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.