The Technicalities
Caution :

Do not attempt to build any of the circuits shown in the following diagrams. They and the descriptions are intended only to show the priciples of operation, and for clarity they omit many of the details which would be needed for a practical circuit.
Also be warned that circuits involving flash tubes contain lethal high voltages which remain even after the batteries or any other power sources are removed. Only a qualified technician should work on flashgun internals.


Basic

The tube of a flashgun is lit by a current passing briefly through the xenon gas which the glass tube contains. A voltage of at least about 300 volts is required for this, and in addition the gas first needs to be ionised. Once the current is flowing the gas remains ionised, but the ionisation needs to be kick-started. This is done by a momentary application of several thousand volts to an external electrode on the tube.

It is possible to supply the ~300 volts from an external battery pack, but most flashguns use low voltage internal batteries, typically of 3, 6 or 9 volts, even if an external supply is also an option. The battery voltage, which is of course DC, is raised by first passing it to an inverter which turns it into AC, then to a transformer which raises the voltage to a high value but still AC, and it is then rectified to the high DC voltage required. The alternating stage is needed because transformers do not work on DC, and it is the inverter and transformer vibrating at the AC frequency which produce the characteristic whining sound of a flashgun charging.


Fig 1 shows a basic flashgun circuit. The 6 volt internal battery supplies, via the unit's on-off switch SW1, 6 volt DC to 300 volt DC conversion circuit represented here by a box. The conversion circuit involves the inverter-transformer-rectifier sequence mentioned above and is not described further here because it is a standard arrangement not specific to flashguns.

The 300 volts is then supplied via the resistor R1 and coil L1 to one end of the xenon flash tube FT1, the other end of the tube being grounded, but the tube does not fire yet because it is not ionised yet. The ~300 volt supply also fills the large electrolytic capacitor C3 which stores electrical energy as a charge. In this article I shall also refer to it as the flash capacitor and it is the size of this which principally determines the maximum light energy and Guide Number of the flashgun. The purpose of R1 is to limit the charging current to C3 which could otherwise overheat the 6 to 300 volt converter and other components, especially in the early stage of charging.

At the same time the much smaller capacitor C1 is charged via R2, also to 300 volts. The circuit will reach a steady state with both capacitors at full charge and the unit stops whining. The other line from the positive end of C1, through the transformer T1, leads to dead ends for now.

Once the charge in C3 reaches a certain voltage, a neon light LT1 will turn on as an indication that the charge is sufficient for a shot, the "Flash Ready" signal that flashguns invariably have. It does not necessarily mean that C3 has reached its maximum possible charge. In more modern units the neon light is replaced by an LED in series with a zener diode of high voltage rating to drop most of the voltage.

Notice that the voltage reaching the hotshoe foot is 300 volts in this circuit, and could be higher in other examples : this is the trigger voltage that could upset a modern camera.

There will be a microswitch switch in the camera's shutter mechanism (or a solid state equivalent) connecting to the hotshoe foot shown in the diagram, or by a PC (Prontor-Compur) socket. When the shutter fires the two lines through the hot shoe are shorted together briefly, and C1 discharges to ground via the primary coil of the transformer T1. The high value resistor R2 prevents the the main capacitor C3 from discharging any significant amount via this route. SW2 is a push-button test switch that most flashguns have, and it functions in the same way as a camera shutter switch.

T1 is a high gain voltage step-up transformer and it works because the discharge current through it is effectively part of an alternating current cycle; its secondary coil produces the high voltage pulse needed to trigger the xenon tube, it being connected to its external electrode. This triggering current is extremely small and extremely brief, less than 1/10000 second, which is all that is needed to ionise the xenon gas.

Once ionised, the xenon tube discharges current from C3, producing light, until the voltage of C3 falls below a threshold not enough to keep it going even through ionised gas, whereupon the light will cease abruptly. This all occurs in a very brief time, typically between 1/1000 and 1/10000 of a second.

The purpose of the inductive coil L1 is to moderate the otherwise very high current from C3 to the flash tube in the first moments of the flash. Basically, inductive coils oppose any change of current. Conversely, when the current in the flash tube subsequently stops, L1 will produce an unwanted momentary forward voltage surge in an attempt to keep the current going; this is harmlessly shorted out by the backward facing D1.

Fig 1 and its description above are appropriate for very simple early electronic flashguns of around 1960 and later cheap units, such as the built-in flash of point-and-shoot cameras. The fixed full "power" light output was accepted because photographers were previously used to fixed output flash bulbs, and the electronic flashguns were less powerful than flash bulbs anyway.

From around 1975 many cameras with a hot shoe began to feature a "Flash Ready" indicating light in the viewfinder, and if they had an electronic shutter this could also set it to its flash sync speed. This required an additional contact in the hotshoe and at this point, unfortunately, different camera brands adopted different standards with regard to the position of the contact and the nature of the signal. This is covered in the section on hotshoe layouts.


Auto Control

By around 1970 flashguns were acquiring "Auto" control of their light output, meaning that a photosensor on the unit meters the flash lighting of the subject and terminates it when enough has been received. "Auto" can mean many things but this was what it meant in this context, and the word was usually proudly engraved somewhere on the flashgun body if it had the capability. However the flash unit needs to "know" what the camera lens aperture and film speed are, and in practice it advises the user what aperture to use by means of a scale, given the film speed. Most units give the user a choice of two or three apertures. Auto mode frees the user from estimating distance and from using the Flash Formula (with or without help from a table or slide rule) and was a huge advance when it was introduced. The option to fire the flash without photosensor control is usually retained (and has been to the present day) and from that time this option has been referred to as "Manual" mode.

Fig 2 shows an early type of the additional circuitry introduced by the Auto mode, which is an extension of the circuit in Fig 1. A xenon flash tube cannot be turned off by a direct electrical signal in the way it can be turned on, so the only way to stop it is to interrupt its electricity supply. When semiconductor technology was not very advanced, this was done by diverting the current into a second "dark" flash tube, FT2 in Fig 2, within the body of the flashgun so that the light from it did not escape.

This second flash tube, known as the quench tube, requires a trigger arrangement similar to that of the main flash tube, but the signal to trigger the quench tube comes from the photosensor part of the circuit. The quench tube is wired directly across the main tube and once triggered the electrical charge from the main capacitor diverts into it preferentially, because it has a shorter distance between its electrodes.


The photosensor part of the circuit, to the right in Fig 2, shows the principle. The photosensor PD1, a photodiode in this case, is supplied with 6 volts from the battery and passes current to a small capacitor C4 in proportion to the light falling on it, so the capacitor voltage rises in that proportion summated over time. R10 has a relatively high resistance that has little influence on this process, but is there to discharge C4 between shots. This voltage is compared with a reference voltage (at the top of R9) representing the amount of light required.

The reference voltage is set by the potential divider formed by R11 and R9. Switch SW3 allows R12 or R13 to be chosen instead of R11, creating different reference voltages for different Auto levels.

The voltage on C4 and the reference voltage are compared in the operational amplifier (op amp) OA1. When the former voltage exceeds the latter, the op amp sends a positive voltage to the gate of the silicon controlled rectifier SCR1. This "fires" SCR1, the equivalent to being turned on like a switch, grounding the coils of transformer T2, which creates the very high trigger voltage for the quench tube FT2 in the same way as the flash tube FT1 was triggered earlier.

Once the quench tube is fired it preferentially takes current away from the flash tube, which therefore extinguishes and de-ionises, and drains down the remaining charge in the flash capacitor C3 until it falls below a threshhold level and it too extinguishes and de-ionises. The flashgun then re-charges for the next shot.

SCRs are like swiches that are turned on by a low voltage being applied to their gate terminal. They have the characteristic that once turned on they cannot be turned off except by removing the main current supply, and in this respect they are a solid-state equivalent of a xenon tube, with the advantage that the voltage required to trigger them is low compared with the thousands of volts needed to trigger a xenon tube, as well as being cheaper and more robust. It might be asked why the quench xenon tube cannot be entirely replaced by an SCR, but at that time an SCR large enough to dump all the energy of the main capacitor would have been expensive, and large. That would be solved by later energy-saving designs.

Most Auto flashguns have an indicating light to show that the lighting level was adequate. The light is arranged to come on if and only if the flash quench signal is generated. In some older designs, a small window in the body allowed the user to see the brief flash from the quench tube itself, which, although it could not be seen by a user with their eye at the viewfinder, it could be seen when making a test button firing beforehand.

Later units use an LED to confirm adequate flash, glowing for a couple seconds after the shot. A possible arrangement is shown within Fig 2. Capacitor C6 is charged up between shots via R21. When the quench SCR1 fires, in addition to grounding T1 it allows C6 to discharge to ground through LED1, R22 and D2, LED1 giving the "Flash OK" indication. D2 protects it from the 300 volts in T2 prior to the quench. Typically, the LED fades as the charge in C6 runs down, after which the current through R21, LED1, R22 and D2 is below the holding current of SCR1.

Because they use up the whole of the charge every shot, these Auto but non-energy saving designs are characterised by taking as long to re-charge after a shot with a low light energy requirement (such as for a close subject) as they do after a shot with a higher energy requirement. Apart from slowing down the taking shots in a situation requiring rapid fire, they would also consume batteries faster.

Fig 2 above showed a method of changing Auto levels electronically, by switching between different resistor values to create different reference voltages. However there was a simpler method used on cheaper flashguns, which was to use a single reference voltage and use a slider in front of the photocell which allowed different amounts of light to pass at its different positions, typically two or three. This was usually by it having holes of different diameter. The less light the slider let through, the longer the flash would shine for; less light reaching the photosensor was appropriate for a smaller lens aperture allowing less light to reach the film. The slider typically also had a Manual position which simply blanked off light completely, forcing the unit to output its full light energy in an attempt to satisfy the sensor circuit. The slider was simple, cheap and reliable but there was a physical limit to the number of levels it could offer, partly because of space limitation and partly because it could push the photosensor out of its useful physical range of light intensity.



Manual Control

In a flashgun with Auto control, an additional single full power manual level is easily arranged by either disconnecting the photosensor with a switch or by blanking it off with a slider as has already been mentioned. Such an arrangement was very common on cheaper flashguns.

However, if more than the single full output manual flash level is offered, a step up in complexity is required. The photosensor related circuity will be switched out entirely and replaced by some form of timing circuit, possibly involving a capacitor in series with a choice of resistors (and possibly a choice of other capacitors). In the case of some Vivitar units of 1970's design the photosensor and its circuit were physically pulled out of the unit as a module and replaced by a manual module sold as an optional extra.

Typically multiple manual levels are a range of reductions from full in intervals of one photographic stop, ie full, 1/2, 1/4 etc. Some of the most powerful flashguns go down to 1/128 of full energy, a seven stop range. Contrary to expectation perhaps, manual levels can be more demanding in design than auto modes because the latter are self-regulating to an extent; for example the auto sensor will compensate for a less than full charge on the flash capacitor. If simple capacitor-resistor timing is used, the light output from a manual discharge will depend on the initial charge on the flash capacitor which will depend on how long since it has been switched on or since the last shot, because flash units generally can be fired before they are fully charged. In fact the charging process is asymptotic.


Fig 5 shows the principle of a possible manual control circuit, which again is an extension of the basic circuit of Fig 1. It is similar to the Auto circuit in Fig 2 except that the photosensor part is replaced with a timing circuit. This will work on the principle of how long a capacitor will take to charge through a resistor.

In Fig 5 any of the three resistors R18-20 may be selected by switch SW4 through which to charge the capacitor C8, giving a choice of three manual flash levels. The process is managed by the timer control circuit represented by the square marked "Timer" in which there will probably be an integrated circuit such as a 555. The timing is triggered to start at the same time as the flash by the same grounding of the hotshoe centre contact. C5 allows only a pulse through which is all the timer needs to trigger.

A more sophisticated circuit would allow for the possible different levels of initial charge in the flash capcitor C3. One method would be to use a photosensor circuit similar to that of the Auto control system above, but with the sensor close to the flash tube and aimed at it, rather than at the subject, perhaps through a pin hole in the reflector.


Energy Saving

The next step forward in flashgun design after the introduction of the control of energy level was energy saving. This is achieved by turning off the current from the flash capacitor after the flash tube has emitted enough light, saving the rest of the charge towards the next shot. One method of doing this is to send the output of the quench tube to a small capacitor rather than to ground. Capacitors offer zero resistance (strictly speaking, impedance) when current first starts to flow into them, but in a very short time they become a dead-end like a cul-de-sac, but enough time for the current diversion away from the flash tube for its xenon gas to lose its ionisation, and therefore become unable to resume its current flow.


Fig 6 shows the circuit of the once popular Vivitar 283, one of the first energy-saving flashguns. It uses a quench tube in a slightly different way. It has a thyristor SCR1 and a quench tube QT1 in parallel with it, both leading to ground from the main flash tube FT1. SCR1 conducts the current during the flash.

The metering circuit at the lower right of Fig 5 triggers the quench tube Q1 when enough light has been received. This diverts the current leaving the flash tube to diode D8, capacitor C15, through Q1, to ground. While the flash current is diverted away from SCR1, the latter is given a negative signal to its gate to ensure it will not turn on again. Meanwhile, C15 in the diversion path is small (3.6 uF) compared with the main flash capacitor C9 (1150uF) so almost immediately it fills with charge and stops the diversionary flow. At this point all current from C9 has ceased, so no more energy is wasted. The promotional headline of the 283 was that it used an energy-saving thyristor (in fact it used two) but it still employed a dark quench tube so it was a in fact a transitional design.

Only a tiny amount of electrical charge (and therefore energy) will have passed into the quench capacitor, a level that can be handled by a small SCR, so the quench tube itself could be replaced by an SCR. triggered from the metering circuit. However the later invention of the gate turn-off thyristor (GTO) led to a further improved step in flashgun design. Unlike the SCR which could only be turned on, and not off, the GTO has the capability of being turned on or off by the application of a positive or negative signal to its gate, and was effectively a very high speed solid state relay.


Fig 7 is a simplified version of the flash tube part of a circuit using a gate turnoff thyristor GTO1 to turn off the flash when the exposure meter decides it has had enough. The GTO thyristor is in the cathode or ground line of the flash tube and it also needs a signal to turn it on, at the same time as the flash tube is turned on by the usual high voltage trigger arrangement.

Between flashes, C5 and C6 will charge up through the relatively high resistance R4. The flash tube cathode and GTO1 anode are effectively grounded by R17. When the flash tube is triggered its cathode voltage will rise momentarily causing some of C5's charge to flow back through R14 - C6 - R15 to ground, sending a positive pulse to the gate of the GTO which starts it conducting the main flash current. The SCR thyristor SCR1 does not conduct at this time.

When the flash needs to be stopped, the metering circuit will send a positive quench signal to the gate of SCR1. This turns SCR1 on, which pulls the charge out of C6 though R15 and R14 causing a momentary negative voltage at the gate of SCR1. This stops the flash current flow.



Low Sync Voltage


For all the issues that high trigger voltages have caused, a safe low voltage at the flashgun foot can be arranged easily and inexpensively.

One possible method is shown in Fig 8 which is a modification of the circuits shown in Figs 1 and 2, although to save space this diagram omits everything to the right of the flash tube.

Instead of the camera shutter grounding T1 via the hot shoe directly, T1 is grounded by the thyristor SCR2. When the camera shutter fires (or the test switch SW2 is pressed) R23 is grounded which causes a small current to flow through it from the base of the PNP transistor TR2. This triggers a current from TR2's collector into R24 and C7. The latter allows only a brief pulse of current to pass through to the gate of SCR2 (the grounding by the shutter is brief anyway) but enough to fire it, which grounds T1 which fires the flash. C7 also protects the camera against 300 volts in the unlikely event of SCR2 failing with a short between its anode and gate. In this circuit the voltage in the unit's foot cannot exceed the 6 volts of the battery.



TTL Control


From around 1980 the more expensive cameras acquired the ability to control the flash exposure through the lens [TTL], and this feature was subsequently extended to models lower down the range. TTL flash usually employed a sensor at the bottom of the mirror box aimed up at an angle to measure the film surface illumination during the flash. The measurement was thus moved from the photosensor on the flashgun body to the camera itself. In many cameras the same sensor was also employed for exposure measurement without flash.

The advantages for the user were that it was no longer necessary for them to communicate the lens aperture between the flashgun and the camera by making settings, and also that they could set any lens aperture on the camera that they wanted instead of being restricted to a few prescribed values. For example the user could set the camera lens to its maximum aperture of f1.4, which they probably could not with an older Auto system unless they were using very slow film.

There was also the advantage for flashgun makers that, in time, they no longer had to provide light metering, because the camera did it. So the cost of the photosensor and its circuitry and switching was saved, although it is debatable whether this saving was passed on to the buyer. All the flashgun designer needed to do was to receive and act on the quench signal from the camera.

Nevertheless, for many years non-TTL cameras were still in use and still being made, so there was a transitional period while the better flashguns offered all three of the basic exposure modes - Manual, Auto and TTL. Even after the Auto mode was dropped, a Manual mode or modes were still included because it is required for special uses such as when firing into a flash brolly or softbox. As has already been mentioned, it is simple to offer a full power Manual flash by arranging to disconnect the quench signal.

TTL required a further contact in the hotshoe to carry the quench signal from the camera, again with no standardisation between camera makers.

Be aware that many commentators refer to TTL control as simply "Auto", and it is indeed an automatic mode, but it can therefore be confused with the earlier Auto mode described above.

Pre-Flash TTL

For digital cameras, TTL evolved into a version which added a low power pre-flash, which for various camera brands is termed i-TTL, P-TTL, E-TTL etc. The result of the pre-flash is evaluated through the camera lens by the same metering system used before a shot for natural light metering, and the energy of the main flash is set accordingly. Pre-flash will not be covered much here because this article is about film era flashguns, but the buyer needs to be aware of the system.

Digital cameras moved away from film-era TTL because the latter relied on the matt film surface from which the sensor measured exposure. However the surface of the digital sensor that replaces film is specular and creates diffraction patterns, so cannot be relied upon as a measuring surface.

Pre-flash systems require even greater integration between camera and flash units, and needless to say the systems are both proprietory and incompatible with film era TTL systems. Typically, a modern flashgun will offer pre-flash TTL and Manual modes only.