0
\$\begingroup\$

Im researching on carbon film resistors and film resistors in general, hence I would like to know certain things:

  1. Resistivity of carbon the film is made-up of? Does the resistivity depend on the type of allotrope of carbon? If yes what allotrope of carbon is used as the film? - I can't find this on the internet.

  2. What is the length of the film deposition before it is cut into a helical coil?

  3. What shape is the cross-section of the helical film in the resistor?

  4. How is the helical film designed to provide a certain resistance value. For eg. if I want a carbon film resistor to possess a value of 10k ohm, how should I cut the film deposition? How many turns must the cut film have and what should be the cross-sectional area (thickness) of each turn? What should be the gap between each turn?

I need to know these things in order to calculate the number of turns, thickness of turns and gap between each turn for a given resistance value.

\$\endgroup\$
7
  • \$\begingroup\$ It's definitely not diamond. \$\endgroup\$
    – Andy aka
    Commented Oct 10, 2021 at 12:41
  • \$\begingroup\$ It all depends on the resistor. Each manufacturer may publish some of this information and keep other information as a trade secret. \$\endgroup\$
    – user16324
    Commented Oct 10, 2021 at 12:57
  • \$\begingroup\$ @user_1818839 but isn't there a general information open to public? \$\endgroup\$
    – Somanna
    Commented Oct 10, 2021 at 13:02
  • \$\begingroup\$ Well there's the obvious equations connecting bulk resistivity, dimensions, and resistance. en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity - plus the equally obvious trick of fitting a great length into a small space by winding it into a helix. Gap between turns is just insulation. What more do you need? \$\endgroup\$
    – user16324
    Commented Oct 10, 2021 at 13:07
  • \$\begingroup\$ @user_1818839 1,2 and 3 in my question is required to answer 4. \$\endgroup\$
    – Somanna
    Commented Oct 10, 2021 at 13:31

1 Answer 1

Reset to default
2
\$\begingroup\$

Thin film resistors, whether carbon film, metal film, printed and fired ink, tantalum film, have a resistivity that's measured in 'ohms per square'. This depends on the thickness and the bulk resistivity of the film material.

Any given resistor manufacturer may have a few stock film resistivities that they're good at making, and then they will change the resistor geometry to get the desired terminal impedance.

If your film is (for instance) 50 Ω per square, and you want a 150 Ω resistor, then you use a piece of film three times longer than it's wide. That may well fit onto a standard resistor body directly. If you want a 10 kΩ resistor using the same film, then it needs to be 200 times longer than it's wide, which will probably need to be spiralled or otherwise meandered onto a resistor body.

\$\endgroup\$
1
  • \$\begingroup\$ For what it’s worth, many years ago I analyzed Philips 5% leaded ‘carbon film’ resistors, and the lowest values were actually metal film, with corresponding low tempcos. \$\endgroup\$
    – Spehro Pefhany
    Commented Oct 10, 2021 at 16:15

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