Xenon Strobe

March 1, 2011 - category: Alarms and security

Description:

This is a very simple strobe circuit which I have not seen anywhere else. My inspiration came from a strobe circuit that was in a school fire alarm buzzer - the kind that has a built-in strobe light on top.

WARNING: This circuit contains voltages and currents which can KILL if you are not careful. Charged capacitors will SURPRISE YOU! They can hold a lethal charge for hours! If you don't know much about working with line (and higher) voltages or if you aren't crazy (like me) then DO NOT attempt to construct this circuit. I CANNOT BE RESPONSIBLE if you electrocute yourself to death! That said, let's have some fun!

Xenon Strobe circuit diagram

It's a pretty simple circuit which some of you may be able to figure out, but I'll include this description for beginners and others of you who may not be as familiar with the characteristics of Xenon tubes.

As the power comes in, it passes through the large current limiting resistor which we will refer to as R1. This resistor's purpose is to prevent the current input from the AC line from maintaining the arc in the Xenon tube. If this did occur, the high current would place excessive wear on the electrodes. Also, undesirable metal haze would be deposited on the inside of the tube which would reflect heat back and further shorten its life.

Continuing, D1, D2, C1 and C2 form a voltage doubler which rectifies and steps up the voltage to around 340V. This voltage does not establish an arc in the Xenon tube because it is insufficient to ionize the Xenon gas. The charging time of C3 is adjusted via the potentiometer; the lower limit of which is controlled by R3. This controls our flash rate. Interestingly, C3 also charges through the primary of the trigger transformer. When C3reaches 280-320V, the SIDAC breaks over and dumps this energy into the primary of the trigger transformer. This primary voltage is stepped up through transformer action to around 2kV and is directed to the outside of the Xenon tube's glass envelope; ionizing the gas inside and making it more conductive. Once this occurs, the larger energy stored in C1 and C2 is able to further ionize the gas and create a bright arc which is maintained by the strong current. The arc will last until the voltage and current in C1 and C2 drop below levels sufficient to maintain it. This happens very quickly, on the order of a few milli- or nanoseconds, giving us our flash. The remaining charge in C1 and C2 will typically be around 50V.

Xenon Strobe
Xenon Strobe

Notes:

The polarity of the Xenon tube is important. If it is inserted into the circuit backwards, its life will be drastically reduced. The positive terminal is usually denoted with a red dot on the glass envelope. If your red dot has rubbed off, just look for the larger terminal, the one that looks like a wrapped up light bulb filament, the duller looking one, or the one at the end opposite the trigger electrode. This will be the negative terminal.

Some note should be made of eye protection. The Xenon strobe tube puts out hard UV so it's not a good idea to look straight into it while it's operating. One solution to this is to mount the tube or even the whole assembly behind UV blocking plastic. You will notice this on the commercial units. Lexan (polycarbonate) works well for this purpose. This is what a lot of UV blocking sunglasses are made of.

You may notice that I did not include a fuse in the schematic. This is because I like to live dangerously (cheaply). It is in the parts list. Use it!

By far, the most difficult component to find will be the SIDAC. These are also known as bidirectional thyristor breakover diodes. It's basically the same thing as a higher voltage DIAC or 2 high voltage back to back zener diodes. The ones I use are the Teccor K3000F1. I have also used 120V units such as the NTE 6419. These are less desirable because this runs the trigger transformer at a lower voltage and would require a higher value capacitor and/or potentiometer and R3. With only 1500-2000V to ionize the Xenon gas in the tube, it will not trigger reliably as the tube ages. If this is all you can find in your area, however, it does work and is what I used in my original circuits.

I don't recommend using a different value for C3 since using a smaller one would yield a weaker trigger pulse and using a larger one reduces the penetrating ability of the trigger pulse and could possibly overload the trigger transformer.

You will probably want to mount your Xenon Tube in a 3 pin socket instead of soldering it; since it will eventually need to be replaced.

A lot of the commercial strobes use polyester or polypropylene capacitors for C1 and C2 instead of the electrolytic. There are several possible reasons for this. One would be the lower ESR thereby giving a shorter flash duration. Another could be that they give a more desirable arc. Personally, I would recommend using them if you can. They are not as easy to find in >1 micro Farad sizes as the electrolytic and are thus probably more expensive. One possible source of such bipolar capacitors is motor run capacitors which can be found in old electric motors and air conditioners. These units have the added feature of having an internal discharge resistor. Building the circuit as it is in the diagram can be done using an old flash or disposable/cheap camera, a couple CRT televisions or monitors and little else. Going all out will get you a higher quality strobe in the long run but that would be up to you.

Those of you who live in areas where you have 240V service will notice that the circuit has been adapted to run at 120VAC. For 240V operation, instead of the doubler arrangement, use a bridge rectifier, a single 10uF 350 or 400V capacitor and a 470 ohm resistor for R1. You may also have to adjust the sizes of the potentiometer and R3 to regain acceptable flash rate adjustment.

One final note is to use some RTV silicone or flexible epoxy as strain relief. You will want to do this on the capacitors, Xenon tube socket, and power cord connections because otherwise they will strain and rip out very easily.

Parts Inventory:

  • C1, C2: 22u 200V aluminum electrolytic
  • C3: 470n 400V polyester or polypropylene
  • D1, D2: 200V or greater general purpose rectifiers
  • F1: 750mA instant glass fuse
  • FT1: Xenon Strobe tube; 300-400V, 4 watt second, 3kV trigger, such as the Radio Shack #272-1145, Electronic Goldmine A1034, or Mouser Electronics 361-4425
  • R1: 220R, 5W axial wirewound
  • R2: 1M potentiometer
  • R3: 220K, 1/4W carbon
  • SIDAC: (Littelfuse) Teccor K3000F1, 300V breakover
  • T1: 3kV or higher trigger coil for Xenon Strobe tubes
  • Other: 20x25 Plated Prototyping Board, 3 pin socket, AC power cord, Lexan (polycarbonate) plastic, RTV silicone or epoxy to reduce strain

Author and source

author: Robert Dvoracek

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