I'm building a decade resistance box using 2W resistors, from 10x1M ohm to 10x0.1 ohm. However, due to some contact resistance, the 0.1 to 1 ohm resistance will likely turn out to be ~1.1 ohm to 2 ohms (the rest of which will be in a tolerable margin of error). Is there a way I can 'offset' the ~1 ohm of wiper and contact resistance with some active components, with an adjustable potentiometer for fine tuning?

I'm thinking -1 ohm +/- 0.5 ohms ought to do, and ideally it would handle up to 2W just like the other resistors. Is such a circuit possible?

  • 2
    \$\begingroup\$ Find lower contact resistances. \$\endgroup\$
    – user16324
    Nov 29, 2015 at 19:53
  • \$\begingroup\$ I can't, really. Even passive units costing thousands of dollars have residual resistances of around 1 ohm and report their accuracy after having subtracted their 'zero resistance'. I'm looking for a way to do this internally, if such a thing is possible. \$\endgroup\$
    – Ehryk
    Nov 29, 2015 at 20:01
  • \$\begingroup\$ @Ehryk: My 1953 Siemens resistance decade has a total contact resistance of 80mOhm. Most modern mid price range resistance boxes I know of have contact resistances around 300mOhm. Even my cheap 1% quickly slapped together resistance decade has a resistance of 800mOhm. Maybe you were not looking at the right place? \$\endgroup\$
    – PlasmaHH
    Nov 29, 2015 at 20:16
  • \$\begingroup\$ Perhaps, but I'm building my own and I'd like to be able to trim out whatever is there so that I get a 'true' 0.1, 0.2, 0.3 ohm with an acceptable percentage of error (the resistors are all 1%). Is this possible? \$\endgroup\$
    – Ehryk
    Nov 29, 2015 at 21:58
  • \$\begingroup\$ I.E. even with 300mOhm, your '0.1 ohm' setting would resultin 0.4 ohms, no? 400% error? \$\endgroup\$
    – Ehryk
    Nov 29, 2015 at 21:59

1 Answer 1


That positive passive resistance you have in your box is very 'pure'. Low noise, wide bandwidth, very low reactive components (depending on construction), floating with respect to ground, and very robust against transient high voltages and currents, without clipping or croaking.

You can rig up an opamp or two, with power followers, that can force the current pulled and pushed into/out of a pair of nodes to be proportional to a scale factor of their differential voltage, and that scale factor can be positive or negative, and controllable by a pot.

However, that circuit will have limited bandwidth, add noise, will have stray impedances to ground, and will clip for small overloads and will die with large overloads, depending on how conservatively you design it.

Were I to try such a thing, my biggest problem would be deciding on a specification. I'd start off with a capacity of 1v and 1amp, then think again and it would grow to until it was too large to build.

What can you do with good low Rdson FETs for switches, you could manage mOhms? But it will still be non-zero, and positive.

  • \$\begingroup\$ Hmm, what would the implications of 1000V and 10A be? Would it be possible to have some components 'switch in' in parallel only when needed? Perhaps I'll just do a separate 0.1 - 0.9 ohm unit, if the alternative isn't feasible. \$\endgroup\$
    – Ehryk
    Nov 30, 2015 at 6:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

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