Previously, we’ve seen the safe sync’s current flow path on a schematic. We’ve also discussed how C1 stores the energy to trigger the flash, and that the time to charge C1 depends on the values of C1 and the power source resistors. Let’s see that from an oscilloscope’s point of view.
In this circuit, the triac is the digital switch that connects the wires of the flash together. This causes the flash to release its energy and light up the subject.
The manufacturer of a triac documents the conditions necessary to trigger it. In the case of the L601E3 1 amp sensitive triac, the gate threshold is a 1.3 V, 3 mA, for 2.5 µs. Those are all worst-case maximums; in practice the triac will trigger at a lower voltage, lower current, and shorter length of time. We design the circuit to exceed the maximum values to guarantee that it triggers.
The circuit permits a power source as low as 3 V. In the worst case, capacitor C1 will likely charge up to a voltage of about:
Lowest voltage for C1 = lowest battery - D1 diode drop for small currents
Lowest voltage for C1 = 3.0 V - 0.2 V = 2.8 V
That’s more than 1.3 V (triac gate threshold). So, we’re good there. Now let’s calculate current:
Starting current for C1 = lowest voltage for C1 ÷ resistor R1
Starting current for C1 = 2.8 V ÷ 18 Ω = 0.157 A = 157 mA
Fine. So, we have plenty of current. It isn’t really going to be that much, because there is a voltage drop across the triac gate. Also, the current and voltage will decline rapidly as C1 discharges.
The time it takes to discharge a capacitor to 36.8% of its voltage is defined as the RC time constant.
time in seconds = R1 resistance in ohms × C1 capacitance in farads
time in seconds = 18 Ω × 0.00000068 F = 0.00001224 s = 12.24 µs
That is much more than the 2.5 µs needed. It is reasonable to assume that capacitor will be above 1.3 V (gate threshold) or 46% of the capacitor voltage for at least 2.5 µS. (We'll see for sure in the images below.)
Without changing R3, R4, or R5, if you increase R1 and decrease C1, you can decrease the charge time while keeping the discharge time steady. But, that will decrease the trigger current since C1 will have less energy. So, you need to be careful when substituting values, as you need to balance multiple requirements.
The green top line of the oscilloscope trace shows the flash voltage. The black bottom line shows the voltage at the gate of the triac (U1). The left image is the circuit powered by the flash. The right image is the circuit powered by a 5 V power source acting as the battery.
Triac triggering from two power sources: flash (left), 5 V battery (right).
The black line starts near 0 V and drops down, because the capacitor power is flowing backwards through the triac gate, which is a negative voltage relatively speaking. This is a perfectly acceptable way to trigger a triac.
Violet represents the region where we must exceed the triac trigger parameters of 1.3 volts and 2.5 microseconds. As you can see, the gate voltage is in the violet region in both cases, although the flash-based circuit is borderline. That particular flash was old and provided a fairly weak charge voltage to begin with. In fact, the safe sync circuit used in these oscilloscope traces had a 10 ohm resistor for R1. These results caused me to switch to an 18 ohm resistor to keep the voltage from dropping so quickly.
Now we will zoom way out to a time period 100 times longer.
Oscilloscope traces of flash discharge.
Both of the black lines follow approximately the same curve and period of time, although there are minor differences at the beginning. Instead, what is interesting is that the green line representing the flash trigger wires shows that the flash was unable to discharge as rapidly when it was also the source of power for the safe-sync. Again, this was an old cheap flash.
In both cases, the flash hasn’t even started recharging for the length of the trace (9 * 0.2 ms = 1.8 ms). Not much of a surprise since the light from an electronic flash typically lasts between 1 and 5 ms. (There are high-speed flashes and high-speed sync controllers for faster shutter speeds and for focal plane shutters.)
The lesson from this oscilloscope trace is: you want to have the option of a battery power source for maximum compatibility with accessories, particularly old ones. On the other hand, my brother’s studio flash worked fine as a power source, which is why it is also nice to have the choice to not worry about another battery.
Finally, let’s look at an example of the circuit implemented on a breadboard.