Question 1.29: A current of 100 mA (±20%) is to be drawn from a 28 V DC sup......
A current of 100 \mathrm{~mA}( \pm 20 \%) is to be drawn from a 28 \mathrm{~V} DC supply. What value and type of resistor should be used in this application?
The value of resistance required must first be calculated using Ohm’s Law:
R=V / I=28 \mathrm{~V} / 100 \mathrm{~mA}=280 \Omega
The nearest preferred value from the E12 series is 270 \Omega (which will actually produce a current of 103.7 \mathrm{~mA} (i.e., within \pm 4 \%> of the desired value). If a resistor of \pm 10 \% tolerance is used, current will be within the range 94 \mathrm{~mA} to 115 \mathrm{~mA} (well within the \pm 20 \% accuracy specified).
The power dissipated in the resistor (calculated using P=I \times V ) will be 2.9 \mathrm{~W} and thus a component rated at 3 \mathrm{~W} (or more) will be required. This would normally be a vitreous enamel coated wirewound resistor (see Table 1.8).
| Table 1.8: Characteristics of common types of resistors | ||||||
| Resistor type | Property | |||||
| Metal clad | Vitreous wirewound |
Ceramic wirewound |
Metal oxide |
Metal film |
Carbon film |
|
| 0.05 to 10k | 0.1 to 22k | 0.47 to 22k | 10 to 10 M | 1 to 1M | 10 to 10M |
Resistance range ( Ω) |
| ±5 | ±5 | ±5 | ±2 | ±1 | ±5 | Typical tolerance (%) |
| 10 to 300 | 2 to 4 | 4 to 17 | 0.25 to 0.5 | 0.125 to 0.5 |
0.25 to 2 | Power rating (W) |
| +50 | +75 | +250 | +250 | +50 to +100 |
-250 | Temperature coefficient (ppm/°C) |
| Good | Good | Good | Excellent | Excellent | Fair | Stability |
| n.a. | n.a. | n.a. | Excellent | Excellent | Fair | Noise performance |
| -55 to +200 |
-45 to +125 |
-45 to +125 |
-45 to +125 |
-45 to +125 |
-45 to +125 |
Ambient temperature range (°C) |
| Very high power applications |
Power supplies, loads, medium and high-power applications |
Amplifiers, test equipment, etc., requiring low-noise high-tolerance components |
General-Purpose | Typical applications | ||