Capacitive circuit for one fluorescent lamp
A capacitive single-lamp circuit contains a matched series capacitor as well as a conventional choke in the lamp branch. The capacitor does not replace the choke. Both components form part of the circuit design for the intended fluorescent lamp.
Here, capacitive describes the behaviour of the matched branch. It does not mean that a fluorescent lamp may be connected directly to the supply through an arbitrary capacitor.
Construction and operation
Components and their functions
The lamp has two electrodes, each with two external connections. A suitable glow starter supports preheating and ignition. The choke limits operating current and contributes to starting.
The series capacitor affects the entire branch, including lamp current. Lamp type, choke and capacitor therefore need to match. An unsuitable capacitor can create unfavourable operating conditions, additional losses or component overload.
Interactive circuit diagram
The capacitive single-lamp circuit adds a matched series capacitor to a conventional fluorescent branch. In the diagram, C1 is in series with ballast L1 and lamp E1. The ballast remains part of the circuit; C1 does not replace it.
Starter S1 connects the free electrode terminals for preheating. The small capacitor C2 across the starter has a different purpose: interference suppression. A series capacitor is also different from a compensation capacitor connected across the supply.
Switch on to start, or work through the sequence one step at a time.
C1: series capacitor · C2: suppression · Slow start sequence
Simplified learning diagram: Capacitive circuit for one fluorescent lamp. A functional illustration, not a universal terminal layout.
Here, C1 is the series capacitor and C2 provides starter suppression. An additional parallel-compensation capacitor is not drawn in this learning diagram. Labels are local to each drawing: in the DUO model, C3 identifies the suppression capacitor at the second starter. Position and function, rather than the number alone, determine its role.
Required capacitance and voltage rating follow the intended design. The voltage across an individual component in this AC circuit must not simply be assumed to equal the supply voltage. There is no universal capacitance or voltage rating for every fitting.
Starting and normal operation
The starter temporarily closes the preheating path through the lamp filaments. When it opens again, the choke's voltage pulse helps strike the lamp. After successful ignition, the starter remains open during normal operation.
The capacitor remains in the intended operating-current path. It does not automatically stop influencing the circuit after starting. Its effect must also be considered when replacing the lamp, choke or starter.
Series compensation and parallel compensation differ
| Capacitor arrangement | Purpose | Important distinction |
|---|---|---|
| In series within the matched lamp branch | Changes branch-current behaviour; part of this capacitive circuit | Not an arbitrary addition to ordinary lamp wiring |
| In parallel with the supply of a suitable fitting | Reduces reactive power drawn from the supply | Does not create a series-connected lamp branch |
| Small suppression capacitor at the starter | Reduces radio interference | Does not compensate the entire luminaire |
A correctly designed capacitive branch can draw a current that leads the voltage. A duo circuit combines such a branch with an inductive branch, allowing their reactive-power contributions to partly offset each other.
One capacitive branch does not automatically compensate an entire installation. A better power factor is not direct evidence of reduced active energy consumption. With distorted currents, overall power factor must be distinguished from displacement factor cos φ.
Electronic ballasts and LED tubes
An electronic ballast is not simply a replacement for the series capacitor. It operates approved lamps according to its own design and connection diagram. Dimming and lamp compatibility are product-specific. The existing equipment photograph shows an explicitly non-dimmable example, not a universal compatibility list.
An LED tube intended for conventional ballast operation is not automatically suitable for a capacitive branch containing a series capacitor. An LED replacement starter does not establish that either. The actual tube and luminaire documentation determines whether the arrangement may remain or requires professional modification.
Identify the starting stages and components
The animation shows starter glow, a closed preheating contact, opening with an ignition pulse, and subsequent lamp operation. C1 remains in the AC branch during both the illustrated preheating stage and normal operation. No conducting bridge is drawn between its plates.
Use “Next step” to compare the contact and lamp discharge manually. The automatic start is only an example; its times are not calculated from real component values. The inductive single-lamp circuit explains the same basic start without the series capacitor.
Avoid common mix-ups
- Here C1 is in the lamp branch, not across the whole supply.
- Here C2 is the starter's suppression capacitor.
- A number can identify a different local component in another drawing: the series capacitor in the DUO diagram is called C2.
- Component values cannot be taken from a symbol alone or copied from another luminaire.
Functional checks and limits
The model lets you follow the starter closing, the ignition pulse appearing and the lamp discharge starting. PE remains unchanged. The functional terminal numbers do not specify the layout of a real product.
The illustration calculates neither capacitor charge nor residual voltage, phase angles or instantaneous values. OFF only means that the model switch is open. It is not proof of discharge or safe isolation. Any LED conversion must also suit the actual luminaire and an approved conversion arrangement.
Technical references
DIAL: circuit arrangements distinguishes series, parallel and suppression capacitors; Schneider Electric explains magnetic-ballast starting.
Circuit diagram and explanation
Reading the circuit diagram
C1 identifies the series capacitor, L1 the choke, E1 the lamp and S1 the starter. Here L1 is a component reference, not a requirement for an additional supply phase.
The lamp branch connects between L and N. C1 is in its current path rather than simply across the supply. The four tube-contact numbers explain the drawing. Determine physical connections from the actual luminaire's documentation.
The protective conductor is shown for the designated conductive luminaire parts. Its requirements depend on the protection class and construction. It does not connect to the glass tube.
Connection principles and practical details
Stored charge and component replacement
A capacitor may retain energy after isolation. Establish whether the actual equipment has a discharge arrangement and how it works. An arbitrary waiting period alone does not universally prove a safe condition.
Discharge and verification require the intended professional procedures. Shorting the capacitor with a screwdriver is not an appropriate general method. Safe isolation must also address stored energy.
Replacement selection involves more than the printed capacitance. Construction, permitted operating conditions and approval for the application also matter. Bridging or omitting the series capacitor changes the circuit.
Functional checks
Following professional work, assess whether the lamp starts correctly and remains stable. Required electrical inspection and testing are separate. A glowing tube does not confirm correct component matching or effective protective measures.
- Do the lamp, starter, choke and series capacitor match the equipment specifications?
- Are there visible defects, unusual heating, smells or repeated ignition attempts?
- Are the intended component fixings and electrical connections intact?
- Have the inspections and measurements required for the work performed been completed and assessed?
These questions are not instructions to test exposed live supply connections. Capacitor assessment in particular requires an appropriate procedure. A component that looks normal is not necessarily serviceable.
Advantages and limitations
In a correctly designed duo arrangement, the capacitive branch can offset part of an inductive branch's reactive contribution. Together, the lamps can also produce less pronounced combined light fluctuations than similar branches operating together.
This does not guarantee flicker-free lighting, a specific phase angle or a particular energy saving. Correct matching and capacitor ageing are limitations. The choke still dissipates power and may produce audible noise.
This circuit is best understood as part of existing conventional lighting technology. A new solution requires assessment of the complete lighting task, including safety, light quality and system compatibility.
Questions and related information
Common questions
Does the capacitor alone act as the ballast?
No. The illustrated circuit still requires the choke. The capacitor forms part of a jointly matched arrangement.
Is every capacitor in a fitting a series capacitor?
No. Parallel compensation and interference suppression serve different purposes. The circuit and component documentation establish its function.
Does the capacitor automatically reduce electricity bills?
That conclusion cannot be drawn from its presence. Reactive power and power factor must be distinguished from active power, losses and the applicable billing arrangement.
The inductive single-lamp circuit explains operation without the series capacitor. Old fluorescent lamps contain mercury and require the designated collection route. Related safety information is available under basic electrical knowledge.
Sources and further documentation
Electrical safety: scope, competence and precautions
Put electrical safety first.
- These explanations and simplified diagrams support learning. They are not complete installation designs, site-specific risk assessments or a substitute for competent professional assessment.
- Electrical work can be life-threatening. It requires the competence and authorisation appropriate to the actual task, installation and jurisdiction.
- Errors and omissions can occur. Check the actual equipment documentation, applicable requirements and the complete installation before relying on an example; a working device is not proof of electrical safety.
- Personal protective equipment must suit the assessed hazards and task. It does not replace isolation, appropriate tools, competence or a safe system of work.
- Do not work on exposed live parts on the basis of these guides. Identify every relevant source, isolate it, secure against reconnection and prove dead using suitable equipment and procedures before access.
- Responsibilities for design, installation and verification must be established for the actual work. A website statement does not determine liability or remove duties under applicable law.
- This summary is not exhaustive. Where the circuit, condition or protective arrangement is uncertain, obtain suitable professional assistance before work proceeds.
- Complete the required inspection, testing and documentation before use, and maintain the installation and equipment appropriately.
Electrical terms explained
Electric current
The rate of flow of electric charge, measured in amperes (A). Conventional current direction follows positive charge flow, which is opposite to electron drift in a metallic conductor.
Voltage
The electric potential difference between two points, measured in volts (V). Always identify the reference points, such as line-to-neutral or line-to-earth; a voltage is not a property of one isolated terminal alone.
Resistance
The opposition associated with resistive current flow, measured in ohms (Ω). For a component behaving as an ohmic resistance under the stated conditions, U = IR.
Electrical power
The rate of electrical energy transfer, measured in watts (W). P = UI applies to steady DC; in a sinusoidal single-phase AC circuit, active power is UI cos φ using RMS values. The general average depends on voltage and current waveforms.
Ohm's law
For an ohmic resistance, current is proportional to the applied voltage: I = U/R. Resistance can vary with temperature, and non-linear devices do not necessarily obey one constant-resistance relationship.
Kirchhoff's laws
The current law expresses charge conservation at a circuit node. The voltage law states that the algebraic sum of voltages around a closed loop is zero within the applicable circuit model; induced effects must be represented appropriately.
Direct current (DC)
Current that maintains one direction. Its magnitude may be constant or vary; pulsating DC is still distinguished from current that reverses direction.
Alternating current (AC)
Current whose direction reverses with time, commonly periodically. Public supplies normally use approximately sinusoidal AC, while electronic equipment can produce other waveforms.
Frequency
The number of cycles per second of a periodic quantity, measured in hertz (Hz). Many European public supplies operate at a nominal 50 Hz.
Phase difference
The relative position of periodic quantities of the same frequency, often expressed in degrees or radians. For sinusoidal quantities, a time displacement can be related to that phase angle.
Circuit diagram
A drawing representing circuit components and their electrical connections using symbols. It is distinct from a physical layout or a complete installation design.
Capacitor
A component that stores energy in an electric field. Its capacitance relates stored charge to voltage under the stated conditions; a practical capacitor also has voltage, current and other operating limits.
Strappers in lighting circuits
The pair of conductors linking the changeover contacts in conventional two-way lighting switching, including intermediate switching where used. They carry the alternative switching paths and are not transformer windings; identify their actual functions from the circuit.
Coil or winding
A conductor arranged in turns, commonly to produce magnetic effects. Its behaviour includes inductance as well as the resistance and other properties of the actual winding.
Inductance
The relationship between flux linkage and current in the applicable model, measured in henries (H). Changing current produces an induced voltage; for a constant inductance its magnitude relates to L di/dt with the appropriate sign convention.
Capacitance
The relationship between stored charge and voltage, measured in farads (F). For a constant capacitance, changing voltage produces current according to i = C dv/dt.
Transistor
A semiconductor device used for switching, amplification or other control functions. Its operating principle and control quantities depend on whether it is, for example, a bipolar transistor or a field-effect device.
Semiconductor
A material whose electrical behaviour can be controlled through composition, electric fields, light or other conditions. Semiconductor devices use this behaviour for functions such as rectification, switching and amplification.
Diode
A device with a strongly directional current-voltage characteristic. Real diodes have forward losses, reverse leakage and limits; they do not act as perfect one-way valves at every voltage.
Relay
A device that changes an output in response to a control input. An electromechanical relay uses a coil and contacts, while a solid-state relay uses electronic switching elements.
Switch
A device that makes, breaks or changes electrical connections. Its rating, contact arrangement and permitted duty determine the applications for which it is suitable.
Electrical network
An interconnection of electrical components and branches. Series, parallel and more complex arrangements can be analysed with an appropriate circuit model.
Transformer
A device transferring energy between magnetically coupled windings through changing flux. Turns ratio influences voltage and current relationships; losses, limits and any conductive connection depend on its construction.
Electromagnet
A magnetic arrangement energised by electric current, commonly using a coil and magnetic core. Contactors, relays and actuators use electromagnetic force for mechanical operation.
Impedance
The relationship between voltage and current phasors in a sinusoidal steady-state model, measured in ohms. It combines resistance and reactance and can vary with frequency.
Short circuit
An unintended low-impedance connection between points at different potentials. Current is limited by the complete source and fault-path impedance, not literally unrestricted.
Electric circuit
An arrangement of sources, conductors and components. Current flows where a complete conductive path exists, although a circuit can also be described in an open or disconnected state.
Series connection
Components in one unbranched path carry the same current. Their voltages add algebraically; the individual voltage shares depend on their electrical behaviour.
Parallel connection
Branches connected between the same two nodes share the same voltage. The branch currents depend on their loads and add algebraically at the connection points.
Fuse
An overcurrent protective device with an element designed to melt and interrupt the circuit under specified conditions. Its rating, characteristic and breaking capacity must suit the application.
Overcurrent protection
Protection against overload and short-circuit effects, commonly using fuses or circuit-breakers. A sensing relay may require a separate interrupting device; an ordinary RCCB does not provide this protection by itself.
Earthing
A deliberate connection of a system point or conductive part to earth. Protective earthing forms part of an electric-shock protection arrangement; it must not be confused with neutral, functional earth or a guarantee of zero touch voltage.
Frequency converter or variable-speed drive
Electronic equipment that produces a controlled output frequency, commonly with controlled voltage, to operate a suitable motor. Its pulsed output, protection and compatibility requirements differ from a direct mains supply.
Electromagnetic compatibility (EMC)
The ability of equipment to operate satisfactorily in its electromagnetic environment without causing unacceptable disturbance to other equipment. Both emissions and immunity matter.
Power loss
Energy dissipated per unit time, commonly as heat in conductors, cores, insulation or semiconductor devices. Ideal inductors and capacitors exchange stored energy without average loss; practical components have additional loss mechanisms.
Short-circuit current
The current resulting from a short circuit. Prospective fault current is assessed to select protective devices with adequate interrupting capability and to check thermal and mechanical effects.
Power factor
The ratio of active power to apparent power. For sinusoidal voltage and current it equals cos φ; with distortion it also reflects waveform effects. It is not the same as energy-conversion efficiency.
Insulation resistance
Resistance measured between specified conductive parts separated by insulation under stated test conditions. The test voltage, connected equipment and interpretation must suit the circuit; it is not a complete assessment of insulation performance on its own.