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A photoflash unit is a Resistor-Capacitor circuit. It utilizes a fundamental property of capacitors. A capacitor opposes an abrupt change in the voltage and this ability of a capacitor is put to use.

Circuit design of a photoflash unit:

A photoflash unit has a simple circuit design: a high-voltage direct current (DC) supply is connected in series with a high-resistance resistor (which we’ll call ‘R1′). This resistor limits the current flow. A capacitor ‘C’ is connected in parallel with a flash lamp. The resistance ‘R2’ of a flash lamp is of small value. The circuit contains a switch between the large resistance ‘R1’ and a small resistance flash lamp ‘R2’, such that it can connect either resistance at any time during the process.

How does it work?

When the switch connects R1, the capacitor begins to become charged. The charging of a capacitor is time consuming due to a large ‘time constant’. The time constant is the product of the resistance ‘R1’ and the capacitance ‘C’, given by the following expression:

Time Constant = Resistance of large Resistance * Capacitance

        T = R1*C

Charging process:

During the charging process, the potential of the capacitor starts rising gradually. Initially it has the value of zero but by the charging it rises to a steady value of ‘Vs’. As the voltage increases, the value of the electric current passing through it decreases from peak value to zero. This limiting of current happens due to the large resistance R1. The charging time of a capacitor is approximately equal to five times the time constant. That is;

Charging time of capacitor = 5*Time constant

Discharging process:

The discharging process of a capacitor takes place when switch connects with the flash lamp (with small resistance ‘R2’). The low resistance of the flash lamp allows a high discharge electric current to flow in a brief period of time. The discharging time is almost five times the product of small resistance ‘R2’ and the capacitance ‘C’, given by the following relation:

                        Discharging time of a capacitor = 5* (R2*C)

The photoflash unit circuit emits a high current pulse of short duration during the complete charging and discharging process of this simple Resistor- Capacitor Circuit. Such an RC circuit has many other practical applications, including Radar Transmitter Tubes and Electric Spot Welding.