Using Fluorescent Ballasts to Power Xenon Strobes

Using the 40 Watt 2-Lamp Ballast

The 40 watt 2-lamp rapid start ballast is a convenient way to obtain high voltage with some current limiting. The current limiting effect of the intentional leakage inductance in the ballast does a very good job of not letting the energy storage capacitor recharge too quickly after a flash. This avoids problems from the flashtube failing to extinguish.

UPDATE 9/9/2012: My testing was with an "iron core" or "magnetic" ballast, as opposed to an electronic one.

To use this ballast (or at least the popular 120 volt 60 Hz AC versions that were widespread in the USA), simply connect one of the two blue wires and one of the two red wires to the AC terminals of a suitable bridge rectifier. Such a bridge rectifier can be made with 1N4007 diodes or Radio Shack's 276-1114 1,000 volt diodes. The bridge rectifier feeds the energy storage capacitor. The white and black wires get AC. Ignore the yellow wires.

Use a fullwave rectifier, not a halfwave one. These ballasts usually have a capacitor in series with the secondary of the transformer inside them, so they can only put out AC and not DC.

The red wires have a small voltage across them, and so do the blue wires. You probably want to use whichever blue wire and whichever red wire give you more voltage.

WARNING The secondary winding may not be isolated from the line. In the ballast I tried this with, one of the blue wires seemed to be shorted to the black one.

This charges the energy storage capacitor to about 410-450 volts DC. If the capacitance is small (a few uF or tens of uF), its voltage may overshoot to about 500 volts when it is recharged after a flash. You can get average power levels of a few tens of watts, but the capacitor may not fully reach 450 volts (especially if capacitor is large enough to avoid the voltage overshoot).

This scheme is appropriate for:

1. Strobes using smaller, straight camera flash tubes with an outside diameter around 3-4 mm. and an arc length of 18-32 mm. Energy levels should generally be around a joule to a few joules, indicating capacitor values from about 8 to about 30 uF. Average power levels up to a few watts seem to be safe if the flashtube is reasonably ventillated.

2. Strobes using the smaller U-shaped flashtubes with a tubing outside diameter around 6 mm. (1/4 inch) and arc lengths around 4 to 5 cm. (1.6 to 2 inches). Energy levels should be a few joules. This means capacitor values should be from about 15 to about 50 uF. Average power levels up to several watts seem to be safe if the flashtube is reasonably ventillated.

3. Strobes with "medium" U-shaped flashtubes with tubing diameter around 6 mm. (1/4 inch) and arc lengths around 6 to 8 cm (2.4 to 3.2 inches). At 450-500 volts, these flashtubes do well with higher energy levels from several joules to around 25 joules. This indicates capacitor values from 50 to 250 uF. These flashtubes seem to safely take average power levels up to around 10 watts if they are reasonably ventillated.

4. Strobes with professional duty flashtubes having a tubing outside diameter around 7.5-10 mm. (.3-.4 inch) and arc lengths around 7.5 to 13 cm. (around 3 to 5 inches), as well as the FT-6 (available from a few camera shops). This includes the "Lectric Lites" LL2009 (available from Electronic Goldmine, cat. no. G6941, USA phone no. (800)-445-0697) and the Photogenic C4-5 (Available from B&H, USA phone numbers (800)-947-9950 and (212)-444-6600). Energy levels can be from 20 to at least 100 joules for the LL2009. The FT-6 can take 500 joules. This indicates capacitor values from 200 uF to as high as 1,000 to 5,000 uF. The C4-5 takes similar energy levels or even higher ones up to 1,000 joules.

These flashtubes can take as much average power as the ballast can deliver if some forced air cooling is used. The C4-5 does not require forced air cooling if flash energy is at least 60 joules or the lamp is not flashed continuously more than once per second.

The ballast may overheat without forced air cooling, especially if average power exceeds 40 watts. Avergae power will almost certainly be limited by the ballast to less than 80 watts.

Go here for more info on big flashtubes and 450 volt capacitors.

A voltage Boosting Trick with a Choke

If you already have a DC power supply and a small capacitor, but you want it to be charged to a somewhat higher voltage than the power supply delivers, then you can add a large capacitor and a choke type fluorescent ballast (15 or 20 watt ballast typically) to get more voltage.

       |         choke     diode   |
       |                           |
      === large C                 === small C
       |                           |
       |                           |
       |                           |
      gnd                         gnd

One requirement is that the voltage is sufficient to have the flashtube reliably flash before it is boosted with the choke trick.

How this works:

Initially, both capacitors are charged to the lower voltage.
The flashtube suddenly discharges the small capacitor. The large capacitor recharges the capacitor through the choke. The current flowing through the choke has a "momentum" effect, which builds and continues to increase until the small capacitor's voltage is almost as high as the large capacitor's voltage. The "momentum" of the choke current keeps it flowing until the small capacitor has a voltage substantially higher than the large one has.

The diode keeps the small capacitor from discharging back to the large one afterwards.

Ideally, the voltage can be nearly doubled in this way. In practice, a voltage increase of about 50 percent seems typical. Values of the small capacitor from about 1 to several uF seem to work best.

The flash rate must be slow enough that the choke current completely stops flowing between flashes. Since the voltage boost cycle only takes several to maybe a few tens of milliseconds, this should usually not be a problem.

This scheme is good for having the energy storage capacitor (the small one) not rapidly recharge until the flashtube extinguishes. Its voltage will increase only slightly and gradually during the first half millisecond to about a millisecond after a flash, and increase more rapidly in the following few milliseconds. This is good for helping the flashtube becoming nonconductive after a flash, in sutuations when this becomes an issue.

Written by Don Klipstein.

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