Understanding Xenon Flash Tube Circuits

by Lewis Loflin

  

  

A flashtube, also called a flash lamp, is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations. Flashtubes are made of a length of thin glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes and conducts a high voltage pulse to produce intense light. Flashtubes are used mostly for photographic purposes but are also employed in scientific, medical and industrial applications such as lasers.

The flash lamp comprises a hermetically sealed glass tube, which is filled with xenon, and electrodes to carry electrical current to the gas. Additionally, a high voltage power source is necessary to ionize the gas. A charged capacitor is usually used for this purpose so as to allow very speedy delivery of very high electrical current when the lamp is triggered. (See fig. 1 above.) Flashtubes require high operating and triggering voltages and caution must be observed when using them.

Xenon flashtube outline
Fig. 1 Basic construction of a xenon flashtube.

The glass envelope is often made of fused quartz, borosilicate, or Pyrex. The electrodes protrude into each end of the tube. For low electrode wear the electrodes are usually made of tungsten, which has the highest melting point of any metal. Cathodes are often made from porous tungsten filled with a barium compound and the structure of cathode has to be tailored for the application. The cathode undergoes destruction from positively charged xenon ion bombardment. Anodes are usually made from pure tungsten.

Basic xenon flashtude circuit
Fig. 2

Fig. 2 shows the thee main components of a flashtube circuit. This consists of a high voltage power supply, the flashtube itself, a high voltage trigger transformers, and a photoflash capacitor.

The tubes I used required 300 volts to operate correctly. One can use an inverter, a device that steps up a low voltage battery to a high voltage (this is used in cameras), or a voltage doubler circuit for a 120-volt AC line. I used a homemade inverter circuit using a 12-volt center-tapped transformer as shown in the photos below. See the following:

Capacitor C1 is charges through resistor R2. When the switch is pressed C1 quickly discharges though T2 creating a high voltage pulse that ionizes the gas in the tube, causing C3 to discharge through the tube, thus crating a bright flash.

Note that while a standard electrolytic capacitor can be used, specially made photoflash capacitors should be used. They can withstand the high discharge rate. Resistor R1 must be used to keep the power supply from swamping and destroying the tube. In my setup C3 charges up to 330 volts and discharges down to 100 volts.

Using a photoflash capacitor that's too large can destroy the tube from overheating and thermal shock. The trigger transformer is usually matched to the tube.

Flashtube circuit triggered by a SCR
Using an SCR allows the use of low-voltage electronics to control the flash. Make note of gate sensitivity.
See Basic Triacs and SCRs

Flash strobe controlled by neon lamps
Another variation of triggering an SCR through one or two NE-2 neon lamps.

Flashtube circuit triggered by SIDAC
Here we use a Sidac. The flash rate is controlled by R2 and C1.
See How to use SIDACs and their operation

Flashtube test setup with HV power supply.
My experimental flashtube test setup.

My flashtube test setup
Closer view of my flashtube board.
commercial flashtube board
A board pulled from a flasher used on a state truck.

Xenon flashtube
Picture of a horseshoe shaped flashtube. Flashtubes come in a number of shapes and sizes.

Flashtube trigger transformer
One type of trigger transformer.