Medium Power Resistive Decade Load Box
PART NO. 2171997
This kit will allow the hobbyist or engineer to build a 5W Decade Resistance Box with 1% accuracy from 0.1 ohm to 999.9 ohms. The
box can be set to any resistance within the listed range in 0.1 ohm steps. Once completed it will serve as a variable resistor for circuit
creation and debug or as a high power load for testing circuits ability to drive resistive loads. This is ideal for engineering laboratories,
university and college laboratories, high school physics laboratories, and garages or home shops.
Time Required: 3 hours depending on experience
Experience Level: Intermediate
Required tools and parts:
Drill bits, 1/8 in to 7/16 in
Small crescent wrench
Phillips screwdriver
Soldering Iron
Metal file or nibbler
Allen Wrench
Wire stripper
** Some components in the BOM are not included, see below.
Bill of Materials:
Qty
Jameco SKU
Component Name
7
659956
RESISTOR 0.1 OHM 5W
7
660130
RESISTOR 1.0 OHM 5W
7
660375
RESISTOR 10 OHM 5W
4
576501
Switch Rotary SP12T
4
264998
KNOB,1/4 INCH SHAFT,33.4MM
1
1582170
Red Banana Jack
1
1582188
Black Banana Jack
1
209358
CASE,METAL,7.5 INCH X9.8 INCH X3.2 INCH
100
2094389
SCREW,PAN HEAD,PPN4-40X1/4,SOCKET PHILLIPS PAN HEAD
100
40943
NUT,HEX,4-40,! ZINC PLATED STEEL (100)
100
106826
FLAT WASHER,4,9/32(OD),! ZINC PLATED STEEL,ROHS (100)
1
36792
WIRE,HOOK-UP,SOLID,22AWG,BLACK,100'
Step 1 - Resistor placement
Take the base of the box and all of the 5W or 10W resistors and place them into the base with the lowest value resistor on the right
side of the box. Place the 0.1 ohm resistor into a row evenly spaced along the right side of the base of the box. Then place the 1
ohm resistors in a straight line evenly spaced next to the 0.1 ohm resistors without allowing them to touch. Next place the 10 ohm in
line and the 100 ohm resistors in a line on the left side of the base of the box as shown below:
Step 2 - Resistor placement marking
Carefully without disturbing other resistors mark the inside of each hole in the
aluminum cases (mounting holes) of each resistor with a felt tip pen. A Sharpie extra
long felt sharp point marker works best.
Step 3 - Divet Resistor Mounting Holes
Remove Resistors from the box. Using a hole punch to divet each felt marker mark in
the center of the marks made from step two. The hole punch should be placed in the
center of each mounting hole marked.
Step 4 - Drill Resistor Mounting Holes
Drill holes in the Box at each divet location of the resistors you've used to create your
box using an 1/8" drill bit.
Step 5 - Drill Switch Holes for switch mounting
Take the dial paper and poke a small hole in the center of each hole without tearing the paper. Take the box side wall on the side
with the 10W resistors, hold the dial picture over the outer side of the front side that has the clearance left for easier wiring. Holding
the dial picture in the center of the side mark the center hole of each using a felt marker. Be sure to keep the paper level so the
switches will be mounted level with respect to each other and the box. Punch the marks using the center punch. Drill out the four
holes on the front of the box incrementally increasing the drill size for the four switches and the label using no greater than a 7/16" drill
bit.
Step 6 - Add no turn notch into box front wall
Insert the Power switches partially with the small No-turn lug directly below the hole, mark the location of the turn stop (lug). Drill out
one side of the No-turn lug area and use a nibbler or file to open a slit big enough for the no-turn lug to slide in, but small enough so
that it doesn't allow play in the No-turn lug so that the switch won't turn when inserted into the box and operated after assembly.
Step 7 - Resistor Mounting and series connections
Mount each resistor into place and then solder pairs of resistors
together with a wire to each pair alternating the side as you go down
the lines (100, 10, 1, 0.1) as shown until a wire connects to each
resistor on each row.
Step 8 - Install the banana jacks into the back side of the box.
a. Measure the width of the box without the lid. Subtract 3/4" from
the measurement and divide by two. This is the distance from each
edge of the box that the banane jacks will be installed (length wise).
Mark a vertical line at this distance from each side of the box.
b. Measure the height of the box without the lid. Divide by two. Mark
this distance horizontally through the vertical lines drawn in the
previous step.
c. Use the hole punch to make divets where the lines intersect. The
divets should be level and 3/4" apart in the center of the back side of
the box.
d. Drill out the banana jack holes, insert the banana jacks and screw
the bolts that hold each banana jack in place to the box wall.
Step 9 - Install and wire the switches
Install the switches into the front of the box and begin wiring the switches into place following the schematic below (Tie pin 11 to pin
12 of each of the four switches):
Step 10 - Finalize wiring of switches
The following image shows a wired box with pin 2 shown:
Step 11 - Verify wiring of switches
Place an ohmeter on Pin 1 of the switches and on pin 2 and cycle through each position and verify the resistance increments by the
correct amount with each step.
Step 12 - Connect the wires to the banana jacks.
Connect the wires to the banana jacks from by adding the two wires shown in the schematic tied to the banana jack. The 100 ohm
row should tie to the black jack and the 0.1 ohm row should tie to the red jack.
Step 13 - Close the box lid
Screws the lid onto the base of the box.
Step 14 - Prepare 0.1 ohm curve
Print the 0.1 ohm resistor safe operating curve.
Laminate the graph and attach to the back right side of
the top of the box.
Step 15 - Prepare 1 ohm curve
Print the 1 ohm resistor safe operating curve. Laminate
the graph and attach to the back left side of the top of
the box.
Step 16 - Prepare 10 ohm curve
Print the 10 ohm resistor safe operating curve.
Laminate the curve and attach to the front right side of
the top of the box.
Step 17 - Prepare 100 ohm curve
Print the 100 ohm resistor safe operating curve.
Laminate the curve and attach to the front left side of
the top of the box.
Step 18 - Finalize the box
Cut 1/2" holes in the center of the resistance indicator paper in the center of each circle. Laminate the resistance indicator papers
and cut out 7/16" holes within each half inch hole previously cut. Place the resistance indicator paper over each switch and mount to
the box beneath the nuts that hold the switches in place. Add each knob to the switches and tighten them down. Now place the box
into service.