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SAFE-T-FLASH: A Safe Flash Trigger for 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! W e 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 58  Silicon Chip 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. siliconchip.com.au By Ross Tester your Digital SLR Camera 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 10MW Tektronix TX3 DMM while the second was a much cheaper model which, as it turned out, had an impedance of only 3MW. 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 10MW (ie, with the Tektronix DMM) and 5MW (Tektronix DMM in parallel with a 10MW 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 4.5MW! 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 50MW 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 50MW 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. The answers are on the ’net . . . NOT! As part of the research for this feature, we spent many hours on the internet looking for the experience of others. Several websites (including www.botzilla.com/photo/ strobevolts.html, http://photo.net/bboard/q-and-a-fetchmsg?msg_id=00KBWJ and http://aaronlinsdau.com/gear/ articles/flashvoltage.html) had pages and pages of strobe sync voltage readings. These were taken by photographers all around the world on a huge variety of strobes and offcamera flashguns (many of which we’ve never heard of). After our investigations, we would bet London to a brick that all of the sync voltage readings are wrong. Most were siliconchip.com.au 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). April 2008  59 How DO you determine the source voltage and impedance? The sync source of the Balcar electronic flash described in this article is the classic “black box”. It had an unknown (high) source voltage and an unknown (high) source impedance. When you have two unknown values, how do you proceed? The first step is to draw the equivalent circuit, as shown below. RO + IO VO RL VL - Inside the “black box” is a voltage source VO, connected in series with the output impedance RO. This is connected to the “outside world” to the load RL. The next is to measure the voltage across RL. Then repeat that step for a different value of RL. We now resort to Kirchoff’s Voltage Law which states that the sum of the electrical potential differences around a closed circuit must be zero. So we draw up an equation based on that law (also known as Kirchoff’s loop or mesh rule): VO = IORO + VL (1) Since the same current (IO) flows around the whole loop, we can calculate: IO = VL/RL (2) and we substitute that into equation (1) to get: VO = (VL/RL)RO + VL (3) We then take the voltage measurements for 10MW (224V) and 5MW loads (171V) and substitute them into equation (3) to get two new equations: VO = (224V/10MW)RO + 224 VO = (171V/5MW)RO + 171 (4) (5) We then calculate the value for IO in each of the equations and substitute its value into (4) and (5). This gives: VO = (2.24 x 10-5)RO + 224 VO = (3.42 x 10-5)RO + 171 (6) (7) To solve these simultaneous equations to find a value for RO, subtract equation (6) from (7) to get: 0 = 1.18 x 10-5RO - 53 (8) Therefore: RO = 53/1.18 x 10-5 = 4.49MW We can then substitute this value for RO into equations (6) or (7) to calculate the value of VO and the result is 324V. This is the true value for the open circuit voltage of the sync circuit; something that could not obtained by any direct measurement. 60  Silicon Chip Fig.1: here’s the actual firing of the Balcar strobe flash, with only the high impedance (50MW) probe of our LeCroy DSO connected. The ripple on the trace is actually 50Hz hum. Note the maximum voltage reading of 317V. recorded as being done with a DMM, usually of unknown pedigree. By the web posters’ own admission, at least a few of them were done with an analog multimeter. To prove the point, we measured the Balcar sync voltage with two different analog multimeters. One, a typical model with 20,000W/V impedance, gave us a reading of 210V on its 500V range and 160V on its 250V range. The second, nominally 20,000W/V but dropping to 10,000W/V on its highest (300V) scale, gave us readings of just 70V on the 300V scale and 54V on its 100V scale. Table 1 shows the actual voltage readings with various analog meters. These results are further confirmation of the high charging impedance of the Balcar sync circuit and of course, are utterly misleading as an indication of the true voltage. But based on their meter readings alone, most internet posters would say (and do say!) it would safe to use the Balcar flash with a Nikon. However, we know the true voltage is over 320V and most definitely not safe. The conclusion? You simply cannot use a multimeter – analog or digital – to accurately measure voltage in such a high impedance circuit. They load the circuit too much to produce an accurate reading. (Old timers may remember the same problem when trying to measure screen voltages in valve circuits. It was even worse back then when the average meter was just 1000W or 2000W/V!) Beware of JISP By the way, if you spend much time trawling through websites, as we did, you’ll find there is a LOT of serious misinformation on the internet – JISP (“Jumbled Interpretation of Scientific Phenomena”) as a SILICON CHIP sub-editor used to call it. Like this gem: “beware of flash units with trigger (sync) voltages of 300V because these can kill you!” Or “there is no way that (brand X flashgun) trigger voltage can exceed 6V because it is powered by four “AA” batteries and 4 x 1.5 = 6V.” Hmmmm! One chap even put into print “I am a graduate electronics engineer from such-and-such university, so I am competent siliconchip.com.au ~250-300V DC-DC INVERTER BATTERY ~4-10kV DUMP CAPACITOR SYNC XENON FLASH TUBE CT TRIGGER TRANSFORMER Fig.2: a somewhat-simplified diagram of an electronic flash which shows where the sync or trigger voltage comes from. The DC-DC inverter (or power supply in the case of a mains-powered studio flash) provides the high voltage from which the sync voltage is derived. When the flash is triggered, capacitor CT discharges through the trigger transformer, generating a high voltage which in turn ionises the gas in the flashtube. The dump capacitor then discharges through the tube. in what I am doing” and then proceeded to measure sync voltages with a multimeter! But it gets worse . . . So far we’ve been talking about our particular set-up with a Nikon Digital SLR. But other brands, such as Canon, Olympus, etc have rather significantly lower maximum sync voltages – in fact, the two brands mentioned have a maximum of just 6V. And the net is full of tales of woe about fried digital SLR cameras where their owners have unwittingly connected a flash or strobe with a high-voltage sync. If the camera can be repaired (and apparently that’s often a big IF!), the repair bill can be huge: one report I read said that it was virtually as much as buying a new camera body! We’ve singled out Canon and Olympus because they appear to have the lowest sync voltages. But we’ve seen others in the 6-12V range and yet more stating a maximum of 20V. If you own a digital camera, we strongly recommend you look in the instruction manual for its maximum before using any off-camera flash. If the manual doesn’t tell you, call the local distributors and ask them! By the way, there is an international standard for sync voltages – ISO10330 1992-11. It states the sync voltage should be between 3.5V and 24V. Most new flashguns and strobes are made to this standard so a brand new set-up should be fairly safe – unless you happen to be using a DSLR with a 6V limit and a strobe with 20V+ sync! Not just digitals You might think the problem is confined to digital Table 1: the various voltage Impedance readings with a range of analog multimeters. What this 50MW table proves is that you cannot 10MW rely on any meter reading in a 5MW high-impedance circuit. Many have been trapped by this 3MW “little” problem! 2MW siliconchip.com.au It’s a lot easier to troubleshoot (and to change values if required) before you pack it into a tiny “case”. You can then use these components in your final version. The resistor you may need to change is the 270kW, in this pic partially hidden by the 220nF capacitor. Lowering this resistor will lower the sync (trigger) voltage. cameras, with their solid-state flash sync circuitry (in most cases, an open-collector transistor circuit). But you would be wrong. Film cameras, at least until quite recently, almost always had a mechanical flash sync, with a pair of very fine contacts brought together at the appropriate moment to fire the flash once the shutter opened. I mentioned my Minolta film cameras earlier. Despite being over 40 years old, they had done sterling service (in a former life I was a wedding photographer) and I had a very good lens collection to suit them. The main reason I managed to extract such a long life out of them was that every year, each of these went in for service and a good clean-out. The last time I put them in, I mentioned to the technician that one in particular sometimes had unreliable flash firing. The technician returned that camera in a plastic bag in pieces, the bag labelled as being “BER” – beyond economic repair. I was told that the flash sync contacts were essentially missing in action and that it would cost much more than the camera was worth to obtain the spare parts and replace them. The other two cameras were cleaned and repaired but I was told that they too were way beyond reliable service life. Their contacts were still operational – but only just. Having now found that there has been over 300V across those flash contacts ever since I started doing SILICON CHIP photography, I’m not surprised they were pitted and burned. I’m actually surprised they weren’t welded! Incidentally, it was this that convinced us to make the switch to digital at SILICON CHIP. That and the time it took to scan 35mm slides or negatives for use in the magazine! Scale     Voltage (scope) 310V 500V 210V 250V 160V 300V (10kW/V) 70V 100V (20kW/V) 54V Our trigger circuit Fig.3 shows the Safe-T-Flash, a circuit we developed to ensure that the strobe sync voltage presented to the Nikon was absolutely safe. With a minor amendment, it can also be used on cameras with a much lower sync voltage (such as the 6V of Canons April 2008  61 A C106D + FLASH UNIT SYNC – SC 2008 K in shopping centres) tend to charge an arm and a leg for these relatively obscure items, especially if you buy “genuine” (eg, Nikon branded hot shoe adaptor ~$60. Large camera store model? $19.95!). Trust us, the cheaper variety work just as well! 6.8M CAMERA HOT SHOE G – + 1k safe-t-flash 220nF Polarity 270k G A K Fig.3: the circuit could hardly be any simpler – the voltage is limited to safe levels and the SCR fires the flash. This circuit is effectively a switch in series with the sync lead. and Olympuses – or should that be Olympi?). The circuit is simplicity itself. A voltage divider across the sync supply charges a 220nF capacitor to a much lower voltage than the original sync voltage. When the shutter is released, discharges instantly into the gate of an SCR connected across the sync supply. This then almost instantly turns on, shorting out the sync and firing the flash in the normal way. We said almost instantly – we’re talking microseconds here, very much faster than the 1/250th second sync speed of a modern digital camera. So using this circuit will have no effect on exposure times or flash timing. The voltage divider we used (6.8MW and 270kW) gives about 7.5V from a 320V sync supply. These two resistor values can be changed if (a) the strobe/flash you use has a lower sync voltage (most modern ones do) or (b) if your digital camera has a low maximum sync voltage. For example, replacing the 270kW with 180kW will give about 5V with a 320V sync – ideal for Canon and Olympus. If your sync is lower than 300V, you’ll need to select the resistor to suit. The SCR is a “garden variety” type, albeit with a highenough rating to deal with 300V+ sync voltages. We used a C106D, a plastic-pack (TO126) type with a 400V rating. The 1kW resistor from gate to cathode keeps the gate tied low until it receives a “fair dinkum” trigger from the camera. Otherwise, induced voltages on the sometimes-relativelylong sync leads could lead to false triggering. Speaking of sync leads, you’re going to need one – either a new one or perhaps (if you’re like me!) you’ll find a couple of pensioned-off ones in the bottom of your camera bag or drawer! And with most DSLRs, you’ll also need a hot-shoe-toPC-terminal adaptor. Both of these are relatively easy to obtain at camera stores. But be careful – some stores (particularly “consumer” camera chain stores Many DSLRs do not have an “X” (sync) connector but do have provision for a hotshoe adaptor, such as this one shown with sync lead attached. 62  Silicon Chip There are two voltage polarities to check. First is the sync voltage. From our Balcar flash, the tip of the 6.5mm plug is negative and the body positive – just the opposite of what might be expected (sync leads sold for Balcar flash units take this into account). Make sure you construct the circuit with the polarity that suits your strobe/flash. The second is the polarity of the camera flash trigger. It makes sense to connect the more positive side (even if you’re only measuring millivolts, which is quite possible) to the voltage divider/capacitor side and the negative to the 1kW resistor/SCR cathode side. Before construction It’s much easier to make any changes to the circuit (which you might have to do) before the components are packed into a small space. So the first thing to do is to “tack together” your SAFE-T-FLASH without trying to miniaturise it, to ensure it is going to work with your particular strobe/ flash and camera. When finished and checked, connect your strobe/flash (only) at this stage, turn it on and measure the voltage across the lower (in our case 270kW) resistor. Depending on the voltage divider you have chosen and the sync voltage of your flash, it should be quite low – certainly no more than 20V or so but it could be just a few volts if you have chosen a lower value resistor to suit your system or if your strobe has a lower voltage sync. If all appears well, short out the sync terminals in your circuit. The flash should fire immediately. Repeat this several times just to make sure the flash doesn’t misfire. Now connect the two wires in the sync lead from your camera to the two sync terminals – as we mentioned before, the more positive wire goes to the voltage divider/capacitor. Fire off a shot or two to ensure that the flash still works. If it does, you’re ready to build the final version. If it doesn’t (or if the previous test didn’t work), you either have a mistake to correct or perhaps a resistor to change to achieve the required voltage. Parts List – SAFE-T-FLASH 1 connector to suit your flashgun or strobe (prototype used a Jaycar PP-0176 6.5mm stereo plug) 1 sync lead to suit your camera with appropriate PC male (sync) plug 1 hotshoe-to-female-PC converter, if required 1 C106D 400V SCR (or equivalent) 1 220nF 60V monolithic capacitor Resistors (0.25W or 0.5W metal film) 1 6.8MW 1 270kW 1 1kW Spaghetti insulation, insulation tape, potting compound, etc, as required. siliconchip.com.au The SAFE-T-FLASH built onto the 6.5mm plug. We provided insulation wherever there was a risk of shorting (including the red insulation tape covering the body). The 220nF capacitor is under the SCR. Again, refer to camera and strobe/flash manufacturer’s websites and/or distributors, agents, repair shops, etc for more detailed info. However, remember our warning earlier about misinformation on some websites! Construction We built our SAFE-T-FLASH inside a 6.5mm plug because these are the sync connectors used on our Balcar studio flash. Each manufacturer has their own “standard” and it’s quite possible (in fact, probable) that this option will not be available to you because we don’t know of too many manufacturers who use the 6.5mm plug. Other ideas are building it inside a “hot shoe” adaptor, or perhaps simply as a “lump in the sync cable” – eg, insulated with heatshrink tubing. Another possibility is one that I used many years ago when making an optical slave flash trigger for a Metz flashgun, which (along with quite a few other flashguns and strobes) uses a 2-pin (US-style 110V) sync plug. Mount the components on the back of the plug and “pot” them in epoxy adhesive – once you’ve confirmed it works properly, of course. 5-Minute Araldite makes a great potting compound if you make some type of container/mould to hold it while it is still runny. But we’ll leave that part up to you and your particular flash – our photos show how ours was constructed inside the 6.5mm plug. We used a right-angle stereo plug (Jaycar PP-0176) not because we needed stereo – in fact, exactly the opposite – but because this style plug has plenty of room inside and the “lid” is plastic. The mono version doesn’t have much room at all and is also all-metal construction, which could be a problem with shorts! If using the 6.5mm stereo plug, you will need to connect the ring and body together to convert it back into a mono plug – and hope that the point of contact inside the socket doesn’t line up exactly with the insulator between the two! Yes, it is unlikely (it didn’t on ours) but you never know when Murphy is going to strike . . . We simply soldered the appropriate tag down onto the plug body. The surface had to be scratched a little to remove siliconchip.com.au Here’s another view, this time from the underside. Note that this is a stereo plug – the ring (the bit between the two black insulating disks) must be connected to the plug body. the plating to get the solder to take. This then became the main positive connection point. As there are only three resistors, a capacitor and an SCR inside the plug (and also due to the fact that many constructors won’t be using the 6.5mm plug anyway) we haven’t shown any form of wiring diagram. The close-up photos should give you all the info you need. Just take care that no leads can short to any others or the plug cover, remembering that when the cover is screwed on some compression is possible. We covered any leads which might short with insulation (actually removed from other wires and slid onto the leads). You will note that we also covered the inside of the metal plug body with insulation tape – just in case. Also note that the back of the SCR has a metal face which is connected to the anode. Make sure that nothing can short to this (we used it upside-down so that the anode was on top, against the plastic lid of the 6.5mm plug). As we have already tested the “large” version of the circuit and made any component adjustments needed, your miniature version should work perfectly if you haven’t made any mistakes or allowed components to short. Remember that when you put the back of the plug on, it may compress the components so that they do short – again, use spaghetti insulation if there is any danger of SC this happening. Finally, the finished SAFE-T-FLASH with the “case” screwed onto the 6.5mm plug. The opposite end of the cable goes to the PC (sync) connector. April 2008  63