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Since the dawn of the solid state logging tool era there have been many downhole logging casing collar locator (CCL) amplifier designs ranging from the classic single transistor SIE design to overly complex designs with many transistors, and even a few with op-amps. There are good and bad designs; some are truly awful. Sometimes logging CCLs are built into tools, and sometimes they are used separately as is the case with most cement bond tools. The CCL log is generally displayed between tracks one and two, sometimes called the depth track, and usually to the right of said depth track, on the suggested American Petroleum Institute (API) log format. However, the CCL record is occasionally displayed elsewhere on certain specialty logs. AnaLog Services, Inc. frequently replaces and/or upgrades downhole CCL amplifiers for our customers. Several different logging CCL amplifier designs are discussed below. The SIE single transistor logging CCL amplifier is an enduring design, copied by many. A few designs have higher gain, but the SIE circuit performs well in most applications. Most SIE logging CCLs left the factory with resistors R2 and R3 installed in reversed order (see schematic below); correcting this error will improve performance. We usually replace the 2N2102 transistor with the tougher (higher voltage rating) 2N3439, resulting in a practically "bullet proof" circuit. The single transistor SIE CCL circuit should draw around 10 or 12 milliamps at 120 volts, though currents up to 15 milliamps are seen in stock factory tools. Emitter resistor R4 is selected to control current draw and lends stability to the circuit. Many units left the factory during the "boom days" with no emitter resistor. Our friend and tool guru Paul Knight has suggested the use of silicon diodes in place of the emitter resistor. See our complete schematic of the
SIE CCL amplifier (pdf file). |
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One of the better downhole CCL amplifier circuits ever devised is the current version of Comprobe's TIP120 Darlington circuit. Early versions of the TIP120 circuit may not be stable at moderately high temperatures and require a few minor modifications. In addition, the TIP122 transistor is a higher voltage device, the use of which makes the circuit somewhat more resistant to over-voltage damage. Interestingly, many of the Comprobe TIP120 circuits left the factory with the same resistor pair reversed as in the SIE units; could the TIP120 circuit have been borrowed from SIE, replicating the error that appears on the old SIE schematics? Earlier Comprobe downhole CCL amplifiers, including one version that actually contained an op amp, are disasters and should be replaced with the TIP120 circuit, the SIE circuit, or the AnaLog / Baer amplifier circuit. Until recently, many Comprobe tools had the CCL circuit board insulated with large diameter heat shrink tubing and merely dropped into a cavity in the tool sub, a source of amusement for the easily entertained service technicians out there who often refer to this approach as "bagged electronics" (here is our replacement). More recent production has the board and power resistor affixed separately to the chassis (reportedly to prevent thermal runaway, and thus enhance stability at higher temperatures). See our schematic of the
TIP-120 Downhole CCL Amplifier (pdf file). The Gearhart-Owen / Mineral Logging Systems COSMOS downhole CCL amplifier is truly pathetic. There are some fixes, but none improve performance significantly. Replacing the stock 36 volt, 5 watt zener diode with a 39 volt, 5 watt component improves CCL performance slightly. When it fries, the U2T105 can be replaced with a S2T105 or a 2N6351 transistor. Remove the stock U2T105 Darlington transistor if installing a replacement downhole CCL amplifier. A replacement circuit is available from AnaLog Services, Inc. that will work at the stock voltage or with 120 volt conversions. The SIE single transistor 2N2102 / 2N3439 style circuit can be installed as a replacement only when also converting to 120 volt operation. See
GO/MLS COSMOS Tool Notes for more information on these fine old tools. GO / GOI / MLS had a few other downhole CCL amplifier designs, but all seem rather lackluster compared to the better designs available. This
cascode amplifier circuit is of some interest since it can be implemented on a very small circuit board and was used in some small logging CCLs (it uses two transistors, but no power resistor or capacitors). Titan is relatively new to the logging tool business, but their downhole CCL amplifier circuit is interesting. It is the first we have seen using a MOSFET, in a TO-3 package no less. AnaLog Services, Inc. sells and installs several different downhole CCL amplifiers. We offer the AnaLog / Baer design which is arguably the very best downhole CCL amplifier ever conceived. It is available for retrofit use or for incorporation into new tools. We also offer two
TIP120 Design Printed Circuit Boards (PCBs), and a
COSMOS Upgrade PCB that is a direct replacement for the pitiful GO/GOI/MLS COSMOS CCL amplifier. See Application Note CCL-2 for more information about the various uses of this cute little upgrade pcb. For the SIE and TIP120 variants, we always liked the Clarostat VC3D enamel coated power resistor, but they have become more difficult to find now that Honeywell has acquired Clarostat; the Ohmite 23J is a small vitreous enamel coated equivalent to the VC3D that is an excellent substitute. See also
Downhole Logging CCL Amplifier Schematics
and Casing Collar Locator (CCL) Printed Circuit Boards (PCBs). |
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See also CCL Overview, CCL Coil Trivia, CCL Magnet Trivia, and CCL Magnet Wire Trivia. If you need more information on downhole logging CCL amplifiers, please feel free to
contact us. 10-99 |
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