Now that we have a completed design (see Lab Resistor Substitution Box – Part 1: Design), we need to select appropriate components so that we can begin building. First, we’ll define some basic parameters (specifications) that our components need to meet.
Don’t order the knobs I recommend, they do not fit. They should according to the description, but they don’t. I’ve ordred some replacements from Mouser and will see how they work.After defining the component specs, we’ll visit a few different supplier websites and perform some parametric searches to find the right components at the best price. (see the video segment).
Resistor Specifications
We’ve already specified in our requirements that we want the box to be able to handle 3W maximum power and that we want it to have a tolerance (accuracy) of 1%. So we will be choosing 3W, 1% resistors. Also, since we are going to solder the resistors directly to the switch terminals we want to select resistors with axial leads (leads that come out of the ends of the resistor). We’ll need nine each in values of 1Ω, 10Ω, 100Ω, 1kΩ, 10kΩ, and 100kΩ.
Note: Whenever more than one resistor is selected in the circuit, the power dissipation will be shared between resistors. Depending on the value selected using the rotary switches, the box can handle more than 3 watts. The 3W limit is per resistor. However, the resistance and number of resistors selected changes as you dial up the desired value so you should never exceed the maximum power rating of 3W.
Rotary Switch Specifications
We’ve already defined many of the specifications for our rotary switches, 10, 11, or 12 position (10 position minimum), single pole, single deck, non-shorting. But what about current handling?
It turns out that the box will draw maximum current when we are at maximum power (3W based on our specifications) and minimum resistance (1Ω). We can apply Ohms Law to determine the current draw at maximum power under this worse case condition.
We see from the above calculations that we need to select rotary switches that can handle at least 1.73A. We’ll round up to 2A and add that specification to our selection criteria.
Switch Knobs
There are a few different ways that we can ensure a clean, professional appearance for our resistor substitution box. For one, we could create or order custom decals to label our switches. While this provides the most options it’s also the most expensive. A sheet of custom decals can cost up to $100 delivered.
Another option is to use the same rub on dry transfer letters and lines that model railroaders use on freight cars and locomotives. These come in a variety of letter and line styles, colors, sizes, etc. and cost about $6 per sheet. The issue with this approach is that they can be hard to align consistently – it’s very difficult to label a graticule and 10 separate values around each of the rotary switch knobs.
However, if we could minimize the number of dry transfers we have to apply – If for instance, we don’t need a graticule or numbers around each knob – then this would be a great solution.
To achieve this, we’ll select knobs with a skirt that already has numbers imprinted. Then we only need a single short line and the number place (x1, x10, x100, x1k, x10k, and x100k) applied to the case at the top of each knob.
Whichever knobs you choose, make sure they fit the shaft on whatever rotary switches you’ve chosen. Most shafts are 1/4 (0.25) inch / 6mm in diameter but some differ. Just check to make sure the knob fits your rotary switches before you order.
Project Case (Enclosure)
There are no specific requirements for the enclosure but following a few recommendations will make assembly easier. We want to pick a case that is large enough to fit all of the components comfortably and still allows us to solder them easily. In addition, we want to be able to mount the switches and still have easy access to them from the rear. A case with a detachable front panel will achieve this. We’ll look for a suitable example.
Binding Posts
The only specific requirements for the binding posts is that they be insulated from the case. We want to pick some good quality binding posts that have both screw and banana plug connections. You can choose whatever color you like. In fact, since resistors aren’t polarized (and neither is our box) you don’t have to have different colors for the two terminals. Nor do you need to label them.
Hook Up Wire
The only other item we need is some stranded wire to hook it all up (switch to switch and to the binding posts). We want to select wire whose current carrying capability matches our minimum current requirement of 1.73A. Let’s consult a wire ampacity table.
We can see from the table that we need a minimum of 24 gauge wire. Feel free to use either 24, 20, or 18 gauge wire. Whichever of those three gauges you have on hand is fine. I’ll include the wire in our Bill of Materials, just in case.
Wire has it’s own resistance so the larger the diameter (smaller AWG) the lower the resistance. It won’t make a difference at higher selected resistor values but it may at smaller selected values. Using 18 AWG will keep it lower than the 24 or 22. You also want to keep the lengths as short as possible.
Component Sources
There are a great number of component suppliers. Use whichever you are most familiar or comfortable with. Here are some that I use regularly.
Digi-Key, Arrow, and Mouser carry hundreds of thousands, if not millions, of products and are geared toward manufacturers and professional designers/engineers – they still sell to the public and offer reasonable prices even in small quantities. They also include good parametric search capabilities which we’ll take advantage of for our search. Arrow offers the added advantage of offering free same day, overnight shipping in the U.S. on many items. This will often offset a slightly higher price.
Others, like Jameco, Spark Fun, and Adafruit carry a much more limited selection and are geared toward hobbyists. These sites offer a lot of learning and tutorial information but are usually more expensive than the larger suppliers.
Don’t forget Amazon and eBay. Especially for things like binding posts, enclosures, knobs, and hook up wire. You may even find suitable switches at a good price. Although I doubt we’d find all of the resistors we need on either of those sites. As a prime member, I almost always check Amazon first. The free shipping often offsets a small price difference.
Selecting The Actual Components
Now comes the fun, but sometimes tedious task of searching through the various supplier websites to find and order suitable components at the best price. We’ll do this by visiting supplier websites and performing parametric or other searches.
See the video linked in at the beginning of this post. This way I don’t have to include dozens and dozens of screenshots here. You can watch the video to learn how to perform a parametric search for components like our rotary switches and resistors. If you just want to order the parts, see the recommended Bill of Materials (BOM) and associated links below.
Bill of Materials (BOM)
Below is the BOM and some links to the supplier pages for the components and other parts I’ve selected for our resistor substitution box project.
Amazon Parts
Project Case/Enclosure $ 22.99 Numbered Knobs $ 10.99 Binding Posts $ 9.99 20 AWG Hookup Wire $ 16.99
Arrow Electronics
Rotary Switches $ 33.96 10Ω Resistors $ 4.71 1kΩ Resistors $ 7.13 10KΩ Resistors $ 2.14
Mouser
Conclusion
So how did we do at staying within our <$100.00 budget. Well, we went a bit over at $124.33. But our original target assumed that we already had the hookup wire – which I do. If we take off the $16.99 for the wire we come in at $107.34. Also, keep in mind that we’ve selected a premium extruded aluminum case for this build. If we opt for a plastic case instead we easily meet the sub $100.00 target. And, with a little more searching we might even find better prices on some of the components.
Also, if you don’t mind building a 1W version, you can save quite a bit on the switches and the resistors, get well under $80.00, and still have a much better resistance substitution box than you could buy for almost twice the price.
Well, that will do it for this episode. Go ahead and order your parts if you’d like and we’ll start the assembly in the next installment which will be posted shortly after the parts arrive. See you then.
In the mean time, if you enjoyed this article, please like and subscribe. As always, thoughtful, on topic comments are always appreciated.
Cheers
Dominick
