2
\$\begingroup\$

Carbon film resistors are basically an insulating (probably ceramic) rod covered in a carbon film that is twisted around it, as shown in the picture below:

enter image description here

However, this seems very similar with a coil. I know that on DC, there is no problem, but this shouldn't have some other effects on AC, like a coil? Are there any situations where this kind of resistors are avoided in AC applications?

P.S. I am sorry for using a vague terminology, but I haven't studied AC for a very long time, so I don't completely understand it and the exact effects various components have. But I know that a coil have a different effect on the current than a resistor, and I have some intuition about them.

\$\endgroup\$
  • 1
    \$\begingroup\$ The words you want are "inductive" and inductance. As Maxthon says: Yes, they do have inductance but it is usually a minor effect compared to their resistance. At "radio frequencies" it may matter. Order of inductance is probably in the 10's to 100's of nanoHenry range (at a guess). \$\endgroup\$ – Russell McMahon Dec 25 '14 at 3:00
  • 1
    \$\begingroup\$ I want to comment to the OP that these type resistors are not made by "twisting carbon film around a ceramic rod". In fact they start out as ceramic rods that are coated end to end with a continuous layer of resistive material. The rods then have the end caps and leads attached. Then they are fixtured into a machine similar to a lathe where the assembly can be turned whilst simultaneously monitoring the resistance between the two leads. A cutter or laser is then used to start cutting a groove in the coating by advancing along one side while the assembly turns slowly. (continued). \$\endgroup\$ – Michael Karas Dec 25 '14 at 5:01
  • \$\begingroup\$ (continued from above). While the groove cutting proceeds the resistance is measured until it gets to the desired value. When within the tolerance desired the cutting stops leaving what looks like a spiral of resistive material around the ceramic rod. A fine tuning step may be applied for high precision components. Finally the components are coated and marked with value rings or inked numbers. \$\endgroup\$ – Michael Karas Dec 25 '14 at 5:06
6
\$\begingroup\$

There is inductance and there is capacitance between the spiral turns and from end-to-end. It's generally not important until you get to VHF frequencies, but of course the effect is relatively larger on higher resistances for the capacitance and the inductance effect is relatively higher on very low resistances.

For example, if you use this calculator, a coil with 8 turns 2mm diameter and 7mm long would have an inductance of 0.04uH, so at 100MHz and Xl/R = 0.1, R < 250\$\Omega\$. Note that if you have a current sense resistor of very low value, even a fraction of a microhenry inductance will start to have a noticeable effect at moderate frequencies. A 10m\$\Omega\$ resistor with 40nH inductance would be affected similarly at only 4kHz.

Parasitic capacitance works similarly from the other end of the frequency scale- a 10M\$\Omega\$ resistor with 0.4 pF of end-to-end capacitance would be be affected similarly at only 4kHz.

\$\endgroup\$
  • 1
    \$\begingroup\$ I've been told that they sometimes still use old-school carbon composition resistors in very high frequency (microwave?) applications specifically to avoid this problem with parasitic inductance. \$\endgroup\$ – Warren Young Dec 25 '14 at 7:03
  • \$\begingroup\$ @WarrenYoung A 'modern" chip resistor may be better in this respect. These are often LASER trimmed in various ways. Methods vary with some having less impact on RF performance than others. See eg Useful document pictures? | Advanced aspects Use of - Vishay .... \$\endgroup\$ – Russell McMahon Dec 26 '14 at 7:32
  • \$\begingroup\$ @RussellMcMahon As I understand it, the physically smaller the resistor the less effect the overall dimensions as well as the laser trimming cuts and such like have at very high frequencies. Then we have these babies- 50GHz. \$\endgroup\$ – Spehro Pefhany Dec 26 '14 at 9:03
5
\$\begingroup\$

Indeed it is a coil and it do have some effects on AC, but at least for mains power and below the inductance is so small that the effects are too negligible to be noticed anyways.

Even a piece of straight wire have some inductance. And some times, especially in HVAC transmission lines in power grids, this inductance spells several percent of loss. However that is VERY HIGH voltage and VERY SIGNIFICANT power, but that is out of our scope.

\$\endgroup\$
1
\$\begingroup\$

This answer addresses the initial question briefly but is added mainly to provide information on the modern equivalent to the spiral cut resistor which is being asked about. In most cases the through hole tubular resistor would not be used and an "SMD" equivalent probably would.

Question [paraphrased]: Carbon film resistors are basically an insulating (probably ceramic) rod covered in a carbon film in a spiral around it. However, this seems very similar to a coil.
Are there any situations where this kind of resistor is avoided in AC applications?

The applicable terminology for having properties like a coil are "inductive" and inductance.

As Maxthon says: Yes, they do have inductance but it is usually a minor effect compared to their resistance.

At "radio frequencies" it may matter.
Order of inductance is probably in the 10's to 100's of nanoHenry range. [This range is similar to what Spehro indicated in his answer.]

Related - the modern equivalent:

The following relates to a modern equivalent - the surface mount "chip resistor". These are available in carbon film (older) and metal oxide film (most newer ones) and also work by utilising a layer of conductor which is "fired" onto an insulating surface. They also usually have resistances which are to broadly distributed to be suitable for use directly and so they also are "trimmed". Because the resistor element is (usually) a flat rectangle the trimming does not use a spiral but instead either material is trimmed off one side of the rectangle or (more usually) a range of patterns are cut into the rectangle to increase the overall resistance. For experimental purposes (usually) the resistive layer may be abraded (eg by "sanding or scraping) to decrease its thickness and so increase its resistance.

The various methods of altering resistance have various obvious and less obvious effects on resistance characteristics. Long term resistance values, amount of "drift" and rate of drift are process dependent and to obtain high accuracy long term stable resistors significant effort may be required.

The usual zillion pictures - each picture links to a related page

'Modern" chip resistors are often LASER trimmed in various ways. Methods vary with some methos having less impact on RF performance than others.

See eg Useful document. Pictures were missing when I looked but this might be due to Win 8.1 / Boxing Day too much food blues / Murphy / reboot needed - looks like a worthwhile page if pictures appear.

Advanced resistor trimming aspects +/- 0.02% target.

  • Manufacturers are now being required to shrink the case size as well as improve the gain adjustment capability, final trimmed accuracy, and repeatability. In order to accomplish these goals simultaneously and in a manner consistent with the high throughput requirements of standard TFCR production lines, a new generation of green (532nm) laser system has been developed. Unlike previous IR laser based systems, the green wavelength affords a significant reduction in the spot size of the laser and a corresponding reduction in the kerf width and thermal effects. Combined with a unique serpentine cutting routine, this provides the basis for a significant increase in the gain adjustments and an increase in final resistor accuracy and repeatability.

Use of user trimmable resistors - Vishay Why what you get may not be what you want if you do it wrong and how to do it right.

Useful 1 page resistor trimming summary This is a company's 'what we can do page' but shows offered trim patterns and achievable results.

Basic trimming cuts

At 1st glance this looks like "just" a Vishay 6 page resistor spec sheet - Post insertion trimmable with instructions but contains instructions on how to post insertion LASER trim the products.

LASER trimming machinery - awesome stuff

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.