Electronics notes/Resistors

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This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

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See also Category:Electronics.


Types

See also:


Precision/tolerance

Resistor precision refers to the fact a production process will produce resistors with different resistance. (and not the abilities of a single resistor)

For example, if you grab a random 100 Ohm resistor

from a 5% batch, you would see a value somewhere between 95 and 105 Ohm.
from a 1% batch, you'll know it has a value between 99 and 101 Ohm.
from a 0.1% or 0.01% resistor, well, you get the idea.


Getting a resistor value exactly right doesn't matter in most parts of most circuits, which is why you can use cheaper components from the less accurate production line.

And sometimes it does matter. A common example is current-sensing resistors. Since the point is I=V/R (you measure the drop across a known resistor), your calculations's precision relies on knowing the resistance, which is why these resistors are often 0.1%.

And in some circuits (often a specific analog part of it) two or more components must match each other closely.

Yes, you could do that by just measuring individual resistors, and sorting them into smaller ranges. It's just usually not worth your time or extra work in automated production, or arguably even in DIY, so it's cheaper and easier overall (process/production-wise) to buy components for the parts where it does matter.


When the value matters, temperature behavious often also matters. Precision resistors are not necessarily better behaved here - though products can be, e.g. current sensing resistors.



Resistor marking

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Resistors can be marked with their...

  • resistance (pretty much always)
  • tolerance (regularly)
    • indicates how close you can expect the real resistance to be to the marked one - can be as high as 10% and 20%, or as low as 0.05%.
    • For example, a 470 Ohm 5% resistor can be expected have a value within 446..493 Ohm
    • For many applications (e.g. current limiting) the exact resistance does not matter much, or can be calibrated to (e.g. voltage dividing before an ADC), so the 5% tolerance that the bulk of basic cheap carbon resistors have is good enough
  • quality/reliability (sometimes)
    • indicates the failure rate per 1000 hours of continuous use at the rated power(verify)


Marking systems:

Color band codes (mainly on through-hole resistors)

  • one black band
    • (near-)zero-Ohm resistor/link, a wire conductor in component form. Used in board designs where it may be a good idea for this to be replaceable with some other value later, to connect something only in testing, to manufacture with one step/machine less (avoid also placing jumpers), and perhaps other reasons.
    • http://en.wikipedia.org/wiki/Zero-ohm_link
  • color color color color (the most common)
    • 2 digits, 1 multiplier, 1 tolerance (The tolerance uses different color set - which is useful to see what side is the start of the series)
    • You can think of this as scientific notation, digit digit E digit. For example, orange orange brown is 3 3 E 1, =33*101, =330 Ohm
    • http://en.wikipedia.org/wiki/Resistor#Four-band_resistors
  • color color color (three band)
    • the previous system but with with 20% tolerance
  • color color color color color can be...
    • four-band system + quality band (2 bands of value, 1 multiplier, 1 tolerance, 1 quality)
    • precision resistors (3 bands of value, 1 multiplier, 1 tolerance)
      • same idea as 3-color system, but with one more digit - and so with the multiplier one less. e.g. 330 ohm as orange orange black black (330*100), whereas in 3-color it would be orange orange brown (33*101)


  • color color color color color color can be...
value value value multiplier tolerance TCR


See also:


Numbers, mainly on SMD resistors (note: sometimes confusable with SMD capacitors with similar marking)

  • 0
    • (near-)zero Ohm resistor, often for replaceability [1]
  • number number number
    • same system as color bands, without the color-namber mapping. For example:
    • 330 is 33 Ohm (33 * 100)
    • 221 is 220 Ohm (22 * 101)
    • 223 is 22 kOhm (22 * 103)
    • 105 is 1 MOhm (10 * 105)
  • number number number number
    • like the last - but with three digits, one multiplier
    • e.g. 4992 is 49.9 kOhm


  • number number letter
    • EIA-96 marking, for 1%
    • where letter in F, E, D, C, B, A, X or S, Y or R (for *100000 .. * 0.01)
    • and the number should be looked up in a table
  • letter number number
    • similar to EIA-96 marking - same multiplier, but different lookups
    • ...and the section within that lookup table tells you whether it's 2% (01..24), 5% (25..48), or 10% (49..60)

(What about EIA 48, 24, 12, 6?)


When there's enough space, such as on power resistor heatsinks, you also see the BS 1852 system (note: also used for capacitance), which roughly comes down either...

Either...

  • letter B number number number number
    • letter - wattage (B=1/8W, C=1/4W, E=1/2W, G=1W)
    • B
    • three numbers: value value multiplier
    • tolerance (5=5%, 2=20%, 1=10%)
    • Examples:
      • EB1041 for 1/2W, 100kOhm, 10% tolerance
      • CB3932 for 1/4W, 39kOhm, 20% tolerance

Or...

  • an R before/in/after a number - for small values
    • e.g. R56 (or 0R56) for 0.56Ohm
    • 8R2 is 8.2 ohms
    • 100R for 100 Ohm
  • a K, or M within/after a number - larger values
    • 2K7 for 2.7 kOhm
    • 32K7 for 32.7 kOhm
    • 1M0 for 1.0 MOhm
  • ...with a possible an additional suffix letter refers to tolerance
    • (M=20%, K=10%, J=5%, G=2%, and F=1% D=.5% C=.25 B=.1%)
    • Examples:
      • 3K2G for 3.2 kOhm, 2% tolerance
      • 32KK for 32 kOhm, 10% tolerance
      • 32K7F for 32.7 kOhm, 1% tolerance


Other systems

  • MIL-R-11 or MIL-R-39008 (look like RC05, RCR05)


See also:

Resistor behaviour

The below two are typically secondary, but are interesting to know about.


Variation with temperature (behaviour)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Resistance varies with temperature.

This is mostly because of the electron activity, and for most of the range the resistance varies proportional to temperature in Kelvin (being a true-zero-energy point and all).


Around human-comfortable temperatures you can model this variation with something like

R = R0 (1 + ⍺(T-T0))

...where

  • R0 is a reference resistance
  • T0 is a reference temperature (e.g. a room temperature like 20°C or 25°C, occasionally 0°C)
  • , the temperature coefficient, (unit: 1/°C.), is a small number (often in -0.007 .. 0.06)
    • ...can be positive, meaning more resistance with higher temperatures (e.g. many metals)
    • ...or negative, meaning resistance lowers with rising temperatures (e.g. semiconductors, such as carbon)

Note that this model simplifies away a few things and isn't ideal over large ranges or for extreme accuracy.


Thin film: 5..25 ppm / K ( 0.000005 .. 0.000025) Thick film: 200..250 ppm / K ( 0.000200 .. 0.000250) Carbon: ~-500..-2500 ppm / K (-0.000500 .. -0.002500) (verify)


Thermistors (design/spec)
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Thermistors (thermal and resistors) are resistors that intentionally have a large ⍺


Can be divided into:

  • PTC (positive temperature coefficient) thermistor, or posistor.
  • NTC (negative temperature coefficient) thermistor.


Their behaviour makes them useful for things like:

  • temperature sensors (NTC or PTC)
  • self-regulating heaters
  • resistors that vary with current
    • for inrush limiting (NTC, because high resistance when cold, lower resistance once warm)
    • for self-resetting overcurrent protectors (PTC, because basically a wire when cold, and higher resistance once hot)


Contrast with varistor

Resistor noise (behaviour)

Designs and applications

Current limiting resistors (application)

Power resistors (design/spec)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Power resistors refers to a resistor being able to handle more power (dissipated as heat) than your average resistor.

Power resistors may be able to deal with on the order of 10W or even 50W (while your common carbon resistor can only dissipate 1/8 Watt).


They are usually low-resistance - a few dozen ohm and less.

They are usually physically larger to be able to dissipate the heat - and may come in heat-sinking constructions, depending on their rating - say, TO-220 (a package rated up to 5W), wrapped in ceramic, metal heatsinks, and more. See also an image search for power resistor.


They are regularly used to dissipate noticable power (as heat), in constructions that control considerable voltage or current (or may sometimes need to), or as a test load.


For an example of the last: to repurpose an ATX power supply to a lab power supply (See also Electronics_project_notes/Power_notes#Repurposing_an_ATX_power_supply), one part is fooling it into thinking it has load, which you can do by connecting a 10 ohm power resistor from 5V to ground, sinking (5V/10ohm=) 500mA amp and taking 2.5W (It's probably a good idea to take one rated for 5W since the cooling probably isn't going to be ideal inside a metal box).


Pull-up / pull-down resistors (application)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)
tl;dr

A resistor that effectively makes the sensed state of a wire be (usually either) ground or full voltage level -- basically when it is the largest (frequently only) predictable thing to interact with any such voltage level.


Why

Components like jumpers, photo-diodes, switches (buttons, toggles, reed switches, etc) will, in some of their states, completely break the connection on a wire.

When you have loose wires as inputs to things such as digital ports on ICs, or analog ports on amplifiers or such, this is often a problem: such inputs are not predictable when not tied to anything, basically because the thing that will make the effective decision is an tiny part of such a component -- which in most situations is completely unpredictable.

Unconnected input pins will typically float: their value is defined by effects that you would usually consider negligible.

This behaviour can be influenced by design, manufacturing, proximity of electric fields, proximity of things like your hand, and can cause fluctuation, oscillation, and with unpredictable magnitudes.

While such behaviour doesn't necessarily happen, it is typically very complex to predicting where, when, how, and with what magnitude it does.

Since floating inputs are noisy at best and meaningless at worst, it's usually a good idea to work around that. Avoiding this is usually both simple and cheap.


What

The typical workaround is to tie them to a voltage level with/through a resistor. Typically it is tied to one of the two basic logic levels used on the component you are handing the signal to - which are usually also the logic levels in the circuit as a whole.

A pull-up resistor ties input to a higher voltage (often the supply / logic-high level). A pull-down resistor settles to a lower level (often ground / logic-low).


In both cases, you keep in mind that:

  • we want enough current to make the input robustly at that level for the component sensing it
  • ...that uses current, but we want to waste as little energy as possible
  • since you often effectively make a voltage divider, we want to minimize the effect of the pulling resistor on the actual sensor

Usually this means a resistor on the order of a few kilo-ohms to a few dozen kilo-ohms. 10kOhm is not unusual.


Why this and not simper

Knowing that the gate/IC you use tends to float high you may initially wire them like:

GND ---- switch ---- input pin

This would mean it's stably low while pressed, but floating when not pressed.

Instead, you would probably want it high while not pressed. The first step would be to add a connection:

                 5V
                  |
GND ---- switch --+-- input pin

This solves the case where the switch is open. It asserts the input high (5V) (though may use more current than really necessary to assert it at that logic level). However, pressing the switch will connect 5V to GND with no resistance, shorting out the power supply.

The basic pull-up design looks like:

                 5V
                  |
                  R
                  |
GND ---- switch --+-- input pin

That resistor primarily just fixes the previous diagram. When the switch is open this asserts the pin at high. Closing the switch will create a direct connection between input and GND, asserting the input pin to low (with its near-zero resistance), and places the resistor across the power source.


The current that can flow through that resistor is V/R, and pull-ups are often at least 10kΩ, so on a 5V circuit they'll use on the order of 0.5mA at most.

You can choose higher resistance (47kΩ is not rare), but above 50kΩ (for ~0.1mA on 5V) you have to start checking at the specs for whatever you're feeding this input to - at some point, the current will be too small for the IO pin to keep a stable state. (that said, in some situations it can be much higher)


Note also that you may be making a simple RC circuit, which may affect the switching speed of the thing you are connecting, which can sometimes matter.


A pull-down resistor refers to a very similar design, but the non-pressed state pulls to Gnd instead of 5V.


Usually there is something that points towards one preference or other, such as the circuit type (open-drain, open-collector, etc.), whether a switch is usually open or closed, whether pulling low or high makes more sense for a particular input for other reasons (e.g. for logic ICs).



Summary / notes
  • pulling up/down can be useful when you want input to be stable when a device/sensor is disconnected, and when a logic device cannot source current.
  • Downsides: current will be used, you may affect inductive behaviours
  • component designs may show different floating behaviour. For example, CMOS may float to half Vcc, where it may draw more current than it usually does. TTL will behave better current-wise.
  • Some ICs (e.g. various TTL semiconductors) may have internal pull up resistors, which in the case of programmable ICs may be switchable (but be careful about the behaviour around reset)


See also:

Resistor ladders (application)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Can be used, for example...

  • as a simple DAC
  • to put a few different on/off input onto one analog input.

See also:


Semi-sorted notes

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Resistances of megaohms to (near-)infinity are everywhere, so high-resistance very-low-current paths can always happen on a circuit.

Sometimes this is because of the circuit (design) itself, sometimes it's a side effects from EM fields, moisture, non-cleaned soldering, and more (verify).

The resistance of such unplanned effects are generally on the order of megaohms or more (so micro-Amperes or less), so generally have little effect, but this can matter to high frequency circuits, fast switching, and such.


Potentiometers

Potentiometers, also known as potmeters, pots, variable resistors, and more, are resistors that are adjustable within a given range.


They often come with a shaft (or a hole for a shaft) that you an put a knob on. You can often turn them a bit-less-than-a-single turn, due to it conceptually, and often physically, being a metal wiper on an exposed resistor.


Slider pots are also common in some applications, such as audio mixers and lighting controls.


There is also a variant often hidden inside equipment, trimmers, trimmer pots, preset pots. These you often have to tweak with a screwdriver, because they are mostly used for factory calibration and rarely for everyday tuning.


There are one-turn and multi-turn potmeters. Multi-turn go through their range in many rotations, typically so that the same movement can be more accurate tweaking. Trimmer pots are frequently multi-turn, user-tweakable pots more often single-turn.

Continuous-turn pots also exist, though in practice encoders are often simpler.


Servo-mount potentiometers is technically about the flat shape of the front end, contrasted with bushing mount, though many servo-mounts pots are continuous-rotation things.



Within their physical range, a potmeter can be:

  • Linear taper:
    • resistivity proportional to angle/distance moved
  • Logarithmic taper:
    • output voltage roughly logarithmic with the angle/distance moved
    • there are approximations (cheaper) and more exact logarithmic pots (more expensive)
    • e.g. used for audio volume adjustment (as that perception is logarithmic)




See also:


Motorized potentiometer

Essentially servos that also expose the potentiometer (servos use potentiometers for position feedback).

Frequently seen in audio equipment.


Rheostats

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

A rheostat is a variable resistor used more to control a load directly, which often implies a low overall resistance of the track/wiper so it can deal with some actual current, and also that it is well behaved in terms of [contact bounce]].


...whereas potmeters are typically tiny-current (measured with high-impedance devices/buffers).

Another difference is that rheostates very typically have only two terminals.


Note that for various purposes, there are more efficient ways to control a load than a rheostat, so they aren't widely used.

There are other sorts of rheostats - consider e.g. liquid rheostats