Christopher, it is not the collector current that is causing the delay. It's the current on the transistor on the other side of isolation. That said, I did oversimplify the problem before. I don't directly drive the collector. I use a buffer chip to prevent too much current being drawn from the micro's IO lines. The buffer chip is fast and not a source of the delay.
Wraper, these chips do seem a little better. The 6N136 says it has a propagation delay of around 1 us. This is the latency from when it goes high on the collector until the switch opens on the isolated side, right? The H11L1M says it has a rise time of 0.1 us and a turn on time of 1 us. Could you clarify what those times are exactly to me? I also only have 2 lines on the flash side of the circuit. Both these chips seem to expect a vcc. So let's say the flash has a 4V and a isolated gnd. How would I hook up Vcc, Vb, Vo, and Gnd for the 6n136? The down side of these chips are they are more expensive and the H11L1M uses 6 pins for a single channel (not sure if it will fit on my circuit board.
These optoisolotor chips seem to allow large currents on the cathode side to drive the LED. I've tried smaller currents and they seem to work just as fast even at 2 mA instead of the max 30 mA currents. I imagine they just list the max current the LED can handle. Does anyone know a general rule on how much current you should drive these devices with?
Hero999, I don't think a pulse transformer would work. I'm not using AC and the switch needs to stay on for long periods (in some cases many seconds).