.
In a higher voltage region, the multiplication factor is higher and discharges occur all along the anode, and the collected charge is limited by the characteristics of the detector. The pulse amplitude is therefore almost constant for increasing supply voltage and independent of the ionization event. This region of the curve is called the Geiger region (or plateau), and is utilized in Geiger-Mueller counters.
As the neutron has no charge, a neutron detector incorporates a neutron-to-ionization particle converter. The incident neutrons are captured by the converter material which then produces (detectable) ionizing particles by a nuclear reaction.
GAMMA RADIATION DETECTION
In gamma detectors, the gamma radiation produces electrons by interaction with the cathode material (the metal detector wall) and with the filling gas. These electrons ionize the gas and the charge carriers are collected by the electrodes, the resulting anode pulse being detected by a load circuit. The original electrons are created by several interaction mechanisms: the photoelectric effect, Compton scattering and pair production.
The mechanism efficiency depends on the energy of the gamma quanta, cathode material and thickness, the filling-gas type and pressure. Detector sensitivity is therefore very dependent on the detector design, but is always energy-dependent. Owing to the dominance of the photoelectric effect, the maximum sensitivity occurs at about 80 keV. At high gamma energies (>500 keV), the sensitivity can fall to one hundredth of the maximum for the same mechanical design. A flatter response can be obtained by using an external metal filter and by the choice of filling gas.
The Gamma Ray tool can be attached to the MWD (measurement while drilling) string below the Directional Sensor in order to measure natural gamma radiation from the formation, which enables basic sand/shale detection. The patented scintillation detector design provides a highly reliable measurement and has been developed to withstand the high levels of shock and vibration usually encountered during drilling operations while maximizing scintillator volume for optimal sensitivity.
Basic electronics for pulse-mode operation consists of a charge amplifier, a high voltage power supply, and a pulse amplitude discriminator.
For more than sixty years, photomultipliers have been used to detect low-energy photons.
CONSTRUCTION AND OPERATING PRINCIPLE
A photomultiplier tube is a non-thermionic vacuum tube, usually made of glass that converts very small light signals into a measurable electric current.
As Fig.1 shows, it comprises:
- A window to admit light
- A semitransparent photocathode made of a thin layer of photoemissive material deposited on the inner surface of the window which emits electrons in response to absorbing photons
- An electron-optical input system of one or more electrodes that accelerate and focus the emitted photoelectrons onto the first dynode of the tube
- An electron multiplier consisting of several electrodes, (dynodes) covered with a layer of secondary emissive material. For each incident electron, each dynode emits several secondary electrons. These are accelerated onto the next dynode by an inter-dynode potential (typically of about 100 V) producing ever more secondary electrons down the multiplier. Electron gains of 10^3 to 10^8 are common and depend on the number of dynodes and the inter-dynode potentials
- An anode grid which collects the electron avalanche, providing an output signal
WHY FIXED VOLTAGE SUPPLY?
The output voltage or pulse amplitude of these detectors depend not only the energy of the detected particle, but also the supply voltage of the detector.
If the detector has a constant, regulated supply, the energy of the detected particles can be determined, meaning that the source elements can be determined by the characteristic energies emitted.
HISTORY OF VOLTAGE SUPPLY REGULATORS
The Victoreen HV Regulator is a high-voltage, low-current device operating in the corona mode of discharge. The same techniques employed in the application of zeners to the design of low-voltage power supplies can be used with the Victoreen HV Regulator to design high-voltage supplies.
The Victoreen HV Regulator provides the high voltage equivalent of the Zener diode. It is a miniature or micro-miniature gas tube operating in the corona mode of discharge. Its characteristics more nearly resemble those of the Zener than they do those of the well-known glow discharge tubes.
Many types of Victoreen HV Regulators operate at currents as low as 5 uA, making them economical as a high voltage reference, but restricting their usefulness to low power applications when employed as a shunt regulator. The "minimum current" of the Victoreen HV Regulator is the value of current below which noise may be generated. Corotron™ is the trademark name for the Victoreen gas tube regulator, and is no longer manufactured.
Alternate High Voltage Equivalent of the Zener Diode
The Codatron® high voltage zener is designed to give superior regulation over rapid temperature changes, and is for use in high temperature environments. The regulation of the Codatron® over rapid temperature changes is superior to any other currently manufactured high voltage regulator. The Codatron® regulator acts like a high voltage zener, but with low noise and a low temperature coefficient (TC).
The Codatron® high voltage zener has been optimized to operate at approximately 60 microamps, the current favored by many well logging tool technicians, but functions well over a wide range of currents up to a maximum current rating of 500 microamps. A much higher pulse or transient current rating is allowed due to the unique electrical characteristics of the Codatron®.
A small positive temperature coefficient has been included in the design to partly compensate downhole logging detector temperature response characteristics. The Codatron® was designed for high temperature downhole well logging tools, but can be used as a direct replacement for the Victoreen Corotron® in most applications. Shunt capacitors may be used, since the Codatron® has a positive resistance characteristic at all operating currents.
- Available in standard Victoreen Corotron® voltages and in custom voltages.
- Standard model specifications good from -55°C (-67°F) to +177°C (350°F)
- Grade "A" model specifications good from -75°C (-103°F) to +204°C (400°F)
- Nominal voltage rating specified at 100°C (212°F)
- Operating current range: 20 uA to 200 uA, -75°C (-103°F) to +204°C (400°F)
- Minimum shunt current for regulation: 2 uA, 0°C (32°F) to 75°C (167°F)
- Maximum shunt current: 500 uA, -75°C (-103°F) to +177°C (350°F)
- Recommended operating current: 60 uA
- Excellent peak current rating
- Stable at all operating currents by design
- Precision tolerance
- Excellent voltage regulation 20 uA to 500 uA
- Low noise generation and no self-oscillation
- Slight positive temperature coefficient added for downhole detector compensation
- ECCN number (export commodity control number): EAR99
- Harmonized Tariff number: 8541100050
- Schedule B Reg Number: 9032.89.3000
- Approximate weight (to 1250V): 0.2 oz.
Do you prefer the standard version good from -55°C (-67°F) to +177°C (350°F) or the grade "A" version good from -75°C (-103°F) to +204°C (400°F)?