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What Are Series and Parallel Circuits?

Lights can be connected in series or parallel circuits. Lights that are connected in series all use the same circuit, while lights connected in parallel each have their own circuit.

circuit black and white

The most important information at a glance:

  • series circuit: lights are all connected to the same circuit
  • tandem series circuit: a type of series circuit where two lights are connected to a single ballast
  • parallel circuit: each light has its own circuit
  • duo parallel circuit: a type of parallel circuit where two lights are connected in parallel (one inductive and one capacitive)


Schaltung m-o Stromzweig.svg
Top: Series circuit with two resistors;
Bottom: Parallel circuit with two resistors


By Saure - Own work, CC0, Link

What Is a Series Circuit?


In a series circuit, all components are connected to the same single circuit. This means that the same current flows through all the connected components and that they share the current. You can connect as many components as the power supply allows.


A very common example of a series circuit is a string of lights. If you connect a string of ten lights to an outlet of 230 V, for example, each light gets 23 volts. The voltage is evenly distributed between all of the components. If one light blows, the entire string of lights won't light up.


Series Circuit for Gas Discharge Lamps


If gas discharge lamps have the same nominal circuit, they can be connected in series. Make sure to use the right ballast so that the current limitation is not exceeded.


Series Circuit for Incandescent Lights


The nominal circuit for incandescent lights also needs to be identical to connect them in series.


What Is a Tandem Circuit?


A tandem circuit is a type of series circuit. Two lights, such as fluorescent tubes, are connected to one ballast. However, each tube still needs its own starter. The starter needs to be suitable for use in a tandem circuit. Suitable starters contain the designation “series” or the abbreviation SER in the product name.


Some of the starters we stock that are suitable for series/tandem circuits include:

Osram Starter 1

Osram Starter 4-22W

Osram Starter 2

Osram 4-22W Safety

Philips Starter 1

Philips S2 4-22W

Philips Starter 2

Philips S2E 18-22W


Single starters are not suitable for use with series/tandem circuits because they do not work with shared mains voltages.


Tandem circuit for LED


If you want to make the switch from fluorescent tubes to LED tubes with tandem circuits, rewiring is needed. Please consult an expert for this.


What Is a Parallel Circuit?


A parallel circuit connects two or more bipolar components. It's important to only connect identical poles to each other.


Each light in a parallel circuit has its own circuit. The individual currents add up to the total current. The voltage for each light is the same. As opposed to a series circuit, if one light malfunctions in a parallel circuit, the other lights still light up.


Parallel Circuit For Gas Discharge Lamps


Gas discharge lamps can only be indirectly connected in parallel. The required ballast can be connected in series. The lamp and ballast together can then be connected in parallel.


What Is a Duo Circuit?


A duo circuit connects two fluorescent tube branches. One branch is inductive and consists of a conventional ballast and a tube. The other branch is capacitive and also consists of a conventional ballast and a tube as well as an additional capacitor for power factor correction. The capacitor is connected in series with the ballast. By using a duo circuit, it's possible to avoid excessive currents.


Definition: What is a capacitor?


A capacitor is an electronic component which can store energy. A light can burn for some time, therefore, even after being switched off.


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