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1/4 W Resistor Kit: Carbon Film Resistors Rated and Colour-Coded to MIL-PRF-22684 Style RL07
A 1/4 W rating is earned, not printed: the US military resistor specification gives its RL07 style 1/4 W at 70 °C only if the resistors survive its tests. Carbon film resistors in a range of values are checked, overloaded for 5 seconds, run for 1,000 hours at 70 °C on a sample, colour-coded with five bands and sorted into a kit.
Result: an assortment of 1/4 W carbon film resistors, ±5 % or ±2 %, each band-coded with its value.
Source: MIL-PRF-22684F, Resistors, Fixed, Film (Insulated) (US Department of Defense, 2006), style RL07. Not given here: the RL07 body and lead sizes and the allowed change in the life test, which are in the RL07 detail sheet. Unknown: the band paint; only its colours are fixed. Picture: Wikimedia Commons.
Intermediate
6 weeks
Instructions
1
1
Check each value
Check each value
Measure the dc resistance of every resistor. It must lie within its tolerance of the nominal value: ±5 % or ±2 %. Work out its rated working voltage from E = √(P × R) with P = 1/4 W, but never above 250 V: for example 50 V for 10 kΩ, 9.1 V for 330 Ω. Ours: a Wheatstone bridge.
Materials for this step:
Carbon Film Resistor1 assorted values, as many of each as the kit needsTools needed:
Wheatstone Bridge2
2
Overload for 5 seconds
Overload for 5 seconds
Mount a resistor horizontally in still air, nothing within 3 in. of it except the base 2 in. below. Apply 2.5 times its rated working voltage (but not more than 500 V) for 5 seconds. Measure again 30 to 35 minutes later: the change may not exceed ±(0.5 % + 0.05 Ω), with no arcing, burning or charring.
Tools needed:
DC Power Supply
StopwatchWheatstone Bridge3
3
Run the life test on samples
Run the life test on samples
Solder sample resistors from the batch to lightweight terminals with 1 in. of lead, spaced so none warms another, in still air at 70 °C (+15, -5). Measure them first, then apply the rated working voltage 1 1/2 hours on and 1/2 hour off for 1,000 hours, measuring at the end of an off period after 250, 500 and 1,000 hours. A batch whose samples pass carries the 1/4 W rating. Not given here: the allowed change, which the RL07 detail sheet sets. Ours: an oven as the 70 °C chamber.
Tools needed:
Oven
ThermometerDC Power Supply
Soldering Iron
Solder WireWheatstone Bridge4
4
Paint the colour bands
Paint the colour bands
On a body of any colour but black (tan is preferred), paint five bands: first figure, second figure, multiplier (black 1, brown 10, red 100 ... blue 1,000,000; gold 0.1, silver 0.01), tolerance (gold ±5 %, red ±2 %), and a wider white band, about 1 1/2 times the others, for solderable leads. Figures: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. So 330 Ω ±5 % is orange, orange, brown, gold, white. Unknown: the paint; the colours must be permanent and nonfading.
Materials for this step:
Colour-Band Marking Paint1 set of coloursTools needed:
Fine-pointed Brushes5
5
Sort into the kit
Sort into the kit
Sort the rated, coded resistors by value into the compartments of a box, one value to a compartment.
Tools needed:
Game Component Organizer BoxMaterials
2- 1 assorted values, as many of each as the kit needsPlaceholder
- 1 set of coloursPlaceholder
Tools Required
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