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Mil Spec Resistor Data

Military specification (mil spec) resistors have been used in nearly every well logging tool ever built, and in a good deal of surface electronics as well.  This is true because these high grade resistors are capable of performing dependably at the high temperatures encountered in deep well logging applications.

There are dozens of mil spec resistor styles, and dozens of applicable mil spec or mil performance spec documents (if you cannot find a spec in the format MIL-R-xxxx, try the newer MIL-PRF-xxxx).  Check out the Defense Supply Center, Columbus (DSCC) for more information on military specifications and standards than you ever dreamed existed.  The better mil spec grades have "Established Reliability", with the best having a known failure rate not exceeding .001% per 1000 hours.  Many are also characterized for operation to 175ºC, with the power derrated by half at 125ºC from the 70ºC values.  The following tables summarize information about mil spec resistors commonly encountered in well logging electronics:

TEMPCO

The "Characteristic" column in the above tables gives the temperature coefficient (tempco) specifications available for each of the mil spec resistor styles covered.  In general, the temperature coefficient is the relative change of a physical property when the temperature is changed by 1º C or K.  Tempco values are simply an expression of how stable a resistor ohmic value will be with changing temperature.  Temperature coefficient is especially important in downhole oil well logging tools where a tool may have to operate over a very wide temperature range (cold outdoor temperatures up to say 200º C / ~400º F).  As a simple example, a one (1) kilohm (1,000W) resistor with a 100 ppm tempco can be expected to change ten ohms (10W) in value over a 100º C / 180º F temperature excursion or one percent (1%).  Mil spec resistors are often marked with a "T" code to indicate the tempco rating; here is a list of common codes encountered and the ppm values associated with them:

T-16  =    5 ppm
T-13  =   10 ppm
T-10  =   15 ppm
T-9   =   25 ppm
T-2   =   50 ppm
T-1   =  100 ppm
T-0   =  150 ppm
T-00  =  200 ppm

Power Resistors

For space economy, no tables have been produced for the larger power resistors.  For reference purposes, the RE / RER type mil spec power resistor is aluminum housed and is similar to the Dale RH gold anodized aluminum housed commercial / industrial product.  The RW / RWR type mil spec resistor is either silicone coated or vitreous enamel encapsulated and is similar to the Dale RS silicone coated commercial product.  If you are using Dale RS resistors, take a look at the cheaper CW model (5% tolerance is good enough for most power resistor applications).


Also see our Standard EIA Decade Resistor Values Table and 1% Resistor Color Codes.


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Last 03-13-07