Resistor
From HvWiki
A resistor is a passive electronic component that restrics current by producing a voltage drop between its terminals.
The current, voltage and resistance of a resistor are related by Ohm's law.
V = IR
Where V is the voltage in volts (V), I the current in amperes (A, "amps") and R the resistance in ohms (Ω).
Contents |
Real world resistor equivalent
- R is the DC resistance
- LS is the inductance in the connection wires
- LR is the inductance in the resistor material
- CL is the parasitic capacitance of the resistor
Some resistors like the wirewound types will have a lot of inductance and some may only be suitable for direct current.
How to rate a resistor
The complete answer is complicated and depends on the type of resistor and the design of the circuit,
Some types of resistors have a very low thermal mass and may overheat in a very short time, those should be ratet for the full pulse power. Homogeneous resistors like carbon cylinders will average out the power over several seconds and can within reason be rated for the average power.
If rated at maximum power the resistor will run hot and that is usually not a good idea when it comes to reliability.
See overload
Maximum temperature
Overtemperature will shorten the lifetime of a resistor. Remember that if the solder points gets too hot they will age very fast and fail.
- Epoxy coated 150°C
- Silicone or aluminium 200°C
- Glazed 350°C
Schematic symbol
Resistor marking
Colour code
Rather than having the value of resistance written in numerals on their sides, resistors usually have them encoded in a series of colored bands around them. This makes it easier pick out values when looking at a lot of resistors on a board or in a pile.
| Color | First digit | Second digit | Third digit | Multiplier | Tolerance | Temp. Coeff. |
|---|---|---|---|---|---|---|
| Black | - | 0 | 0 | 1 Ω | 20 % | 200 ppm/K |
| Brown | 1 | 1 | 1 | 10 Ω | 1 % | 100 ppm/K |
| Red | 2 | 2 | 2 | 100 Ω | 2 % | 50 ppm/K |
| Orange | 3 | 3 | 3 | 1 000 Ω | 3 % | 15 ppm/K |
| Yellow | 4 | 4 | 4 | 10 000 Ω | - | 25 ppm/K |
| Green | 5 | 5 | 5 | 100 000 Ω | 0.5 % | - |
| Blue | 6 | 6 | 6 | 1 000 000 Ω | 0.25 % | 10 ppm/K |
| Violet | 7 | 7 | 7 | 10 000 000 Ω | 0.1 % | 5 ppm/K |
| Gray | 8 | 8 | 8 | 100 000 000 Ω | 0.05 % | 1 ppm/K |
| White | 9 | 9 | 9 | 1 000 000 000 Ω | - | - |
| Gold | - | - | - | 0.1 Ω | 5 % | - |
| Silver | - | - | - | 0.01 Ω | 10 % | - |
| None | - | - | - | - | 20 % | - |
The digits from 0 to 9 are each represented by a color. The first few stripes represent actual digits of the resistor value. For a four-band resistor this is the first two; for a five band it's the first three. The next band is the multiplier, the number of zeros to add to the string of digits if you like (it really means "multiply by 10 to the power of n", n being given by the band color). The last band gives the tolerance, or how accurately the resistor matches the value it's supposed to have. A 1% tolerance resistor is more accurate than a 5% tolerance resistor.
So a resistor that's brown-black-red-gold, codes 1-0-2-(5%). This means the value is a 1, then a 0 then 2 zeros, ie, 1000, or 1 kilohm. A blue-grey-green-gold resistor is 6800000 or 6.8M. A brown-black-black-brown-brown is another 1k resistor, this time with 1% tolerance.
The tolerance band is often seperated from the others, which helps to tell which way around the resistor should be read. If a band on the end is gold or silver, this is a dead giveaway that this is the tolerance band.
Alphanumeric
- 0R1 = 0.1 Ω (Letter in place of comma)
- 0E1 = 0.1 Ω (E and R both mean multiplier of 1)
- 4k7 = 4.7 kΩ
- 22M = 22 MΩ
- 100 = 10 Ω (last digit is the multiplier)
- 101 = 100 Ω
- 103 = 10 kΩ
- 4754 = 4.75 MΩ
Resistor preferred values
Manufacturers can't produce absolutely every value of resistor. Instead, standard sets of "preferred" values are made. The most commonly used sets are the E12 and E24 series, generally used by four-band and five-band resistors respectively. You'll notice that E12, for example, has 12 distinct values in its range and likewise the E24 has 24.
| 10 | 15 | 22 | 33 | 47 | 68 |
| 10 | 12 | 15 | 18 | 22 | 27 | 33 | 39 | 47 | 56 | 68 | 82 |
| 10 | 11 | 12 | 13 | 15 | 18 | 20 | 22 | 24 | 27 | 30 | 33 | 36 | 39 | 43 | 47 | 51 | 56 | 62 | 68 | 75 | 82 | 91 |
Since 15 is in the table for the E12 series, that means that 1.5Ω, 15K and 150M resistors should all be available in the E12 series. Most worthwhile electronics shops (sightings of which are still sometimes reported, believe it or not) stock resistors in the full E12 series.
It shouldn't be thought that these standard values are meant to be restricting. Any value you might want will be within 10% of an E12 value. Since E12 is supposed to be used with 10% tolerance resistors, if the E12 isn't close enough, you should simply use better tolerance resistors. In most circuit design, the exact resistor value doesn't matter much.
Zero-ohm resistors
Sometimes you might see resistors with all black stripes. Yes, these are zero-ohm resistors and are equivalent to a bit of wire.
Why use zero-ohm resistors? Sometimes it's purely a matter of aesthetics (or ego), as an avoidance of ugly wire bridges.
More commonly they are used because they are easier to handle - especially for machines. There's no point designing a new machine to handle wire bridges when bridges can be made just like resistors. This is even more true in the case where a factory produces two versions of a circuit, one with a few more features than the other. The two circuits can be designed with exactly the same circuit layout so that producing one circuit rather than the other becomes simply a matter of programming the production line.
