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A Safe Flash Trigger For Your Digital SLR Camera

Many of today's digital SLR cameras risk serious damage if used with an external electronic flash, whether that is a portable type or a large studio "strobe". We found this was the case here at SILICON CHIP so we have produced a flash trigger to ensure the camera's safety. You can do likewise - but beware of the JISP!

By Ross Tester

We use a relatively ancient but perfectly serviceable Balcar studio flash and softbox for all in-house photography, coupled with a Nikon DSLR (digital SLR). The Nikon replaced my three much-loved but 40-year-old Minolta (film) SLRs.

When we changed to the Nikon, there was a minor problem: no sync connector (commonly known as a PC connector but it has nothing to do with personal computers). There was a hot shoe connector though and we obtained a hot shoe-to-PC-socket adaptor to solve that problem.

The second thing we checked was the instruction manual for any warnings about using studio strobes. There were two: (a) the maximum strobe firing voltage that could be applied to the camera was 250V DC and (b) the polarity of the sync lead had to be tip positive.

Hmm! Both of these could be problems. The second certainly was because the phone-type plug which connected to the Balcar flash was tip negative. At least that problem was easily solved.

Then we wanted to know the voltage at the sync terminals. That’s easy, right? We connected a digital multimeter to the sync terminals and it gave a reading of 224V. But a day or so later, when I repeated the test (to be sure, to be sure, etc) it was down to 103V.

Click for larger image
It's an oldie but a goodie - our Balcar A1200 Studio Flash power pack which mates with the flash head and softbox diffuser at top left. The SAFE-T-FLASH trigger we made is in the black 6.5mm plug (highlighted) - it reduces the sync trigger from 300V to around 7.5V (and could go even lower).

Hang about, nothing had changed, so what was happening? Surely not even a large mains variation could make that much difference? Something had changed and I took a few minutes to realise that I had used a different DMM. The first one was a 10MΩ Tektronix TX3 DMM while the second was a much cheaper model which, as it turned out, had an impedance of only 3MΩ.

Could a digital multimeter be loading the camera’s sync circuit by so much? Well, yes it could, since the sync circuit is essentially a capacitor discharge circuit to fire the Xenon flash tube. When the camera’s flash contacts close, they discharge the capacitor to fire the flash tube.

In essence then, the sync circuit is just a capacitor which is charged from a high voltage source. So to find out the open-circuit voltage from the sync circuit and the charging impedance, we decided to make a few more voltage tests with loads of 10MΩ (ie, with the Tektronix DMM) and 5MW (Tektronix DMM in parallel with a 10MΩ resistor). This gave results of 224V and 171V, respectively.

We then set up a pair of simultaneous equations (see panel). When the equations were solved, the results were that the open-circuit voltage was about 324V and the impedance around 4.5MΩ!

Well, 324V was quite alarming and could certainly do damage to any camera. To confirm this high voltage calculation, we decided to make a further voltage measurement using a 50MΩ high-voltage probe with our LeCroy oscilloscope. The scope revealed that the voltage was around 310V. In fact, we had quite a few problems trying to make sensible measurements with the oscilloscope and its 50MΩ probe because the Balcar’s trigger circuit was floating with respect to mains earth and any connections to the scope tended to upset its operation.

However, we were able to confirm that the open-circuit trigger voltage from the Balcar flash was well in excess of 300V.

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