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What follows are some random tips and tricks we have picked up servicing downhole high voltage power supplies in well logging tools. Many different problems can be encountered. Logging tool high voltage power supplies use an oscillator of one type or another to drive the high voltage transformer, essentially producing an ac voltage stepped up by the transformer, and then rectified into high voltage dc. The frequencies these oscillators operate at, or subharmonics thereof, are within the range of human hearing. Checking for the distinctive whine of the high voltage supply in nuclear logging tools becomes an automatic first step when an unhoused tool is powered up on the test bench. High frequency hearing fails earlier in males than in females with advancing age; if you are an older male, find a nice female to tell you if the tool is singing. The Victoreen Corotron glass high voltage regulator diode tube is no longer manufactured, and there is no second source manufacturer. But never fear, there is a solid state replacement available as discussed below. If you have old stock or used Corotrons that are missing the red dot, the inner electrode is the positive lead (Anode). Do not be confused by the schematic symbol for the Corotron that makes the negative lead (Cathode) look like it is the inner electrode. It is sometimes hard to spot a defective Corotron with a compromised glass envelope with the naked eye. Look closely (a magnifying glass may even be necessary) for a common failure mode where a tiny hole is present near the tip end of the tube. The little hole will look like the reverse conical impact hole made by a BB, as if the shot was from inside the tube. See this old Victoreen Tubes Catalog (large pdf file) for reference material on the obsolete Corotron tubes. Some Comprobe tools use a "leaky diode" voltage regulator that will fail sooner or later, and which behaves badly at elevated temperatures; it should be replaced with a real high voltage regulator of some kind. AnaLog Services, Inc. now markets the extended temperature range
Codatron HT™ (formerly called the TitanTwo) and the low temperature coefficient (TC)
Codatron HTZ™ solid state voltage regulators, Victoreen Corotron replacements designed by Robert L. Baer. These regulators are specifically intended for use in well logging tools, but they can replace the Victoreen Corotron in a host of other applications as well. Our Codatron family of regulators are all two lead devices and are direct "drop in" replacements for the Victoreen Corotron. See our
Codatron Regulator Family Overview for more detailed information and access to technical data sheets. Whenever possible and practical, the Corotron or Codatron regulators should be connected to the negative rail of the high voltage power supply (usually chassis ground) by way of a high grade (1% tolerance or better) 10 kilohm (kW) resistor. The shunt current passing through the regulator can thus be easily determined by simply measuring the voltage drop across the 10 kW resistor. 60 microamps (mA) is a good shunt current for the Corotron favored by some of the better technicians, like Paul Knight ( regulators operate nicely at 60 mA, though they work perfectly over a wide current range). The value of the series resistor(s) between the high voltage supply and the regulator can be selected to obtain the desired regulator shunt current. By way of example, a 60 mA shunt regulator current would produce a reading of 0.6 volts across the 10 kW resistor. Arcing and leakage are common problems with downhole high voltage power supplies. The high voltage wiring in many tools is Type E Teflon wire (rated for 600 volts); it has been known to arc especially if the insulation is nicked. The thicker insulation on the Type EE wire (rated for 1000 volts) is a better choice. In especially troublesome situations, Teflon tubing can be slid over the high voltage wiring to provide additional protection (Bell and others did this in many tools). The brown high temperature polyimide printed circuit boards (PCBs) are notorious for their proclivity to absorb moisture. This can be disastrous if a tool "goes wet" since well fluids are usually salty, and ionic contamination is readily absorbed by a polyimide board. Sometimes even a seemingly "clean" PCB will have sufficient ionic contamination to cause problems, especially at elevated temperatures (keep in mind that ion mobility and leakage currents double about every ten degrees Centigrade; this is true for bulk semiconductors, all types of printed circuit boards, and all "insulating" surfaces, even glass and Teflon). Always carefully clean any high voltage downhole PCB, especially if it will be used at high temperature. Pure Grain Alcohol (PGA) a/k/a Ethyl Alcohol or Ethanol is a good choice for flux removal (we have been using it since
1,1,1 was outlawed). Soaking or boiling a PCB in high quality distilled or deionized (DI) water (one megohm/cm quality at a minimum) for a prolonged period may rescue the board. Long soaks in clean Ethanol or in clean consumer grade rubbing alcohol (70%, or better yet 91% Isopropyl Alcohol) may also help. Completely Denatured Alcohol (CDA) sold simply as "denatured alcohol" in stores should be avoided for electronics use due to the undesirable additives used for denaturing the Ethanol. For more information, see our Cleaning Secrets Revealed page. Adding neon glow lamps between the high voltage supply and a shunt regulator to boost voltage has been used by several manufacturers. GO / GOI / MLS used a single neon lamp to boost the voltage about the same as what is lost in the filter stages following the regulator, and it provides a nifty visual indication that the high voltage power supply and the regulator are functioning. To save money SIE used a string of four or five neon lamps to boost the voltage in their gamma ray / neutron tools, thus deriving a higher and lower high voltage output for the two detectors using only one Corotron. Some NE-2 lamp types may be too noisy for this application; we have had good luck with the Chicago Miniature A1B lamp (a technician friend likes the A9A). Each neon glow lamp produces a boost of 50-60 volts or a little more, depending on lamp characteristics. Another trick is to tie the shunt regulator to the line voltage rather than ground, still using the 10 kW current measuring resistor if space permits. The effect of this practice is to raise the regulated voltage the same amount as the line voltage. Many Comprobe tools have a jumper arrangement to allow using either ground or line for the Corotron negative lead. These voltage boosting tricks work equally well for the
Codatron Regulator Family as they did with the Victoreen Corotron. 03-05-03 |
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