Digitally Triggered Xenon Flash

I’m thinking of taking strobe pictures again, but the results of the LED strobe tach experiment showed that I need many more LEDs, much brighter LEDs, or entirely different technology. The Big Box o’ Xenon Tubes disgorged some surplus camera flash units that seemed amenable to hackage.

The canonical digital trigger uses an optocoupled triac, so I soldered a MOC3022, taken from a random assortment of various optocouplers, across the trigger leads:

Xenon flash - MOC3022 triac
Xenon flash – MOC3022 triac

Alas, that didn’t trigger the flash reliably. It may well be that the triac’s leakage current drains the small trigger capacitor below the voltage required to produce a suitable trigger pulse, but I was unwilling to poke around in the thing.

The clip leads go off to a DVM set to the 600 VDC range, which is, I think, the first time the range switch has ever lingered in that position. The 250 µF 330 V capacitor charges to about 300 V, depending on the mojo of the single AA cell powering it, and discharges to about 50 V after the arc quenches. The neon bulb lights when the capacitor goes above 280 V.

The reed relay assortment emitted an ancient Clare 1A05C relay with, as nearly as I could make out from the fragmentary datasheets available nowadays, barely adequate specs:

Xenon flash - PRME 1A05C relay
Xenon flash – PRME 1A05C relay

Unfortunately (and as I rather expected), the first shot welded the contacts together.

A somewhat larger Axicom (aka Tyco) V23079A1011B301 (I’m not making that up) relay had better specs: 220 VDC / 250 VAC / 2 A contacts. The DC rating isn’t relevant here, because the contacts will break only 50 V after the flash, and the AC rating says it’ll withstand well over 350 V.

As with the other gadgets, a blob of hot melt glue holds it in place:

Xenon flash - Axicom V23079 A1011-B301 relay
Xenon flash – Axicom V23079 A1011-B301 relay

That worked wonderfully well:

Xenon 280 V 250 uF
Xenon 280 V 250 uF

The upper trace comes from a PIN-10AP photodiode in the LED measurement fixture, minus the black cap holding the LED. The photodiode connects directly to the oscilloscope input, so we’re seeing its photovoltaic response rather than the photocurrent, but that’s good enough for now. The pulse is about 1.5 ms long at the 50% level (that’s 1 EV down from the peak) and the tail is pretty much gone by 3 ms.

The 3 ms delay after applying voltage to the coil (lower trace, showing what happens when you use a clip lead as a switch) is well within the 4 ms spec in the datasheet. The release time isn’t relevant, as the capacitor has discharged to 50 V and nothing exciting happens when the contacts open.

Charging the stock 250 µF cap to 280 V stores 10 J = 10 W·s:

10 J = (1/2) (250×10-6) (2802)

Discharged to 50 V, the cap has only 0.3 J left, so most of the energy goes into the arc.

Swapping a 1 µF 600 V film capacitor for the electrolytic cap narrows the pulse:

Xenon 350 V 1 uF
Xenon 350 V 1 uF

A 1 µF cap should reduce the stored energy by a factor of 250 to 0.4 J, but the booster charged it to 350 V = 0.6 J:

0.6 J = (1/2) (1×10-6) (3502)

The test setup, a term that barely applies in this situation, isn’t stable enough to say anything about the relative light output, but it’s certainly not an order of magnitude worse than the 10 J shot (some data and curves from an OEM). The pulse width is maybe 100 µs, just about what I used with the LEDs, but whether the lamp produces enough illumination remains to be seen; it should be brighter than the LEDs.

The boost circuit requires about ten seconds to recharge the 250 µF cap and maybe 250 ms for the 1 µF cap. The Axicom relay can operate at 50 Hz at no load, which definitely won’t constrain the flash rate. The trigger energy at the contacts should be about the same for either flash capacitor, because it comes from a much smaller capacitor charged to the same voltage; buzzing away at a high rep rate will chew up the contacts fairly quickly.