What is the PC port amperage trigger limit for a Canon DLSR?

Question

I want to use my Graflite flash bulb flash on my Canon 5D Mk III.

1. I know that modern DLSRs have a limit to the voltage and current that a flash can use when triggering, before it damages the camera.
2. I know that most similar questions concern using older electronic strobes / flashes which can have too high a trigger voltage. My Canon for example can handle a maximum of 6 volts trigger voltage. That info is freely available.
3. Camera flashes that use actual flash bulbs however, mostly have the opposite problem. In the case of my Graflite flash, the voltage is only 3 volts, BUT the triggering amperage / current is much higher than electronic flashes.
4. I have built a circuit (No the Wein Safe-Sync does not work for this situation) to isolate the Graflite triggering current from the Canon's PC port.

My problem is that I can find nowhere, what the Canon 5D Mk III current limit in milliamps. I have called Canon more than once and they say that information is proprietary, and refuse to provide it.

So here is the question: I need to know if someone can tell me what the maximum milliamps a Canon DLSR can handle during the flash triggering sequence from the PC port.

I have my triggering circuit (That part that will connect into the Canon PC port) down to 4.85V & 70mA. But is that low enough?

I assume that someone with an electronic oscilloscope could connect a later model Canon flash to that scope and see just what triggering current is output from the flash. So, if anyone has such a scope and a Canon flash, I would really like to have that answer also.

Answer 1

The Canon engineers are probably correct in saying that the PC port is a low-voltage port. I would not recommend putting much current at all through that port.

There are a couple of ways of solving your problem:

• Add a single transistor on the input to your flash. Then, your flash trigger power will become negligible, because the transistor will be triggered through the camera, and all the high-current flow will be within the device itself.

• Use the hotshoe. The center pin of the hotshoe can be used to trigger most flashes. I can't say with certainty what Canon uses for the hotshoe mount in the 5D Mark III, either, but according to a discussion thread on Electronics point, the XT uses a 2SK2715 MOSFET for the flash trigger, and it's probably a good bet that they use the same part on all their cameras. The 2SK2715 is made by ROHM Semiconductor, and according to the spec sheet, it is rated at 500V at up to 2A. Canon downrated their maximum spec to 250V, either as a hedge against slightly substandard parts or to allow them to source other equivalent parts that may have looser tolerances. And, of course, a part designed to handle up to 1 kilowatt triggering power laughs at your 0.3395 Watt triggering current requirements, and any even-semi-plausible replacement MOSFET that handles such high voltages will have similar specs.

If you're worried about it, use one of those approaches.

Answer 2

My Canon 50D says the PC Sync port is good for 250V, any polarity.

If the current of the flash unit sync input does not fall below about 3-5 mA the output will remain asserted forever. Sounds like it is driven by a bidirectional thyristor, triac or some such thing which has a recovery threshold.

The electrical characteristic is different for + & - polarities, even though they say either is OK. It pulls down to about 650 mV for + polarity, and about -140 mV for - polarity.

The hot shoe sync seems more robust for pulling heavier loads and pulling closer to zero V. It also recovers and turns off independently of whatever the load current is. The manual says don't use a high voltage flash on the hot shoe since it may not fire, but no warning about any potential damage if you do.

No indication of allowed current is given for the hot shoe either. Seemed to work fine up to about 34 mA, but I didn't try to go beyond that. I have an MT-24EX flash I could check the current on. That would reveal a lower limit of allowable current, but not anything about what the max might be.

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Here is the measured hot shoe current for my MT-24EX at 2mA/div and 50mA/div:

The polarity should be understood to be current going into the camera hot shoe (2mA steady state, 120mA x 1usec initial transient). I should have flipped the orientation of my current probe.

Answer 3

This is not an answer you might want, call it comments, but I needed more bytes.

I always wondered if Canon's 6V sync limit was real? No designer would allow that vulnerability. The same paragraph in the manuals also said "Buy only Canon flashes", but I think it bit them instead.

A Wein SafeSync is just an optical isolator, two separate circuits, input rated to 400 volts that provides safe 6 volt out. I don't know how it might affect flash bulbs, or flash bulbs it.

Speedlights are 4 AA cells, 6 volts. This sync voltage goes to point I'll call "PC" through a high value resistor to limit current to a harmless value. PC is the PC connector, and also the flashes trigger circuit connects internally to PC. The camera shutter sync simply shorts PC to ground (mechanical switch in old days, SCR or similar today). This short drops open 6V at PC to 0 volts (harmless through the resistor). The trigger circuit watches PC sync voltage and triggers flash when it drops to zero. This 6V trigger current is NOT the flash voltage, which is a separate 300 or 500 volt circuit at the flash tube.

Some early electronic flash was different, with dangerous sync voltage, exceeding 200 volts, dangerous to electronics today (the market for Safe-Sync). The old BC flash just charged a capacitor to provide more current for a flash bulb, often with a 22 volt battery.

I think the application simply needs a modern flash.

How is that going to work? Modern cameras have no M or F sync for flash bulbs, which must be triggered several milliseconds early so they can become bright before the shutter opens. If it is an old camera with M or F sync, then no electronics, and the current won't be any issue. But flash bulbs have no use for modern X sync for electronic flash, which is all modern DSLR offer.

You might use a real slow shutter to wait open for the bulb to light up. M sync at 25 milliseconds is an extra 1/40 second waiting for the bulb. That's what M sync is, a 25 millisecond delay. F sync was about 10 or 15 milliseconds. M medium delay, F fast delay, for faster bulbs. But modern electronic X sync is instantaneous. Does your planning include that?