One most likely doesn’t need any assistance in recalling Ohm’s law as,
$$ V = I R $$
Or equivalently,
$$ I = \dfrac{V}{R} $$
Of course this is an idealized expression, as all resistors have a voltage coefficient due to self-heating. A resistor R will dissipate a power under load of
$$ P_d = I^2 R $$
The power dissipation leads to a temperature rise and the temperature coefficient of resistance material leads to a deviation in nominal resistance.
In this blog post, we will take a look at the I-V linearity of a handful of 1 kOhm resistors.
Based on stock at hand, the following 1 kOhm resistors will be studied:
(A) Axial Metal Film (MFR-25FBF52) – 1/4 W
(B) Axial Carbon (CF14 1K 5% R) – 1/4 W
(C) Thick Film 0805 (RC0805JR-101KL) – 1/8 W
(D) Thick Film 0402 (RC0402JR-071KL)- 1/8 W
(E) Cermet 10 Turn Trimmer – 1/2 W
(F) Axial Carbon – 1 W
Measurement Setup
All 6 DUT resistors can be seen mounted on a single-side FR4 board in the image below.
A voltage source Vs1 is programmaticly stepped from 0 to 20V in 100 mV steps. Ammeter Am1 measures the DUT bias current. Voltmeter Vm1 measures the potential across the DUT resistor.
Measurement Results
| DUT ID | Type | \(R_{fit}\) \([\Omega]\) |
|---|---|---|
| (A) | Axial Metal Film | 997.56 |
| (B) | Axial Carbon | 998.14 |
| (C) | Thick Film 0805 | 1007.11 |
| (D) | Thick Film 0402 | 979.15 |
| (E) | Trimmer | 1050.15 |
| (F) | Axial Carbon | 1043.95 |
| DUT Type | Pd Max [mW] | \(\Delta R\) \([\text{ppm}]\) |
|---|---|---|
| Axial Metal Film | 250 | -564 |
| Axial Carbon | 250 | 9677 |
| Thick Film 0805 | 125 | -310 |
| Thick Film 0402 | 125 | -514 |
| Trimmer | 500 | -100 |
| Axial Carbon | 1000 | 30000 (3%) |
Discussion
No consideration was given to thermal EMFs generated during tests. It was assumed thermal gradient from resistance element to hookup leads was matched from both resistor terminals (nulling emf errors).
The 2 surface mount 1 kOhm resistors have far lower thermal resistance from their resistance element to PCB (leadless parts).
Random deviations in the linearity plots for low bias voltages is attributed to measurement noise of the 34401A (100 nArms precision).
The surface mount and metal film resistors continued to have equivalent linearity well being overdriven.
Raw Data
The I-V curves collected for the 6 DUTs can be found as csv files below:
– Metal Film Axial
– Res 0805
– Res 0402
– Carbon Axial 1/8 W
– Trimmer;
– Carbon Axial 1 W
Files are formatted as,
DUT Voltage [V], DUT Current [A]