PRC 2008 Product Catalog - Precision Resistor - #4

One of the shunt values we tested was a ten milliohm, 10-watt part (PLV10AL 0.01Q ±1%) that developed over 30 amperes under full load. Because of its aluminum oxide rectangular case and special low TCR element, the resistance change (under load) was less than ±0.1% ... and there was virtually no measurable EMF. Now that is a breakthrough!

We certainly were impressed with the results ... and we are confident we can help you in a similar manner.

1. Wide variety of shapes & sizes

2. Measurable and predictable voltage/temperature relationship piece-to-piece.

3. Low value/close tolerance combinations.

4. Low TCRs

5. High temperature insulation.

6. Single-joint 4-terminal construction to eliminate lead-out and contact resistance.

7. Assorted diameters of pure copper leads for high current-carrying capacities and lower resistance per circular mil foot.

That's right, you can design, test and fine-tune engineering samples so they perform like a precision instrument. Try a few of our shunts today, because many of the popular values are in-stock for immediate delivery. Better yet, if we don't have exactly what you need in stock, special engineering samples will be manufactured to your specs quickly . and at no cost!

The TCR and temperature are also vital factors when specifying very accurate resistors and must be part of the equation.

Resistance Tolerance - is expressed as (±) plus or minus percent of the nominal value (ohms) required. (All PRC resistors are calibrated and tested within specified limits at 25°C unless otherwise noted.)

TCR - We know there is no TCR (Temperature Coefficient of Resistance) without a change in temperature. That is, no proportional change in resistance without a change in the ambient or some self-generated temperature shift resulting from an excitation of power. This variation in resistance with respect to the change in temperature is expressed in parts per million (ppm), Percent (%) or in ohms/ohm ... per °C

For Example: The TCR for 0±5ppm, also expressed as ±0.000005Q or ±0.0005% per degree C. as with all TCRs at PRC, is calculated between +25°C. and +100°C using the industry standard formula on page 9 ... unless otherwise noted. Therefore, a 1000Q 5ppm resistor is multiplied by 0.000005Q or 0.005Q change/°C

Temperature - is the measure of heat or cold of an object or substance and directly related to the TCR and tolerance. Since the TCR char. is the variation in resistance above or below room ambient (23°C or 25°C) or a span that includes both, it is essential with close resistance tolerances and low TCR requirements, to specify the temperature span of operation and treat all the surrounding conditions affecting the resistor as "one-spec."

Stability is the quality to resist permanent change, and must be designed into the parts from the start. It is very difficult to stabilize or condition a general-purpose resistor and be confident that it will meet a critical application. All stability specs at PRC are designed for precise requirements even if you specify ±1% resistance tolerances.

PRC's precision power resistors are noted for the surge current handling and overload capabilities. However, all catalog ratings are based upon standard ±1% resistance tolerances at +25°C or +125°C depending upon the resistor type. Derating is required for higher temperatures and closer tolerances. Please refer to the derating curves for each type resistor. Usually a larger part or a lower TCR will help, but heat is heat and must be carried off in some manner.

The LVS on page 6 as well as the PLV 4-lead version on page 7 are ideal in voltage drop applications and for accurate current-sensing requirements... because of their stability and flexibility. These unique parts not only offer lower values with closer tolerances, but also lower TCR's (10ppm) . over a wider temperature span. The low TCR feature provides better thermal stability for more dependable measurements under load conditions. In addition, PRC's shunt values provide low EMF with respect to the tinned copper terminals. Thermal effects, EMF and dissimilar metals become part of the resistance readings of low values. Specify low EMF and try to avoid thermal gradients that could cause a large temperature difference across a critical part. Obviously, the more information we have, the better we are able to match parts to your specifications.

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