Professional Electronics Engineer with a specialism in EMC.

Resistors

First, we are going to cover some basics, and we are going to tell some lies, if you know what the lies are, then chances are that you don’t need to read this post… but please do anyway.

Resistors values are measured in Ohms, the symbol used is ‘Ω’ it is the SI derived unit of electrical resistance, named after German physicist George Ohm.

The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere,

What is Electricity?

Electricity is the flow of electrical charge

What is conventional current?

This is our first little lie;

Conventional current is the flow of a positive charge from positive to negative and is the reverse of real electron flow.

Conventional Current
Conventional Current
VIRP Circle (in some countries, Volts is U and not V)
VIRP Circle (in some countries, Volts is U and not V)

Ohm’s Law

For Resistors, the main part of Ohms Law that we care about is V = I * R, (P = I * V, is no less important, however, most of the time you can work out power dissipation by rule of thumb).

Kirchoff’s Law

  • At a circuit node, the total current flowing in equals the total flowing out:
    ΣIin = Σiout⇒ΣI=0
  • Within a circuit mesh, the sum of all voltage drops is zero:
    ΣV = 0

Examples of Resistors

  • Heating element
  • Light bulb
  • Wiring harness (while an ideal wire should have a resistance of 0 Ω, they don’t) I will cover voltage drop at another time.
  • Human body

Uses

  • Resist current flow.
  • Create voltage drops.
  • Dissipate power.

How are Resistors sized?

Resistor Colour code - This is only used on through-hole components. - SMD resistors have a different coding pattern. outside prototyping, hobbyist electronics, and very special applications you are unlikely to come across through-holes these days.
Resistor Colour code – This is only used on through-hole components. – SMD resistors have a different coding pattern. outside prototyping, hobbyist electronics, and very special applications you are unlikely to come across through-holes these days.

Surface mount components often have a similar arrangement to the first 3 bands, but use digits printed on the component. Beware because the rule is not always followed.

SMD resistor code
SMD resistor code

Power Dissipation

Derating Curve, Pmax (Power) vs Temperature.
Derating Curve, Pmax (Power) vs Temperature.

Resistors convert electrical energy to heat.

If a resistor gets too hot then there can be various failure modes depending on several factors. The resistance may change permanently. The resistor may crack and go ‘open circuit’. In some cases, a resistor can get hot enough to melt the solder, which holds it to the PCB.

Resistors change resistance with temperature:

R=R20[1+α(T-20°C)], where α: temperature coefficient, positive

Reliability of Resistors

Most frequent reasons for failure:

  • thermal stress
  • exceeding the maximum operating voltage
  • mechanical and environmental influences

Some failure rates λ per 106 hours (Tamb=80°C, P=0.5W, Pmax=1W) According to MIL-HDBK-217 model:

Composite (insulated)0.000196
Film (insulated)0.000206
Wirewound (accurate)0.000134
Failure rate according to the MIL-HDBK-217 model

Why are higher value resistors less reliable?

When using high resistances in the megaohms (and especially at 10 M ohm and more) environmental contamination such as dust, skin oils, soldering flux residue etc can easily reduce the effective resistance in unpredictable and time-varying ways.

What to consider when choosing a resistor?

Depending on where you are using the resistor, you may need to consider the characteristics of the resistors and the environment you are using them in:

  • Temperature Coefficient,
  • Voltage Coefficient,
  • Noise,
  • Frequency Response,
  • Power Rating,
  • Temperature Rating,
  • Physical Size,
  • Reliability,
  • Material of construction,

Leave a comment

Your email address will not be published.

31 thoughts on “Resistors”