Sceadwian
Banned
Was browsing around today, and found a DIY page for high speed photography. I've always known the flash duration was the limiting factor for this type of photography, but this page has some actual oscilloscope images of the flash duration as measured from a photo diode from a decentflash and I was trying to understand how these waveforms are related to the circuitry.
The Sigma EF-500 flash - Light duration vs. output power | DIYPhotography.net
Starting with the slowest one it seems pretty straight forward, the initial rising lump is the start of the arc in the flash tube which peaks as the arc is fully formed and becomes fully conductive, and slowly sloping drop is consistent with a capacitor discharge curve, so regardless of the power level the flash is charged to the same voltage, but with shorter flash durations it seems that there's a timed cutoff (quiet abrupt) of the discharge. The 1/128th power level doesn't even get to the point where the arc is fully formed.
What in this circuit are they using to so abruptly stop the discharge? On the highest speed one's there seems to be a slight knee but this looks like parasitic capacitance. I'm wondering because the voltage of a typical flash is a few hundred volts from a relatively large capacitor and the cutoffs I see in these scope shots are nearly like hitting a wall. I can't even rule out that the scope shots knee curve for rise/fall times on the highest speed settings are from the photo-diodes junction charge.
The Sigma EF-500 flash - Light duration vs. output power | DIYPhotography.net
Starting with the slowest one it seems pretty straight forward, the initial rising lump is the start of the arc in the flash tube which peaks as the arc is fully formed and becomes fully conductive, and slowly sloping drop is consistent with a capacitor discharge curve, so regardless of the power level the flash is charged to the same voltage, but with shorter flash durations it seems that there's a timed cutoff (quiet abrupt) of the discharge. The 1/128th power level doesn't even get to the point where the arc is fully formed.
What in this circuit are they using to so abruptly stop the discharge? On the highest speed one's there seems to be a slight knee but this looks like parasitic capacitance. I'm wondering because the voltage of a typical flash is a few hundred volts from a relatively large capacitor and the cutoffs I see in these scope shots are nearly like hitting a wall. I can't even rule out that the scope shots knee curve for rise/fall times on the highest speed settings are from the photo-diodes junction charge.
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