Safe Sync to Adapt Digital Camera to Higher Voltage Flash

This circuit adds a datasheet-compliant trigger signal, reversed polarity protection, optional test button, and optional battery operation. U1 L601E3 or MAC97A8 triac. 400 V, 1 A. When U1 is enabled, the + and - wires of the flash are electrically connected, just as though the camera hot shoe had done so. A triac is a semiconductor switch, similar to a bipolar
Safe Sync to Adapt Digital Camera to Higher Voltage Flash - schematic

or MOSFET transistor, except that it won`t conduct in either direction unless turned on at the gate, and it won`t turn off until the current across it subsides. This is beneficial for a flash, since it allows the high voltage to fully dissipate, even if the camera doesn`t assert the trigger signal long enough. R1 18 © resistor. This limits the current passing through the gate of U1; to keep the current below 1 amp and to extend the amount of time it remains triggered in order to meet the 2. 5 µs minimum. Both of these limits are specified by the L601E3 datasheet. Furthermore, this limits the amount of current across the camera hot shoe during discharge. D1 1N4148 or 1N914 diode. This is a one-way path that allows capacitor C1 to charge, but won`t allow it to discharge except through the gate of the triac. (Pictures of the current flow are presented on the next page. ) The type of diode selected has a relatively low voltage drop, in comparison to the 1N4004 diodes, to allow C1 to charge almost completely. D2 5. 6 V Zener diode. This protects the camera by limiting the maximum voltage across the camera shoe to no more than 5. 6 V. A higher-voltage Zener diode runs the risk of damage to the camera. A lower-voltage Zener diode reduces the maximum charge voltage of capacitor C1, which may prevent triac U1 from triggering. R4 and R5 Two 4. 7 M © resistors. These limit the amount of current that can flow from the flash, reducing...

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