Build a trigger coil!

Here is a way to build a trigger coil for many xenon flashtubes.

For best results, first obtain a ferrite core around 1.5 inches (around 4 cm.) or so in length. (longer is OK.) The core should be at least 4 mm. in minimum width or at least 4.5 mm. in diameter if it is round. An "AM Loopstick" core or fraction thereof at least 1.5 inches long is OK. So is the core from the Radio Shack inductor with catalog number 273-102, even if any dimensions of this are slightly short.

A core as short as 1 inch (2.5 cm) will generally work, but slightly longer is preferred if available and will fit where you want to put it.

Please note that the following is for coils taking a primary voltage of around 300 volts. Feel free to adjust the primary turns/wire type/size accordingly if the primary voltage is greatly different.

The primary should consist of a single layer of wire of about 12-15 turns covering about 2 cm. (about .8 inch) of the core. For best results, the primary should be centered on the core, which should protrude from both ends of the primary, preferably by roughly equal amounts. Things should work nearly as well if the core protrudes out both ends of the primary by at least 1/8 inch (3.2 mm) even if unequally. I would suggest using using magnet wire of guage around 16 or well-insulated hookup wire of guage around 20.
If magnet wire (this has thin insulation that is practically painted on) is used, it is recommended to cover the core with a layer or two of paper or tape. Any kind of nonconductive tape is OK.

Now that the primary winding is in place, it should be covered by at least one layer of insulating tape.

Now for the secondary winding!

Use a thin size of "magnet" wire. The guage number of this should be in the 30's or high 20's. Low to mid 30's is preferred.

For many applications, including the basic trigger circuit shown later in this document, connect one end of the wire to be used for the secondary to one of the primary leads. The primary lead that the secondary starts from is the "high" primary lead. The other primary lead is the "low" primary lead.

Wind a layer of the thin wire on the tape that was put over the primary. This layer does not have to be neat, but the ends of this secondary layer should be as close as feasible to being over the ends of the primary.

This secondary layer does not have to be extra neat or extra smooth. Turns can touch each other. A turn can even touch a turn that is a few turns away. However, turns should not touch other turns that are more than roughly 10 turns away.
For example, the 20th turn can safely touch the 23rd. However, I would try to not have the 20th turn touch the 30th, and certainly not the 40th.

After this first secondary layer is complete, wrap a layer or two of electrical tape over it. The tape should go at least .2 inch (5 mm.) beyond the ends of the winding. Now, continue winding. Make a second secondary layer like the first one.

Continue adding secondary layers until they have a total number of turns roughly 15-16 times the number of primary turns. Or, use a turns ratio equal to or slightly more than your desired voltage ratio.

When the secondary is complete, cover it with a layer of electrical tape. Then wind another few turns, near the center of the coil, not too close to either end. Cover this winding with a few layers of electrical tape, but let several inches of thin wire stick out.

An alternative to using the thin wire as the secondary's output lead is to connect the thin wire to a piece of thicker hookup wire. Strip the thin wire however you can (solder, sandpaper, flame, scrape the insulation off with a knife, razor blade or transformer lamination, or do any combination of these). Solder this to the hookup wire. Wind this until the solder joint is on the tape over the last full secondary layer. After this, cover the coil with a few layers of electrical tape.

I have found this to be a somewhat feasible way of building effective, sure-fire trigger coils. These work well at providing the 4-5 KV pulses needed by most smaller flashtubes. By using more turns on the secondary, these should probably be good at providing voltages up to 8 maybe 10 KV and should be good for triggering the somewhat larger flashtubes used in professional photographic flashes.

The long flashtubes used in some photocopy machines and some pulsed lasers may need really high trigger voltages like 20 KV. An auto ignition coil will often work as a trigger coil, producing 20-30 KV pulses by discharging a capacitor into the primary. The capacitor would typically be .1 uF or so and be charged to a few hundred volts, which is much less than the main energy storage capacitor voltage for these long flashtubes.

Another means of getting high trigger voltages is to use a flyback transformer from an old TV. You need one that makes high voltages without a voltage multiplier circuit. The original high voltage winding is the secondary. You may need to put on your own primary, which would need something like 1 turn per (very roughly) 10 volts of primary voltage. The primary will probably work anywhere on the transformer, but things may work a bit better if it is as close to the secondary as possible.

Basic Trigger Circuit

                              )    trigger lead to flashtube
                              ) trigger
                              ) coil
                              ) secondary
To positive end of            )
main storage cap              )
____________    ______________)
            VVVV    |         ) trigger
     47K 1/2W typ.  |         ) coil
                    |         ) primary
 Trigger          -----       |
 Capacitor        _____       \
 (.047 uF typ.)     |          \
                    |           \ trigger switch
To negative end of
main storage cap


Trigger coils typically use and produce high voltages. Even if shock from the trigger coil circuit does not electrocute you, it may cause involuntary body movements that might cause you to contact other high voltage points.

Also note that high voltages can jump through a short distance of air. The spark gap length/voltage relationship varies with electric field distribution, but spark lengths usually range roughly from .28 to .9 mm. per kilovolt of peak voltage at voltages in the 4 to 35 KV range. Significantly longer spark lengths in millimeters per kilovolt can occur at peak voltages near or above 50 KV. This is for air at normal sea-level atmospheric pressure. Longer sparks can occur at high altitudes.

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Written by Donald L. Klipstein.

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