The safe-sync schematic shows the electronic parts and connections needed to allow a lower-voltage digital camera to trigger a higher-voltage studio strobe or flash. This protects the digital camera and permits it to trip external flash units from other manufacturers.
My circuit allows the safe-sync adapter to operate using either the external flash or an external battery as a power source. For the purpose of understanding the circuit, it doesn’t really matter which power source is selected. So, in these next examples, I’ve grayed out the battery portion of the circuit for simplicity.
When the camera wants to trigger the flash, it connects the flash wires at the camera hot shoe. My circuit has a pushbutton that performs that same function, for testing purposes. So, in these next examples, I’ve grayed out the camera hot shoe portion of the circuit for simplicity.
When the circuit is first powered up, and after each time the flash is triggered, the circuit goes through a charging phase to prepare for the next trigger.
Safe-sync charge path shown on a schematic.
A small amount of power drains from the flash, limited by the resistors (R4 and R5), and current flows only in the direction permitted by diodes D1 and D4. This charges up capacitor C1 to just under 5.6 V, which is the limit set by Zener diode D2. Capacitor C1 can’t quite hit the 5.6 V limit, because diode D1 uses a couple of tenths of a volt.
You can see that current isn’t allowed to pass through SW1, because it isn’t pressed right now. It isn’t allowed through D2, because that would be against the direction that the diode is pointing. And it isn’t allowed through U1, because a triac will only turn on when signaled to do so by the gate (G) pin.
Why doesn’t current flow through R1 and U1, causing it to trigger? Well, D1 is a lower-resistance, lower-voltage path.
Eventually, C1 fills up. At that point, it is ready to trigger the flash.
To be technically accurate, when the capacitor is fully charged, then C1 + D1 = 5.6V, which is the point at which the Zener diode is designed to conduct (breakdown or Zener voltage). At that voltage, the Zener diode allows just enough current to flow through it to keep the voltage limited. This works just like the overflow drain on a bathroom sink. As soon as the water level hits the upper limit, it drains off.
Now that the capacitor has charged, look at what happens as the hot shoe wires are connected, when the photographer presses the camera shutter button or the circuit’s test button.
Safe-sync discharge path shown on a schematic.
The capacitor acts like a tiny battery. With the hot shoe wires touching (electrically speaking), the capacitor now has a path to release its energy.
Current flows through SW1, through the gate of U1, and through R1 to complete the circuit. The voltage level and amount of current passing through the triac’s gate causes it to turn on. The triac electrically connects the wires of the flash, allowing it to brighten your photo.
If R1 didn’t exist, it would be like short-circuiting the capacitor, which would release all of its energy instantaneously, if not for the slight resistance of the wires and the capacitor itself. Not only might this damage the capacitor, but the large amperage might damage the camera and would violate the 1 amp limit for the gate of U1. But, equally important, U1 officially needs to have the gate enabled for a certain length of time (2.5 µs) in order to trigger. So, R1 provides both protection and a slowdown in the discharge rate to lengthen the time the trigger signal is provided to the triac.
If D1 didn’t exist (and the bottom of C1 was directly connected to -), then the capacitor would simply discharge into itself without passing through the gate of the triac. D1 allows C1 to charge without flowing through the triac, but forces it to flow through the triac on discharge. Thus, the triac stays off during charging, but turns on when the button is pressed.
As long as the camera hot shoe or test pushbutton (SW1) are pressed, then power from the flash or battery will flow, which wastes power and doesn’t charge C1. Since this is usually a short amount of time, it isn’t a big deal. But, it is worth noting that R3, R4, and R5 are limiting the current from the power source, which protects against too much power regardless of how long the button is pressed.
Unfortunately, R3, R4, and R5 also limit the speed at which C1 can recharge, since those resistors limit the current from the power source. If the camera or pushbutton attempt to trigger the flash before C1 is recharged, then C1 may not have enough energy to trigger the triac (U1). No harm -- but charging starts all over again. So, R3, R4, and R5, along with the capacity of C1, determine how fast the safe-sync can cycle the flash. In most cases, the flash itself will limit the speed of the cycling.
These events are visualized on an oscilloscope on the next page.