A far better approach might be to have a
device which is adjustable over a wide
range that will provide the necessary
value. When looking for a resistance, you
might be tempted to use a
potentiometer; once the circuit functions,
read the value and substitute with a fixed
component. So, you obtain a 10-turn pot
to facilitate fine adjustment.
This approach works as long as the
power requirement of the pot is not
exceeded. Most 10-turn pots will only
take limited power. So, you’re looking at
having to spend a lot of money for a
suitable wattage unit — especially if
you’re designing power supply circuits
(where the power requirements of the
components can be quite large).
A far better compromise is to have a
"box" with suitable pots that can take a
fair amount of current, yet be adjustable
over a wide range. That's where a decade
box can save you.
The original decade boxes consisted of a number of
fixed precision resistors and switches that would add or
subtract the selected values. These boxes were not only
costly, but tended to be very large.
The other day, I had an idea to duplicate the
functionality of the boxes (without the size or price). The
advent of cheaper and cheaper digital meters and the
accuracy they bring lends itself to a much more efficient
alternative.
Previous decade boxes had faceplates with the
resistance values printed on them so you could adjust the
box and then add the values to obtain the resistance
needed. My approach here is one of simplicity. Instead of
having the printed values on the panel and adding up the
values, why not adjust the box and then measure the total
using a digital meter!
So, I set out to build my box. For simplicity and also
low cost, I used linear taper potentiometers. These devices
take up little space and can be had for cheap. In order to
add or subtract the resistance values, I use common
single-pole switches. The box can be as simple or
complicated as you wish. I have limited the total value of
resistance to be 1.1111 megohms (this is derived by
adding one meg, 100K, 10K, 1K, and 100 ohms together).
However, if you like, you can add resistance to either end
to tailor the box to your specific needs.
As stated above, the measurement device is a simple
digital voltmeter with resistance scales. The five-way
binding post connection allows the meter to be connected
when needed, so as not to tie it up. These meters are so
cheap today, that you might want to incorporate a
; SCHEMATIC.
Table 1. Standard
Resistor Values (¹5%)
1.0 10 100 1.0K 10K 100K 1.0M
1.1 11 110 1.1K 11K 110K 1.1M
1.2 12 120 1.2K 12K 120K 1.2M
1.3 13 130 1.3K 13K 130K 1.3M
1.5 15 150 1.5K 15K 150K 1.5M
1.6 16 160 1.6K 16K 160K 1.6M
1.8 18 180 1.8K 18K 180K 1.8M
2.0 20 200 2.0K 20K 200K 2.0M
2. 2 22 220 2.2K 22K 220K 2.2M
2. 4 24 240 2.4K 24K 240K 2.4M
2. 7 27 270 2.7K 27K 270K 2.7M
3.0 30 300 3.0K 30K 300K 3.0M
3. 3 33 330 3.3K 33K 330K 3.3M
3. 6 36 360 3.6K 36K 360K 3.6M
3. 9 39 390 3.9K 39K 390K 3.9M
4. 3 43 430 4.3K 43K 430K 4.3M
4. 7 47 470 4.7K 47K 470K 4.7M
5.1 51 510 5.1K 51K 510K 5.1M
5. 6 56 560 5.6K 56K 560K 5.6M
6. 2 62 620 6.2K 62K 620K 6.2M
6. 8 68 680 6.8K 68K 680K 6.8M
7. 5 75 750 7.5K 75K 750K 7.5M
8. 2 82 820 8.2K 82K 820K 8.2M
9.1 91 910 9.1K 91K 910K 9.1M
April 2014 35
