Checking trigger voltage
Some old flash units are known to have dangerously high trigger voltages. Connecting such units to modern equipment such as digital cameras or wireless trigger may damage the equiment.
The units that it is most likely to have harmful trigger voltages are vintage “generic” units that only have a centre pin on their hot-foot. Newer, dedicated flash units with multiple pins for TTL electronic flash control can generally be considered “safe”. Another clue to look for is the unit's “ready” light indicator. If this is neon, then it is very likely that the unit has a high trigger voltage. If the indicator is of the LED type, it is more likely that the unit is “safe”. But it is always best to check.
This note tells you where to find data about trigger voltages, how you yourself can measure the trigger voltage of an old flash unit, and what trigger voltages can be considered “safe”.
1. Look it up
In many cases, the trigger voltage of a flash unit is listed in the manual. If it is not, you can email the customer support department of the manufacturer of the flash unit, and ask.
Alternatively, you may find the trigger voltage of the flash listed in our product database. Note that some units have manufactured over many years, and the design of the electronics may have changed over time. For such units, we list the smallest and highest voltage reported. However, we have no way of knowing whether our data is complete.
2. Measure it
To make sure, it always best to check the voltage and the polarity of a particular unit yourself.
To check an unknown trigger voltage, you'll need a digital multimeter. For most reliable result, the input impedance of the meter should be 10 MOhm, or higher.
To measure, put fresh batteries in the flash you want to check, and charge the flash until the “ready” lamp (or equivalent) is lit. Then set the range selector on the meter to the most sensitive DC range. To find the right range, start with the highest DC range and go downwards until the meter indicates you are out of range. Then back up one step. This is the most sensitive range. You should now be able to read the flash's trigger voltage from the meter's display.
You meter the voltage between the centre pin and the edge contact. Refer to the photo above if you do not know what these are. If there are more than one pin below the flash, it is the centre one you're interested in. It will make contact with the large round contact in the centre of the camera's hot-shoe. The edge contact may be a small metal square recessed in the hot-foot, or something larger. It is designed to make contact with the metal of the mounting bar on the edge of the camera's hot-shoe. You always connect the negative probe (black on most meters) to the edge contact, and then connect the positive probe (red on most meters) to the centre pin.
The photo above shows me making this measurement on a Nissin Speedlight Di866 using a Caltek CM1100 digital multimeter. Prior to metering, I've set the meter to use the range 0 to 20 volts. I use my hands to connect the points of the positive and negative probes to the metal of the Speedlight's hot-foot. The reading of the multimeter tells me that the Di866 has a trigger voltage equal to 3.42 volts.
Note that older analog voltmeters may have a low input impedance which may result in a voltage drop inside the meter. This will result in a too low reading. For best results, use a digital meter with a impedance of 10 MOhm or more.
Sometimes, the gap between the foot of the flash and the screw for fixing it in the hot-shoe may be too narrow for the meter's probe to reach the metal of the edge contact. If the flash has a pc-socket, you can altermatively measure the trigger voltage from the pc-socket.
A typical pc-socket is shown to the right (real-life diameter is about 3.5 mm). You connect the negative probe (black on most meters) to the edge, and the positive (red on most meters) to the centre of the pc-socket.
In addition to the trigger voltage, the polarity of the flash is important. All modern cameras expects the centre pin of the flash to be positive and the edge contact to be negative.
If things are the other way around (reverse polarity), even a very low voltage may cause harm. If the unit has reverse polarity, you will see this by the voltage displayed as a negative number (a minus-sign is placed in front) on the display of a digital multimeter. Never use a flash with reverse polarity on a modern camera!
3. How much is too much?
A matter of much debate is the maximum trigger voltage that is safe for a flash that is to be used in the hot-shoe on a digital camera, wireless trigger, or other equipment you may want to connect a flash to.
Polarity is also important. The centre pin on the flash hot-foot should have a positive voltage. If it is negative, even low voltages may damage the camera.
Before connecting a third party flash unit to your camera or wireless trigger, you should always check the maximum safe trigger voltage in the manual for your particular camera.
Below is a list of what I believe is the best information about this, however, make sure you read the disclaimer before acting in this information.
For connection through a pc-socket, Canon lists the maximum safe trigger voltage in the manual (e.g. 250 volts). However, this number does not apply to the hot-shoe. Canon does not officially give out information on the safe voltage for hot-shoe mounted flash units (beyond the obvious recommendation that you should only mount one of the Canon's own Speedlites in the hot-shoe).
However, an email from Chuck Westfall (Director, Media & Customer Relationship, Canon USA), posted in this thread in DPreview's Canon EOS 350D/300D forum in April 2005 by Doug Kerr had the following to say about trigger voltages:
The EOS Digital Rebel XT [350D] uses a modified version of the EOS 20D's shutter unit. Consequently, acceptable trigger circuit voltage for both cameras is the same, i.e., 250 volts. Except for the original Digital Rebel [300D], all current EOS digital SLRs (i.e., EOS-1Ds Mark II, EOS-1D Mark II, EOS 20D and EOS Digital Rebel XT) generate their X-sync signals electronically rather than mechanically. This is why they have higher acceptable trigger circuit voltage ratings than earlier models like the D30, D60, 10D and original Digital Rebel [300D]. These older models cannot be modified to achieve a higher trigger circuit voltage rating, since such a modification would require a different shutter mechanism as well as a complete redesign of the supporting circuitry.
In a 2007 interview with The Digital Journalist, Westfall confirms the above and adds some older film SLRs to the list.
I take this to mean that all Canons DSLRs newer than the 350D, as well as all the professional models, can use flash with trigger voltages up to 250 volts in their hot-shoe. However, 6 volts is the safe limit for the D30, D60, 10D, 300D, and Canon's digital compact cameras.
In this thread on RangeFinderForum, the following message from Stefan Staudt, Leica Camera AG is quoted:
The M9 is capable of triggering flash devices using high voltage. It is possible to use flash devices with a voltage up to 600 Volts. But it is important that the positive terminal is on the middle contact of the hot shoe and the ground terminal is on the mounting bar.
Caveat: While I relay the above paragraph, I will add the following note: I personally do not think that 600 volts is safe with any modern camera. Flash trigger circuits are usually designed with a SCR thyristor to trigger the flash. This is an effective and low cost solution, and it will ensure a 400 volts safe trigger voltage (other electronic components may lower this). To go as high as 600 volts, a much more expensive circuit is necessary. Given the cost of Leicas, it may of course use a more expensive circuit, but given that very few flashes (none of them current) have trigger voltages above 250 volts, using an expensive circuit to go to 600 volts in a modern design does not make economic sense.
The Leica M8 and M8.2 is reported to have an identical flash triggering circuit to the M9.
Most Nikon DSLR manuals warns against negative voltages or voltages higher than 250 volts. Here's a typical quote from the the manual for the Nikon D80 (p. 119):
Use only Nikon Speedlights. Negative voltages or voltages over 250 V applied to the accessory shoe could not only prevent normal operation, but may damage the sync circuitry of the camera or flash.
The only verified offical notices about from Olympus I am aware of are the following note about in the EVOLT E-410 FAQ (Q #49):
Thyristor-type flash units can be used with the EVOLT E-410's Manual shooting mode as long as the sync voltage does not exceed 6 VDC. Third-party TTL flash units will not have TTL capability because the contact pins in the camera hot shoe probably won’t align with the contacts on the flash. In addition, the TTL communication with the EVOLT E-410 may damage the camera circuitry or corrupt the camera firmware.
And the following paragraph in the the Olympus Pen E-P3 manual (p. 60):
Using obsolete flash units that apply currents of more than about 24 V to the camera hot shoe X-contact will damage the camera.
However, user iROK reports in a forum message on FourThirdsPhoto.com that he has received the following information in an email from Olympus (Japan):
The E-system is designed to satisfy the ISO10330 (trigger voltage for X contact is 24V or lower). If the flash you use is manufactured under this standards, there will be no problem. The E-1 and E-300 are [designed to] withstand 500V or lower, but it is recommendable to keep 250V as a maximum level.
As for 500 volts being safe, see my caveat under the Leica entry.
On Flickr, user atmbirdie reports getting the following answer from Panasonic tech support:
The camera will not be damaged, even with flash units that had up to 90V on the contacts. Our spec is <= 15V.
While his question was specifically about the LX3, I believe that at least the 15 volts safe limit applies to all Panasonic digital cameras.
Karl Schumacher sent me this message about Pentax DSLRs:
Pentax (Germany) has informed me that the maximum voltage for Pentax DSLRs *istD and *istDS is 30 volts and the maximum safe voltage for the K-series is 25 volts.
For connection through a pc-socket, Sony lists the maximum safe trigger voltage in the manual (e.g. “Use a flash with a sync voltage of 400V or less.” Sony a700 manual, p. 77). However, this number does not apply to the hot-shoe. Sony does not officially give out information on the safe voltage for hot-shoe mounted flash units (beyond the obvious recommendation that you should only mount one of the Sony's own units in the hot-shoe).
However, user georgiaboy reports in a forum message on Dyxum.com that he has received the following information:
I spoke to a Sony tech support guy who said max voltage should not exceed 24 volts.
For Cactus brand radio triggers, the maximum trigger voltage is listed in the manual.
For Cactus V4 and V5, the maximum safe voltage is listed as 300 volts.
Pocket Wizard lists maximum voltage in the owner's manual.
For the dedicated hot-shoe (e.g. the MiniTT1, FlexTT5), voltages should be less than 50 volts. For the 3.5 mm jack flash interface, the MiniTT1, FlexTT5 and Plus II models are listed as being able to handle up to 200 volts. The Plus III is listed as being able to handle up to 300 volts.
According to vendor CowboyStudio, the Seagull SYK range of optical triggers is only guaranteed to work with flashes with a trigger voltage less than 12 volts.
Rebranded version of these triggers are sold as Hama 6967 and Kaiser K1501.
The following information is from UK-based vendor Colinsfoto:
The Sonia peanut slave trigger (can be used alone or attached to a Sonia slave attach hot shoe) should not be used with flashes with a higher trigger voltage than 100 volt.
The Sonia slave with rotating hot-shoe is safe up to 400 volt.
YongNuo RF-602 and RF-603 should not be connected to a flash with a trigger voltage above 12 volts.
The above information about safe trigger voltage limits is believed to be genuine and is reported here in good faith. However, I disclaim any responsibility for your camera if you hook it up with an oddball flash and it fries. If you choose to act on this information, you do so at your own risk.
The ISO 10330 (Photography - Synchronizers, ignition circuits and connectors for cameras and photoflash units - Electrical characteristics and test methods) recommendation says that cameras and flash units should be able to accept trigger voltages up to 24 volts. AFAIK all cameras and modern flashguns (from the 21st century) comply with this.
I've also noted that it as become more and more usual for DSLRs, including low-end models, to use a SCR (Silicon-Controlled Rectifier) thyristor to trigger a hot-shoe mounted flash. This type of electronic component is usually safe up to around 400 volts.
CAUTION: To avoid doing harm to the camera, you should always measure the trigger voltage before using an unknown flash on a DSLR camera to make sure it is within safe limits. Some flashes, and in particular “Made in Japan” vintage editions of the popular Vivitars, may have very high trigger voltages and can damage the camera.
As for technical protection measures, Wein sells a range of safe-sync devices (search eBay for safe-sync) to protect against excessive trigger voltages.
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Once you've measured the voltage, be sure to report your results. I'll add them to out product database along with all other information you may have about the unit.