Photoflash Units

Brooke Clarke 2011 - 2016


Background
    One Time Flash Methods
    Electronic or Strobe Flash
        First Generation
        Graflex Stroboflash IV
        Graflex Strob 250
        Graflex Booster/Analyzer
Synchronization
    Leaf or Diaphragm Shutters
    Focal Plane Shutters
    Flash Powder
    Flash Bulbs
        M
        F
        FP
        X
Patents
Related
Links

Background

One Time Flash Methods

The light sources used for photoflash (Wiki: Flash Photography) work have changed over time.  The early units were chemical flash powders (Wiki).  Next came flash bulbs.  The popular type had a bayonet base and the professional type had a standard Edison screw base (the same as a household lamp).  Next came the Flash Cube (Wiki)  that worked with many high volume consumer cameras.  The big advantage of these was that you got four flashbulbs in one unit and it rotated after each shot allowing for a much shorter time between exposures.

Electronic or Strobe Flash

Unlike the above one time methods where you threw away the flash bulb, strobe flash has a great advantage in that the light comes from a tube that can be fired a large number of times.  See the GR Strobotac web page for more about strobe lights.

First Generation

These used a bank of high voltage batteries and had a very short recycle time.  The batteries charged a capacitor that has the flash tube connected directly across the capacitor.  The typical operating voltage was in the 200 to 600 Volt range which was not enough to trigger the flash tube.  In order to start the ionization of the tube a separate very high voltage (a few thousand volts)  transformer was used which drove a wire wrapped around the flash tube.  Once started the flash lasted until the capacitor voltage drained to below the voltage where the tube would stop conducting.  That's to say that almost all the energy in the capacitor was drained for every flash.  If the battery voltage went down then the flash power would also go down.

Graflex Stroboflash IV (Wiki)

I had one of these and used it with a Hasselblad 500 camera setup.  A battery box holds a bunch of batteries, the final battery voltage may be 510 Volts.  From the Wiki page "It was designed and initially manufactured in 1942 by Strobo Research, a company founded by Edward Farber and Harold Edgerton.."  Edgerton invented the strobe flash tube and the Strobotac.


Model Batteries
Watt Seconds
I
2 x 240V = 480V
50
II 2 x 225V = 450V
EverReady 489
100
III 2 x 225V = 450V
  EverReady 489
200
IV
2 x 225V = 450V
  EverReady 489
50, 100, 150 or 200

Sprague
250 450 DC  (I think that's 250 uF and 450 VDC max)
               Neg
D19054  6913
There are 8 caps and they are wired into pairs so step size is 500 uF per step.
The actual caps measure more like 300 uF each or 600 uF pere step. i.e. 600, 1200, 1800 & 2400 uF
Measured Cap Values
Power
uF
1/4
618
1/2
1262
3/4
1870
Full
2434

There are three active terminals on the connector between the flash head and the battery box.  Looking into the connector on the cord from the battery box with red dot up they are:
Upper right (red wire in battery box): Negative Variable Capacitor terminal
Upper left (white wire in battery box):  Positive battery terminal)
Lower left (black wire in battery box): Positive Variable Capacitor terminal.

Note in order to charge the caps the head must be plugged in.

Looking into the flash head connector with red dot up there is a short between the top two terminals (Neg cap to Positive battery).
This seems to be wrong on two counts, the polarity is reversed and the wire color is wrong.  See Fig 7 below.
Based on trying to charge the caps the polarity above is wrong, i.e. red wire is positive.
Have power supply sitting at 250 Volts at max capacitance.  The current was 45 mA and a minute or two later it's down to 18 mA.
Will wait a while, the increase the voltage to 300V.  After the current dropped to 3 mA (1 Watt power) raised the voltage to 300 and for a few seconds the current limited at 500 mA but then quickly fell to 18 mA.  I've tried to fire the flash at each voltage (200, 250, 300, 350,  ) but so far it has not fired.  Wiki: After Storage

Note:  When I opened the battery box for the first time I notices that tell-tale smell of a resistor that has been overheated.  But there are no resistors.  When first connected to the power supply with the polarity indicated by the capacitor markings [D (+) and triangle (-)] visible smoke came out.  I now think this battery box was wired incorrectly at the factory.

Trigger

The trigger socket is the same as a USA AC mains 2-prong line cord.  That would allow use of the then common AC mains extension cords for flash trigger.
I'm still waiting for the capacitors to form but it appears that the voltage is about 20% of the capacitor voltage.
So, if the caps were charged to their max voltage of 450 V the trigger would be at 90 Volts.
When the trigger is shorted the voltage goes to zero, but when the short is removed the voltage climbs back because the trigger capacitor charging.
Minus is close to the support and Positive is close to the back where the label is located.


Note a 489 (NEDA 728) battery adapter might be made using 25 each 9 Volt batteries, maybe using 5 each of the 45VS adapters.
Fig 1
Graflex
                    Stroboflash IV
Fig 2  2-Prong trigger socket accepts the then common 115VAC extension cords.
Graflex
                    Stroboflash IV
Fig 3 Space available for two batteries is about 4.5" x 6"
Graflex
                    Stroboflash IV
Fig 4  Pencil line between banana plugs is internal short connection.  Although the HV connector has 4 holes only three of them have terminals.  Ground, HV and trigger.  By placing the HV capacitors in the battery box and running a long coiled wire to the flash tube the inductance in the flash circuit is much higher than when there is a physically short path for the connection between the caps and the flash tube.
Graflex
                    Stroboflash IV
Fig 5 Idea for Battery Adpater
Graflex
                    Stroboflash IV
Fig 6 Inside battery box is just a switch and 8 caps.
Graflex
                    Stroboflash IV
Fig 7  Note my analysis had the polarity wrong and it's
been reversed in this photo.  The current is dropping over time
unlike with the wrong polarity is was around 88 mA and staying high.

This is a Consort EV625 Electrophoresis (Wiki) Power Supply
Rated at: 600 V, 500 mA, 150 Watts.  Got it on eBay for about $50 including shipping.  A much lower cost than buying a regular HV bench power supply.  Was sold AS IS, but it seems fine to me.  A little tricky to program, i.e. the first method must be a ramp then the second method can hold the voltage for up to 99h59min.
Graflex
                    Stroboflash IV


Graflex Strob 250

This is a newer lower power version of the Graflex Stroboflash IV.  It uses a single510 Volt  Eveready No. 497 battery.
The box holds only a battery and there is a 2 conductor cable that runs to the flash head.  The connector on the side of the battery box has a male and a female contact.
There may be capacitors in the handle of the flash head since the handle part is not detachable from the head.

Fig 1
Graflex
                    Strob 250 photoflash


Graflex Booster/Analyzer


Reference Graflex Historic Quarterly Vol 14, Issue 3, pg 7 (also has the Graflex SlaveSync).

For use with the Graflex Strobomatic 500 HV and Graflex Strob 250 and by adding the adapter cord Stroboflash power packs can be tested.

This is a small box with three functions.
Boost
A charging function intended to "top up" a carbon zinc type battery.  But this is a controversial concept on a primary battery.
A GE diode marked GE 6RS5GH30BAB1  (with a high forward resistance) rectifies the AC coming out of the power transformer and the half-wave signal is fed to to the battery.   The diode is a spring loaded stack of disks, probably Selenium (Wiki). GEJ-1238A Instructions Selenium Rectifier Stacks - shows the 6RS series, and mentions the low current cells, but no specs.+ 233 VDC on the shrouded (female) contact in boost mode (do not see how that's going to charge a 500 V battery?, maybe the rectifier is no good?).
Battery
Measures battery voltage and can be used to get an idea of how the caps are working.
Load
A way to check the health of a battery by measuring battery voltage with a 3k4 load (500V / 3k4 Ohms) about 150 mA load.

Fig 1
Graflex
                    Booster/Analyzer
Fig 2
Graflex
                    Booster/Analyzer

2668945 Electgrical Measuring INstrument with Permanent Magnet Restoring Means, William E. Pfefler, Electromechanical Instrument Co., Feb 9, 1954 - tag on back of meter

Synchronization

It's important the the light from the flash occurs when the shutter is open otherwise some or all of the light is wasted. 
There are two common types of shutter:

Leaf or Diaphragm shutters (Wiki)

These typically are made as part of the lens assembly.  They are common for view and large format cameras.

Focal Plane shutters (Wiki)

Tese are positioned just in front of the film as part of the camera body, so the lens can be made without a shutter thus at a lower cost.  This is common for 35 mm film cameras.  For the faster shutter speeds one curtain starts moving and a short time later the other curtain starts moving.  This way a narrow gap moves across the film.  If a strobe flash went off during one of the faster shutter speeds only a small part of the film would be exposed, and that same problem exists for conventional flash bulbs.

So to use flash with a focal plane shutter either the shutter speed needs to be slow enough so that the shutter is fully open or a special type of flash bulb is used that has a long peak light output (called FP bulbs).

Flash Powder

The first way to do this was to simply remove the lens cap and ignite the flash powder then return the lens cap.

Flash Bulbs

When flash bulbs came out most cameras had leaf or diaphragm shutters (Wiki) and a provision to use a cable release (Wiki) to trip the shutter. So one of the first camera flash units was the Mendelsohn Speedgun (separate web page) for use with the Graflex (Wiki) Press cameras. There were a number of schemes to get the light synchronized with the shutter opening that all related to the use of the shutter release cable.

The next innovation was to incorporate a electrical PC flash contact (Wiki) into the shutter mechanism.  There may be a flash mode switch associated with the electrical contact.  The flash synchronization modes are:

M

For Medium type flash bulbs, the most common and have peak light about 20 to 25 milliseconds after power is applied.

F

For Fast sync. where the bulb is at peak light output in about 5 milliseconds.

FP (Front Curtain Sync)

For Flat Peak flashbulbs.  The contacts are closed at the start of the exposure, also called front curtain sync.  A problem with this is that if there is streaking (say taking a photo of a car at night where the lights cause a streak) the streaks are in front of the car. 

X

For Electronic Xenon flash where the switch closes at the same time as the shutter is fully opened.  For focal plane shutters the shutter speed needs to be slow enough so that the shutter is fully open, i.e. does not work at the faster shutter speeds.

Rear Curtain Sync

Modern digital cameras have Rear Curtain Sync so that the flash fires at the end of the exposure time thus the streaks are behind the car.

Patents

813148 Magnesium Flash-Lamp, Feb 20, 1906, 431/363
813849 Flash-Light Device, Feb 27, 1906, 431/357
814243 Photographic Flash-Light Apparatus, Mar 6, 1906, 431/357

2204424 Photoflash Synchronizer, Samuel Mendelsohn
RE22338 Photoflash Synchronizer, Samuel Mendelsohn
2332587 Testing Device for Camera Synchronizers, Samuel Mendelsohn, Oct 26, 1943 - depends on human eye.
2358796 Flash photography, Eugene Edgerton Harold, Filed: Dec 17, 1941, Pub:  Sep 26, 1944, 396/206, 315/175, 315/208, 315/358, 315/238, D16/240, 362/3, 315/241.00P -
                AC powered or wet battery vibrator power supply with 2 flash heads.  Camera looks like a 4x5 Graflex.

2588368 Light Integrator, Edgerton Harold E, Filed: May 2, 1947, Pub: Mar 11, 1952, 356/215, 324/102, 250/225, 73/1.56, 250/214.00R, 250/229 - photocell in box with shutter
                Calls:
               
2408764 High-speed flash-photography, Edgerton Harold E, Filed: Jun 14, 1940, Pub: Oct 8, 1946, 396/171, 362/4, 455/66.1, 250/214.00P, 315/230, 455/500, 250/215
                                Referenced by 32 patents. Studio setup (AC only power) 1/10,000 second duration

Related

GR Impedance Bridges
GR Sound measuring equipment
GR Strobotac with patent links
LM631A Amprobe (Meterman) LM631A Digital Light Meter
Macbeth Illuminometer -
Metering strobe flash - Thomas Strobometer Model 1B
Nikon SB25 on camera strobe flash
Nikon on camera flash units
Photo Studio Strobe Flash w/  Einstein 640 & Thomas Strobemeter Model 1B -
Weston Model 594 Photronic Cell light sensor, & Weston 614, 615, 650, 703, 756, 819 & others

Links

Russian w/Google Translator: E640 info on IGBTs 

Brooke's PRC68, Products for Sale, Photography
page created 24 July 2011.