Beginner questions about resistors

[Richardcavell]

This photo is taken from The Art of Electronics, 3rd edition, page 3.

1.  Other than the power rating, do all these resistors work the same way?  That is, they follow Ohm's law over a wide range of possible voltages/currents for both DC and AC. When considering Kirchoff's voltage/current laws, do I have to consider anything else?

2.  The mounting studs - I take it they're to screw the resistor in place, and that they're not electrically conductive?

3,.  Those Dale-type resistors - Is that like a heatsink arrangement around a ceramic core than I can see?  So it's to increase the surface area that is exposed to air?

4.  For the extremely high resistances (2 GOhms and 5 GOhms), is there anything else to consider when placing them into a circuit?  Or they just follow Ohm's law and that's all there is to know?

5.  If one delivers more power than the power rating for that resistor, what typically happens?

6.  For the ceramic resistors, does the current flow through the ceramic material as such?

7. Why does an engineer need to know what material a resistor is made out of?

8.  Is it feasible for a human to solder those surface mount resistor array elements?  Or is it intended that a robot does those?

Richard

[TimFox]

1.  The major difference (except for power capability) is parasitic reactance, especially series inductance of the wirewound units and internal capacitance of the carbon composition units.
3. The “Dale type” parts with metal cases must be attached to external heat sinks, just like power transistors, to operate at full rated power.
4.  High value resistors, especially physically short length units, often show measurable deviation from Ohm’s Law.  Properly specified units will quote a “voltage co-efficient of resistance” in ppm/V to quantify the deviation.
5.  There should be no current through the ceramic, which is an insulating component in the construction.
7.  Real components, including resistors and capacitors, vary depending on the actual construction and materials.  In resistors, there are substantial differences (especially temperature co-efficient) between carbon, cermets, and metal alloys.  In capacitors, dielectric materials vary greatly.  This is important for engineering, past ideal circuit theory.

[mariush]

No, not all resistors behave the same way... some are fusible resistors designed to break under some conditions, some are flame proof, some can have some inductance (some wirewound resistors if they're not wound in a way that would counteract that effect)

Mounting tabs are not always mounting tabs, sometimes are just eyelets to make it easier to solder wires to them.

With very high resistance value parts, you'll want to be careful about flux residue, you may want to have copper removed from circuit board around the terminals, or to have guard traces around it...

5... more power than rating results in resistor overheating and eventually burning up.

6. depends on construction.  construction types of resistors  see https://learnabout-electronics.org/Resistors/resistors_08.php or https://www.nutsvolts.com/magazine/article/resistors_types_and_applications  or

7 because various materials have various pros and cons ... some materials make resistors that drift more with temperature but they're cheaper and your circuit may not care about that so you use those, others may need better specs etc etc

8.. yes, it's very feasible if you have a good soldering iron (soldering station with adjustable temperature)  or alternatively you can buy solder paste and use a hot air gun.

[Sensorcat]

All resistors are approximations to the ideal resistive behaviour the device manufacturers and users want. But we can only get close. Sometimes we are so close that the deviation from ideal behaviour is of no concern in the respective application, sometimes it hurts.

The most important thing is to know that each resistor has a datasheet. A serious developer will always consider this datasheet, know his requirements for the application, and find out if a particular device is a good choice by comparing both. It sounds too simple, but many problems have the root cause that this is not properly done.

If you want to master electronics, you must, among other things, look at a lot of datasheets, learn about the way device properties are explained there, and what the data means. While many understand that doing so is important for active devices, passives are often neglected and treated as if they only have their primary value, resistance for a resistor.

For instance, each resistor comes with a so-called 'parasitic' inductance, simply because current flows through it, and current causes a magnetic field, which stores energy and makes the behaviour time- and frequency-dependent. Since Ohm's law tells nothing about time or frequency, this is an important issue that cannot be avoided. It means non-resistive behaviour in the sense that it is not according to Ohm's law.

7. It is not actually necessary to know the material. The datasheets may or may not tell what it is. But it must tell the behaviour, which depends on the material used.

8. Even the smallest components can be soldered by humans, but you need a microscope to properly see what you're doing.

[MarkT]

For 5, yes resistors overheat, but its a bit more subtle than that.  For instance in pulsed operation you can exceed the continuous power rating for a short time, since the average power over a few seconds is what determines the temperature reached (for smaller resistors that time is shorter, for larger, longer).  And in practice the maximum power rating will come with notes in the datasheet about the conditions in which that can be achieved (ambient temperature, adequate convection etc).

In practice you'd not normally run a resistor more than about 50% of its power rating without good reason as high temperatures lower reliability in the resistor and nearby components.

The factor by which you can exceed the rated power in short pulses depends on the resistor construction - metal thin-film resistors have a lower factor than carbon composition for instance - this data may not always be given in datasheets either.

And one factor worth noting is that resistors have a rated voltage too - this is to do with discharge across the surface of the device, smaller resistors have smaller voltage ratings.  You'd never use an 0402 surface mount resistor for 250V for instance, even if it was many megaohms.

[shapirus]

For instance in pulsed operation you can exceed the continuous power rating for a short time, since the average power over a few seconds is what determines the temperature reached (for smaller resistors that time is shorter, for larger, longer).  And in practice the maximum power rating will come with notes in the datasheet about the conditions in which that can be achieved (ambient temperature, adequate convection etc).

Instantaneous power, or, probably, current, also matters. I have done some tests that showed that resistors (an 0603 thick film 0.1 Ohm one was used in that case) degrade over time (growing resistance) when loaded with very short, like 100-200 ns, and infrequent (1/s) pulses of high current (~50 A), while keeping the average power well below the maximum rated power of the resistor.

I'm not sure if that happens because of instantaneous local overheating or electromigration.

Some resistors (of the more reputable brands, usually) have their max allowed pulsed current specified in the datasheets.

[kevin.gibbs]

6. Ceramic is a dielectric and may be used as a heat sink frame.

[CatalinaWOW]

Only two things to add to or amplify what has been said previously.

You do not need to incorporate non ideal behavior in every calculation.  But you need to know if you have to, which means understand drift, inductance, capacitance, non-linearity etc.  and know how the circuit you are evaluating works (even outside it's normal operating modes) to evaluate if it is important.  For example you may use an op amp in an audio amplifier.  At audio frequencies few if any resistors have any significant effects, but many op amps have gain at frequencies well into the MHz where such effects may become significant.

All resistors in circuits end up having parasitic parallel resistors through insulation and along surfaces.  When you attempt to use very high resistances these paths become important.  Solder flux, fingerprints, dust and other things easily cause leakage paths in the megohm class.