WELCOME IN MC RESISTORI

RESISTORS IN SERIES AND PARALLEL: DIFFERENCES, EXAMPLES, AND REAL-WORLD INDUSTRIAL CASES

Resistors in series and in parallel are two different ways of connecting resistors to obtain a precise result: controlling current, voltage and power dissipation.
In electronics it is a basic, but in the industrial field it becomes daily practice when designing power resistors, resistive banks, braking resistors, anti-condensation resistors or custom-made resistor assemblies.

Here you will find a clear guide with formulas, numerical examples and above all real industrial cases.

Resistors in series: what does it mean?

Resistors are in series when they are connected one after the other, along a single path.

Rules of thumb (series)

• The current is the same across all resistors

• The voltage is divided between the resistors

• Total resistance increases

Series formula

Req = R1 + R2 + R3 + … + Rn

Resistors in parallel: what does it mean?

Resistors are in parallel when they are connected to the same two nodes (multiple branches).

Rules of thumb (parallel)

• The voltage is the same on each branch

• The current splits between the branches

• Total resistance decreases

Parallel formula

Per più resistenze:
1/Req = 1/R1 + 1/R2 + 1/R3 + … + 1/Rn

For two resistors:
Req = (R1 × R2) / (R1 + R2)

Simple examples with numbers (to understand it quickly)

Esempio A — serie

R1 = 100 Ω, R2 = 200 Ω
Req = 300 Ω
With 24 V:

• I = 24 / 300 = 0,08 A

Voltage drops:

• V1 = I×R1 = 0,08×100 = 8 V

• V2 = I×R2 = 0,08×200 = 16 V

Totale 8+16 = 24 V ✅

Esempio B — parallelo

R1 = 100 Ω, R2 = 200 Ω
Req = 66,7 Ω
With 24 V:

• Itot = 24 / 66,7 ≈ 0,36 A

Currents per branch:

• I1 = 24/100 = 0,24 A

• I2 = 24/200 = 0,12 A
  Somma = 0,36 A ✅

The difference that really matters: power and heat

Negli impianti industriali, il punto non è solo “ohm”, ma quanto scalda.

Useful formulas:

• P = V × I

• P = I² × R

• P = V² / R

In series

La corrente è uguale per tutti → dissipa di più la resistenza con valore più alto:
P = I² × R

In parallel

The voltage is the same for all → the resistance with the lowest value dissipates more:
P = V² / R

Questo è il motivo per cui il dimensionamento industriale richiede sempre:

• ohmic value

• rated power

• work cycle

• ventilation / assembly

• ambient and panel temperature

Real industrial cases: when to use series and when to use parallel

1) Frenatura con inverter: serie/parallelo per ottenere l’Ohm giusto

Nelle resistenze di frenatura, l’inverter richiede un valore ohmico preciso per limitare la corrente di frenatura e proteggere il bus DC.

Caso tipico

• Servono 30 Ω ma non esiste un singolo componente “perfetto” in quel formato

• Si realizza un gruppo resistivo con più elementi:

  • in series to increase the Ohms

  • in parallel to increase total dissipated power and manage heat

✅ Vantaggio: valore ohmico preciso + dissipazione distribuita.

2) Load bench / test: parallel to increase total power

In testing and inspection (power supplies, UPS, uninterruptible power supplies, switchgear), resistive banks are used to simulate a load.

Qui il parallelo è molto usato perché:

• riduce la resistenza equivalente (più corrente)

• permette di sommare la potenza dissipabile (più elementi = più W totali)

• facilita modularità e manutenzione (se un ramo cede, spesso il sistema continua a funzionare)

✅ Vantaggio: potenze elevate gestibili e scalabili.

3) Ripartire il calore in un quadro: serie/parallelo “per respirare”

In quadri chiusi o in ambienti caldi, un singolo resistore può diventare un punto caldo critico.

Practical solution:

• usare più resistori per distribuire la dissipazione

• design the group with series/parallel to respect Ohm and Watt

• increase distances and ventilation to reduce heat build-up

✅ Vantaggio: meno hotspot, più affidabilità, vita più lunga.

4) Voltage and signal dividers: series (but be careful with precision)

In industrial instrumentation and control electronics, the series connection is used to create a voltage divider.

Here it matters:

• tolerance (precision)

• stabilità termica

• arises over time

✅ Vantaggio: tensione “scalata” per ingressi analogici e sensori.
⚠️ Nota: in questi casi spesso si preferiscono resistori più precisi (es. film metallico, o soluzioni selezionate).

5) Anti-condensation: different purpose, but the logic of the connections remains

The anti-condensation resistors slightly heat the panel to prevent condensation.
In large systems, it may happen that multiple elements are distributed for:

• coprire più punti del quadro

• evitare un’unica fonte calda

• manage different power supplies

✅ Vantaggio: protezione uniforme.

Common mistakes (that cause sizing to fail)

1. Look only at the Ohms and ignore the Watts

2. Do not declare the duty cycle (continuous or impulsive)

3. Underestimating temperature and ventilation (in a closed environment everything changes)

4. Do not consider distances/insulation in high voltage

5. Using unbalanced parallels in critical applications (distribution criteria are required)

Data Checklist for Quotation (CTA)

To properly design a series/parallel resistor group, prepare this data:

• Application (braking / power / load bank / anti-condensation / other)

• Required value Ω (or acceptable range)

• Required power W and duty cycle (continuous / impulsive, duration and frequency)

• Working voltage (V) and peaks

• Ambiente (temperatura, umidità, polveri, ventilazione, quadro chiuso/aperto)

• Available dimensions and type of fixing

• Cable / Terminal / Connector Length

• Quantità e tempistiche

MC RESISTORS: Customized Industrial Solutions

MC RESISTORI manufactures a wide range of solutions for industrial systems: power resistors, braking resistors, anti-condensation resistors, enamelled resistors, cementated, corrugated tape, wound on ceramic support and specials.