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There are many different designs that have been used for the low voltage power supplies in well logging tools.  It would seem every designer has had their own ideas about what constitutes good design principles.

Bell, Pengo, and others have used switching low voltage power supplies in various tools.  The theoretical advantage of switching power supplies is increased efficiency.  The downside is that these switching supplies require a proprietary transformer that may not be readily available if replacement is required.  Switching downhole low voltage power supplies also violate one of the "laws" of logging tool design, the KISS (Keep It Simple Stupid) principle.  If the designer has used the extra efficiency of a switching supply to power the circuits in the tool (as is usually the case), then it will not be possible to substitute a simple Zener based low voltage supply operating at the same line current; the power budget will simply be too large for the Zener supply.  We are stuck with switching high voltage power supplies because there are no really practical alternatives, and these also share the problem of proprietary transformer availability (also see High Voltage Woes).

Zener diode based low voltage power supplies have been used in logging tools for decades and certainly satisfy the KISS principle.  They are dependable and easily repaired even in very old tools (Zener diodes are available from many manufacturers; see AnaLog's Handy Zener Diode List for part numbers in common use currently).  On this page, "Zener" and "Zener diode" are used interchangeably.

A Zener diode permits current in the forward direction as in a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage or "Zener knee voltage".  Zeners get their name from Clarence Zener, discoverer of this electrical property.  A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener knee voltage.  This is accomplished with special doping of the semiconductor material, and the Zener knee voltage can be controlled rather accurately (tolerances within 0.05% are possible, but the most common commercial tolerances are 5% and 10%).

The GO / GOI / MLS COSMOS Low Voltage Power Supply is a good example of a KISS Zener based low voltage power supply used in COSMOS tools since their introduction in 1969, many of which tools are still in daily use.  This particular schematic also shows the resistor needed to convert a COSMOS tool to 120 volt operation.

The avalanche effect is similar to the Zener effect; in fact both effects are present in diodes properly called Zener diodes or avalanche diodes (all such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode").  In silicon diodes up to about 5.6 volts, the Zener effect is predominant and shows a negative temperature coefficient (tempco).  Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.  In a 5.6 volt diode, the two effects occur together and their tempcos just about cancel each other out.  As the Zener voltage goes up, the positive tempco rises (a 75 volt Zener has about ten times more positive tempco than a 12 volt Zener).

In many tools, the tempco problem inherent in Zener based power supplies can be ignored, but in others a temperature stable power supply may be needed.  Elaborate voltage regulators and voltage references have been used, but these often fail to do their job at elevated temperatures.  There are a number of tricks that exploit the particular tempco characteristics of different diodes to produce a temperature compensated Zener based supply.  Forward based conventional diodes with their negative tempco can be used to counter the positive tempco of Zeners above 5.6 volts, and different voltage Zeners can be combined in a manner that cancels tempco.  It should be noted that the switching supplies discussed above often use simple Zener diode circuits to regulate their outputs, and thus are not immune to the potential problems of Zener tempco.

This KISS Downhole Zener Low Voltage Power Supply uses forward biased conventional silicon diodes in conjunction with a Zener diode to achieve temperature compensation in the 12 volt section of the supply.  Here are three more temperature compensated Zener supplies:  Temperature Compensated Low Voltage Power Supply 1, Temperature Compensated Low Voltage Power Supply 2, and Temperature Compensated Low Voltage Power Supply 3 (also see 106LVPS-1, 106LVPS-2, or 106LVPS-3 Power Supply PCB).

11-27-09
Last 10-20-10