Page 116 - Electromechanical Devices and Components Illustrated Sourcebook

78 Electromechanical Devices & Components Illustrated Sourcebook

Decade Resistance Boxes A low cost decade resistance box may be constructed using
a banana jacks and jumpers, as shown in Figures 4-128 and
For laboratory applications, decade resistance boxes are very 4-129. This unit has the same functionality as the commercial
useful pieces of equipment. They are generally supplied with unit but can be built to carry much higher currents.
a 1 megs-ohm maximum resistance which is switchable in
1-ohm increments. Figure 4-126 shows a typical commercial
decade resistance box. Figure 4-127 shows a schematic repre- Voltage Dividers
sentation of the unit.
A very common use for resistors is as voltage dividers. By
selecting two different value resistors and arranging them as
Case
Terminals
Selector Switches shown in Figure 4-130, a lower output voltage can be gener-
Front Panel ated. The output voltage is based on the ratio of R and R . To
1
2
calculate the output voltage use the following formula:
OUTPUT
4 5 4 5 4 5 4 5 4 5 4 5
3 6 3 6 3 6 3 6 3 6 3 6
V V 1 [R 1 (R 1 R 2 )]
2 7 2 7 2 7 2 7 2 7 2 7 2
1 8 1 8 1 8 1 8 1 8 1 8
Example: V 2 (3 volts) V 1 (12 volts) [R 1 (75 k )
0 9 0 9 0 9 0 9 0 9 0 9
(R 1 (75 k ) R 2 (25 k )
OHMS OHMS × 10 OHMS × 100 OHMS × 1K OHMS × 10K OHMS × 100K
Figure 4-126 1-Mega-Ohm Decade Resistance Box
Figure 4-131 shows a typical voltage divider arrangement.
Low wattage versions of this configuration can be constructed
1 × 10 × 100 × 1K × 10K × 100K × and heat shrunk directly into a feed cable.
Figure 4-132 shows a potentiometer set up as an adjustable
voltage divider.
Another common use for resistors is as current limiting
devices. A resistor is added to a circuit to prevent the possi-
bility of a short-circuit situation. Figure 4-133 shows how a
Terminals
Figure 4-127 Decade Resistance Box Schematic

(M) Banana Plugs

Test Lead Wire

Banana Jumpers

Banana Jacks

0 1 2 3 4 5 6 7 8 9 COMMON
1 X
Case
10 X
Panel
100 X

1K X

10K X

100K X
OUTPUT

Figure 4-128 Decade Resistance Box Using Banana Jumper Plugs