Before we move on to some example applications of the decade resistance,
let’s take a closer look at the components, because they place some limits
on the decade box.
With modern, low-voltage electronics, we often operate under the
assumption that we aren’t using much power, or passing much current.
Sometimes we are reminded of the power or current involved when things
heat up, reset mysteriously, start to smell hot, or catch fire.
To help avoid those situations, let’s explore the main constraints of the
decade resistance.
Resistors
The first place to look is the resistors. The resistors are rated to dissipate
up to ¼ W each. This might not seem like much, but for many circuits, it’s a
reasonable upper limit. Let’s examine the reasons a little more closely.
We know that power in Watts is calculated using.
P = IV
If we don’t know the Voltage or Current terms, but we do know the
resistance, we can substitute Ohm’s law in for the missing I or V term,
resulting in
P = (V^2)/R
or
P = (I^2) * R
We’ll take a shortcut, and assume that the maximum voltage across the
decade resistance is the difference between the positive and negative
supply rails. For some common supply voltages, we’ll calculate the
resistance that draws 1/4W.
Supply Voltage
R that draws 1/4W
Current Through R at 1/4W
3.3V
43.56
Ω
75 mA
5V
100
Ω
50 mA
9V
324
Ω
27 mA
12V
576
Ω
20 mA
30V
(+/- 15V supply)
3600
Ω
8 mA
This table can serve as a guideline for applying the decade resistance. For
a circuit powered by a given voltage, the “R that draws 1/4W” column
indicates the resistance below which you need to consider the power drawn
by the box.
But this is a guideline, and not a hard rule. Some circuits use high voltage
supplies, but don’t put the full voltage across the decade box.
Switches
Additionally, the manufacturer states that the rotary switches are limited to
passing 300 mA.
To Summarize
If you find yourself dialing in resistances below 500 Ohms, take a moment
to work out how much power you’ll be dissipating, and current you’ll be
drawing. The maximum limits for the decade box are overall power
dissipation of ¼ Watt, and the current passed of 300 mA. If you’re unsure,
you can always use an Ammeter in series with the Decade Resistance to
verify the current draw.
Page 12 of 16
