Flexible Bench Load - 20-100 Ohms

Flexible Bench Load - 20-100 Ohms

This is a configurable bench load with five 100 ohm, 1%, 1 W resistors on a circuit board attached to a 3D printed base.

Yes, a high-wattage pot could do the same thing as well or better but where's the fun in that?

Why? I've found that some things just don't work out very well on a breadboard and are better incorporated as stand-alone bench components.  Those can be jumpered to a breadboarded circuit with a couple of wires while sitting apart.  It simplifies the breadboard circuitry and mitigates the crappy contact issues that sometimes plague components on an over-full breadboard.

A 3D printed base makes items like this stable and protects the back side of the board from accidental shorting with other objects on the bench. I think of things like this load module as "black boxes" that can be plugged into the circuit on the breadboard without actually having to be built on the breadboard every time they're needed.

Using jumper caps this board can be configured with any number of the five resistors in parallel.  It is also possible to put some of them in series using Dupont jumper wires.  The latter was not part of my original intent nor did I test it, but it looks like it would be possible if needed.

I kept the resistors up off the board, utilizing the full length of the leads.  This keeps the resistor heat off the board and allows the individual resistors to be separated to allow better heat dissipation.

Without jumper caps or wires, only the center resistor is connected to the I/O pins, so 100 ohms, 1 W in that configuration.

With jumper caps, the board can be set up for...

50 ohms, 2 W (two resistors in parallel)
33 ohms, 3 W (three resistor in parallel)
25 ohms, 4 W (four resistors in parallel)
20 ohms, 5 W (five resistors in parallel)

When using jumper caps, the jumper setup should be the same at each end of the board.

Using Dupont jumper wires, there are other possible configurations.

The board is Vero strip board.  The board edges look like a postage stamp because I broke it to size in a vise.  If you do that, don't drill the screw holes until after the board is to size.  Yes, that's the voice of experience.  

There's an unconnected 2 X 8 black pin header on one edge. Its only function is as a parking spot for eight jumper caps when they're not being used. I'm using jumper caps with handles but the shorter version would work just as well.

I used black and red single pin headers for the intended I/O pins, nearest to each end of the board to help keep things clear. Of course there's no actual polarity requirement with resistors.  I only did it that way to visually discriminate between the I/O pins and the yellow jumper cap locations.

The 3D printed base has a cavity below the board for the solder connections and the screw holes have shouldered pockets for M2.5 brass threaded inserts.  Use a screw to pull them in cold until shouldered; heat setting is not necessary. The board is mounted to the base with M2.5 X 10 mm machine screws; flat washers are optional.  There's no possibility of the screws or washers shorting anything. The screw holes are 1/8" or 3 mm, whichever drill size you happen to have handy.

Note from the images that a cut (blue stripe) is needed on the copper side of the Vero board across the five strips under the resistors.  This is necessary, otherwise the resistors are shorted.

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Printing Notes:

PLA

0.4 mm nozzle

0.1 mm layer height (because of the labeling on the end)

Shells: 0.8 mm

Infill: 16%

Design: Tinkercad

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Materials:

5 each, 100 ohm, 1%, 1 W resistors

4 each, 2.5 mm brass threaded insert, 4 mm long, 3.5 mm OD

4 each, M2.5 X 10 mm machine screw

4 each, M2.5 flat washer,(optional)

Assorted pin headers

Vero board