1. ECE 3301 General Electrical Engineering
Presentation 06
Potentiometers and Power Rating

2. Potentiometers

3. Potentiometers
Current equal to VS/RP passes through the potentiometer.
As the slider is moved from the bottom to the top of the potentiometer, Vout varies from 0 to VS volts. VS RP
Vout

4. Potentiometers
The position of the potentiometer determines the constant k.
k ranges from 0 to 1. VS
Vout
(1 – k)RP
k RP IL
𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑘 𝑅 𝑃 𝑘 𝑅 𝑃 + 1−𝑘 𝑅 𝑃
𝑉 𝑜𝑢𝑡 =𝑘 𝑉 𝑆
This linear operation is only true if IL = 0

5. Potentiometers 𝑅 𝑒𝑞 = 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 +𝑘 𝑅 𝑃
𝑅 𝑒𝑞 = 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 +𝑘 𝑅 𝑃
𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 +𝑘 𝑅 𝑃 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 +𝑘 𝑅 𝑃 + 1−𝑘 𝑅 𝑃
𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 𝑘 𝑅 𝑃 + 𝑅 𝐿 +𝑘 𝑅 𝑃 1−𝑘 𝑅 𝑃 VS
Vout
(1 – k)RP
k RP IL RL

6. Potentiometers 𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 𝑘 𝑅 𝑃 + 𝑅 𝐿 +𝑘 𝑅 𝑃 1−𝑘 𝑅 𝑃
𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑅 𝐿 𝑘 𝑅 𝑃 𝑅 𝐿 𝑘 𝑅 𝑃 + 𝑅 𝐿 +𝑘 𝑅 𝑃 1−𝑘 𝑅 𝑃
𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑘 𝑘+ 1+𝑘 𝑅 𝑃 𝑅 𝐿 1−𝑘
𝑉 𝑜𝑢𝑡 = 𝑉 𝑆 𝑘 1+𝑘 𝑅 𝑃 𝑅 𝐿 − 𝑘 2 𝑅 𝑃 𝑅 𝐿 VS
Vout
(1 – k)RP
k RP IL RL

7. Potentiometers Connection as a Variable Resistor. VS RP VS (1 – k)RP

8. Resistor Power Ratings
In addition to the resistance value of a resistor, each resistor also has a power rating.
The power rating indicates how much power (heat) the resistor can safely dissipate without being damaged, or damaging a circuit board.
The power rating is related to the physical size of the resistor.

12. Power rating
Body length (l)
Body diameter (d)
Lead length (a)
Lead diameter (da)
Watt mm
1/8 (0.125)
3.0 ± 0.3
1.8 ± 0.3
28 ± 3
0.45 ± 0.05
1/4 (0.25)
6.5 ± 0.5
2.5 ± 0.3
0.6 ± 0.05
1/2 (0.5)
8.5 ± 0.5
3.2 ± 0.3 1
11 ± 1
5 ± 0.5
0.8 ± 0.05

13. 3 Figure Resistor Code System
This SMD resistor marking system uses three figures. The first two figures in the code indicate the significant figures, and the third is a multiplier. This is the same as the colored rings used for through-hole resistors.
An SMD resistor with the figures 472 would have a resistance of 47 x 102 ohms, or 4.7kΩ. A resistors marked with 100 is not 100 ohms, but it follows the scheme exactly and it is 10 x 100 or 10 x 1 = 10 Ω.

14. Code
Length (l)
Width (w)
Height (h)
Power
Imperial
Metric
inch mm
Watt
0201
0603
0.024
0.6
0.012
0.3
0.01
0.25
1/20 (0.05)
0402
1005
0.04
1.0
0.02
0.5
0.014
0.35
1/16 (0.062)
1608
0.06
1.55
0.03
0.85
0.018
0.45
1/10 (0.10)
0805
2012
0.08
2.0
0.05
1.2
1/8 (0.125)
1206
3216
0.12
3.2
1.6
0.022
0.55
1/4 (0.25)
1210
3225
0.10
2.5
1/2 (0.50)
1218
3246
0.18
4.6 1
2010
5025
0.20
5.0
3/4 (0.75)
2512
6332
6.3

15. 10 W
100 W
2350 W