Printed on part:

TRW 8624 A-502

955979-57

LPW-15A .5Ω  1%

Manufacturer:  TRW

Type:  Wirewound

Style:  Sandblock

Resistance:  1/2 ohm

Tolerance:  1%

Power:  15 watts (estimate)

Length:  1.83" (46.5 mm)

Width:  .5" (12.7 mm)

Height:  .625" (15.9 mm)

Lead Style:  Axial

Lead Length:  1.3" (33 mm)

    These resistors stand out from others of the same type by virtue of one outstanding characteristic: a tolerance of 1%. This is important since a prime application for this kind of resistor is that of a series current monitoring resistor.  For example, since this resistor is .5 ohm, if the voltage across it is 2 volts, the current through it is 4 amps, if the voltage is 3 volts, the current is 6 amps, etc.  The overall tolerance of the measurement is the same as the tolerance of the resistor itself; in this case, 1%.

    And this resistor can take the heat.  Since the power is I²R, if the resistor is .5 ohm, 15 watt, then the current through it can be as much as the square root of 30, or almost 5.5 amps. 

    Now, having said that, I need to add that I am unsure of the actual wattage of these TRW resistors.  TRW was a big aerospace company that made a lot of advanced good stuff, but they are now out of business. I haven't been able to nail down the wattage.   Similar Resistors of a smaller size are around 10 watts, and slightly larger sizes are 25 watts  or more.  My best estimate is 15 watts, and I think that's fairly conservative. (If anyone knows, I'd sure like to hear from you.)

    One thing I learned during my search for the wattage is that the 1% tolerance of these resistors comes at a premium.  Most of these larger power resistors have a tolerance of only  5% or 10%. 

    I have just a little over a hundred of them, and once they're gone I won't be able to get any more.