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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Activity 1. Getting acquainted with the flash unit

 

Equipment needed:

  • Vivitar 283 flash unit
  • 4 AA batteries or SB-4 AC adapter for flash power
  • PC cord

Some flash accessories that may be useful (but not essential) for the Guidebook activities are described in Appendix B.

 

These activities are written under the assumption that the Vivitar 283 (or 285) flash unit is being used.  While many flash units would suffice, the 283 and its cousin, the 285, are particularly suited to these activities.  If a different flash unit is used, details relating to the architecture of the 283 will not apply in what follows.  However, the fundamental methods of taking high-speed photographs and the general considerations relating to the flash unit will still apply.

This guidebook will not go into detail about how electronic flash units work.  We give below a general introductory reference and an engineering reference in that order.

Lester Lefkowitz, The Kodak Workshop Series: Electronic Flash, (Eastman Kodak, Rochester, 1986).
Harold E. Edgerton, Electronic Flash, Strobe (The MIT Press, Cambridge, MA, 1983).

hspeqp02.JPG (14229 bytes)The 283 flash unit with the SB-4 AC adapter and PC cord is shown to the right.  Use of the AC adapter is recommended to conserve batteries.   The 283 is a rugged flash unit, and handling it carefully will help it last a long time. One vulnerable part is the flash foot; it tends to break off easily. Another is the xenon flash tube. If the unit is dropped, a small crack in the tube would render the unit useless.

hspeqp09.JPG (16022 bytes)Note that the power switch doesn't function when using the adapter. The flash capacitor should begin to energize as soon as the adapter is plugged in. When the orange light on the back of the flash unit comes on (see to the left), the capacitor is almost completely charged. Wait several seconds to provide extra time for charging. Now point the flash at a nearby object and push on the orange light to discharge the flash.

 

There are other ways to discharge the flash. Plug the PC cord into the side of the unit. Be sure that the cord clicks securely into place. The main source of problems in using flash units is the cord. Use something metallic, such as a key or paper clip, to short across the terminals at the end of the cord. Avoid touching the metal when doing this, because you could get a sting from the voltage across the terminals. Also try shorting across the terminals on the flash foot. One terminal is on the bottom and the other on the side of the foot.

 

By shorting across the terminals, you are completing a trigger circuit within the flash unit that initiates the discharge of the main flash capacitor through the flash tube. Once started, the discharge proceeds to completion unless quenched by a means that will be discussed in the next activity. The voltage across the flash terminals is an important consideration when connecting the flash unit to external trigger circuits. For 283s made before 1984, the voltage is 200-300 V. This is typical of older flash units.  Newer models are designed with low-voltage trigger circuits.  For the newer 283s,  this is less than 10 V.

 

Whenever you finish using the flash unit, it is a good idea to discharge the flash capacitor completely after the unit is turned off. To do this, first rotate the dial on the sensor module (labeled auto-thyristor) to M (for manual). Then discharge the unit.

 

hspeqp26.JPG (28788 bytes)In preparation for the next activity, it may be useful to see what's inside the flash unit.  The diagram to the right shows a disassembled Vivitar 283.  The main capacitor stores electrical energy for the flash discharge.  The capacitor can remain charged even when the flash unit is open as in the diagram.  Touching the capacitor contacts in this condition can be very dangerous. Therefore, opening up a flash unit should be left to someone who knows how to do so safely.

 

The power conversion circuit converts the input voltage from batteries or the SB-4 to that needed by the flash circuitry.  The trigger input detects when the flash terminals have been closed and initiates the flash discharge.  The input signal can come either from a PC cord or from the terminals on the flash foot.

 

The trigger circuit steps up the voltage across the flash tube and sends a trigger pulse to the tube to start the breakdown of the xenon gas in the flash tube.

Note the leads from the photosensor, which is attached to the front housing of the flash unit. These leads go to a circuit which measures the intensity of light reflected from the subject and quenches the flash discharge when the right amount of light has been received for exposure of the subject.  This function of this circuit, called the auto-exposure circuit, will be examined in the next activity.

 


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