Attack of the Killer Strobes

Just because a camera has a hot shoe does not mean it can take all flash units (hereafter called simply "flashes").  Early flashes made for predominately mechanical cameras had trigger voltages that could run hundreds of volts!  This extreme voltage was fine for these types of cameras, but more modern sophisticated cameras can be damage by this type of crude voltage. 

What is trigger voltage?  This is the voltage applied to the gas in the flash tube that ionizes that gas.  Once ionized, the gas is able to conduct electricity and an incredible amount of electricity is discharged from the main flash capacitor into the gas tube producing a large amount of light.  This light exposes Grandma Sue with exactly the correct amount of light to produce a great picture.

Why is high trigger voltage bad?  Well, it is not bad intrinsically...the flash really does not care what voltage it is triggered at as long as it does the job of ionizing the gas.  The bad part comes comes from how the flash is triggered.  Mechanical systems simply close the circuit on the flash mechanically.  Modern cameras will close the circuit through electrical means.  If the voltage is too great, some will leak into the camera's circuitry and potentially damage electrical components. This damage may happen gradually and you camera may function for a while using the high voltage flash and suddenly die.  Another failure mode is when the electricity arcs across the contact gap and erodes the contact or produces insulating products that cause the contact to fail.

How much is too much trigger voltage?  For modern cameras, 6v or less is generally considered safe. 6v to 25v is a gray area that may or may not be safe for your camera depending on the make and model. 

How do I tell if my camera is, "modern?"  If your camera is digital, it is modern for the sake of this Instructable.  For film cameras, it gets a little tricky.  Going by manufacture date, cameras built before 1990 are probably more mechanical and should be safe with high trigger voltages.  Any camera built after 1990 should be considered modern. 

The key here is a Volt Ohm Meter or VOM.  Also called multimeter or multitester.  They use to cost $70 ~ $100 and were a significant investment for any test bench.  Now-a-days even extremely accurate digital models are rarely more than $20 or $30.

1.  Flash you need to test.

2.  VOM.

3.  Something to label the flash.

A flash has 2 main electrodes.

1.  The metal electrode on the side of the flash foot is the negative electrode.  Much like your car's electrical system, it connects to the body of the camera.

2.  The small electrode pin on the foot of the flash is the positive electrode.  Sometimes this can be hard to locate because modern flashes has many communication electrodes.  However, the main positive electrode will always be in the center of the foot.  Why does a flash need to communicate with the camera?  Lots of reasons....like any pack hunter, they need to coordinate their collective photographic attack for a scene.  Also the flash sometimes become lonely and despondent without regular communication.

VOM's are often called multimeters as they can read a lot of information.  

The mode you want to use here is DC voltage.  On my meter it is a capital "V" with some straight lines next to it.  The alternating voltage has a sine wave next to it.

Next choose your voltage.  Better to start high and work your way down.  I normally start at 200v.  It may seem high, but some flashes can easily generate 100v trigger voltage (from just a few 1.5v batteries!)  You can go lower for a more accurate reading, but not below the rating on the switch. 

1.  Take the black probe from your VOM and touch it to the electrode on the side of the flash shoe.  If your flash has a tightening nut, you can screw that down and hold the probe in place....freeing up a hand.

2.  Touch the red electrode to the center pin.

3.  The readings on your VOM may fluctuate a few seconds, but then it will settle down.  This is your "Trigger Voltage."

OK, you have a killer flash....now what?  Well you have a few courses of action:

1.  Throw away the flash.  Bad for the environment, bad karma.

2.  Give away the flash.  OK if you make sure that unwary recipient knows that the flash can destroy a modern camera.  Look for the Holga and other toy camera users.  All that plastic will be unaffected by the hundreds of volts coursing through its guts.

3.  Get a limiting device.  Wein Products Inc. sells a very nice hot shoe voltage regulator.  For $50, this product will protect your camera against high voltage flashes.  However, you do sacrifice all the communication you might have on the flash as well.  Usually not a problem.  Most flashes with communication electrodes are modern enough to  have a low trigger voltage.

3a.  Get a PC limiting device.  The flash plug (PC plug [PC stands for Pronto Compur]) will carry the same trigger voltage as your hot shoe only flashes.  You can also buy PC connection cords that will limit the trigger voltage.

4.  Practice good flash hygiene.  Properly label your flashes and only use the correct flash with the correct camera. 

5.  Surgery.  If you are "good with tools" you might want to open the guts of your flash and install a trigger voltage limiting circuit like Zenobe did with a Vivitar 283.  It is not as hard as it sounds, but there is an element of danger.  Flash capacitors can hold literally heart stopping voltages.  Make sure you observe all safety precautions and never trust a capacitor.  Always handle capacitors like they are fully charged, a little paranoia goes a long way.

If you don't want to or don't have the equipment to measure the trigger voltage on your flash,  you can look here for a database of flashes and their trigger voltages.

Enjoy your well lit photos with a flash you can trust!