Homebrew battery-powered flash generator

My mobile flash generator, for use is places with no power outlet in sight.
(Note that the above picture is from a day without wind. Umbrellas on stands are probably not a good idea otherwise...)

This flash generator was designed in April 2006. Most images enlarge when clicked.
Some short points:

This project is a mobile flash generator that provides the functionality of a small studio setup without the need for outlet power. I intend to use it for experiments with mixing flash and ambient light outdoors, or photoshoots in interesting locations. It offers fast recycling (between 0.2 and 1.4 seconds depending on the energy setting). This is of course a highly desired feature, in order not to have to wait for the flash to recharge when shooting under difficult or awkward circumstances.

The generator uses the same flash heads as those described for my other flash system.

This mobile system provides for 100W of modelling light from a flash head, with fan cooling. This requires the head to be wired a bit differently, and fitted with a low-voltage halogen bulb. Without this change, the heads will operate normally, but without modelling light and fan. The modelling light can be individually turned on or off for the two heads. The unit is housed in a Schroff Compac cabinet (3HU/28TU) that I bought second-hand. It has a convenient size for being carried around, and weighs around 8 kg (about half of which is the weight of the battery).

Settings and controls

The energy distribution to the two flash heads is controlled by two independent channels that can be set to full, half or quarter energy using big rocker switches (capacitor bank switching). If only one head is used, a third switch can be used to combine the two channels, making up to 600Ws available. (I admit to borrowing this idea from the panel of the Profoto Acute2 generator :-)

There is also a stepless master energy dial that provides fine adjustment of the light output, from 0.0 to -1.0 f-stop. This is done by voltage regulation, and the range is calibrated for max-, min- and midpoints. This feature is connected to an auto-dump function, which bleeds the capacitor banks when the energy is dialed down. This way a full flash does not need to be wasted in order to get the desired setting.

The generator has a TEST button, and a jack for connecting a sync lead. A buzzer signal, which can be turned on or off, tells when the unit is ready to fire.

The battery status is displayed using three LEDS which, like a traffic light, light up in green, yellow or red depending on the remaining charge. A red light means that the battery should be recharged as soon as possible.

Design details


The generator is powered by a 12V sealed lead-acid battery, with a rated capacity of 12Ah. Its dimensions make it fit very well in the bottom part of the cabinet.

The battery is housed in the bottom of the enclosure.

To recharge the battery, a battery charger is connected to a socket on the top panel. The charger used in this project is a generic model specifically made for lead-acid batteries. It performs the charging at a maximum current of 2A, and stops automatically when the battery is fully charged. A signal lamp on the charger switches from red to green when charging is complete. Both the battery and the charger come from the same shop, each costing around EUR20.

Electronic details

While most of the inside volume of the generator is occupied by the battery and the capacitor banks, the heart of the design is a powerful DC/DC converter. It is a push-pull design operating at 45kHz, and it can provide a peak output power of over 600W.

It was a design goal to keep the circuit simple and compact. A 10x15 cm PCB holds everything - the controller circuit, the DC/DC converter, the charging network, and the auto-dump circuit. The generator contains only two ICs - the first being a SG2525, which controls the converter. The second IC is a PIC16F676 microcontroller, which takes care of everything else. Of course, the high voltage part of the circuit requires wide clearances, but the low voltage side was built as compact as possible.

The topmost PCB holds all control and charging circuits. The step-up transformer and two of the switching MOSFETs can be seen here.

The SG2525 drives four 100A power switches in the converter primary side. It has a soft-start feature which reduces the initial current draw when the capacitors start charging from zero. During recycling the IC operates at maximum duty cycle, until the capacitor voltage has reached the value selected on the master energy dial. Then its output is reduced to brief pulses, providing a 'trickle charge' which maintains a stable voltage in the capacitor banks. The error amplifier in the SG2525 has its output available on one of its pins, which provides information to the PIC microcontroller about the charging status.

The PIC16F676 microcontroller controls the DC/DC converter using the 'shutdown' pin on the SG2525. The microcontroller also handles flash triggering, energy dumping and the visual and audible ready indicators. Furthermore, it measures and displays the remaining battery capacity.

During the brief moment when the generator is recycling, a huge current is drawn from the battery. The fuse used in the connection to the battery (+) has to allow for this. I used a 60 Amp AGU fuse - a type which is normally used in the 12V supply to powerful car audio amplifiers, and looks like a giant version of a regular 20mm glass tube fuse.

Performance and comments

For outdoor photography, this generator provides for some interesting experiments with mixing flash and ambient light. At maximum energy, and a distance of 2 m, I measured f/16 using a white umbrella and f/22 with a silver umbrella (ISO 100). The unit thus provides enough energy for a diffused light source that can compete with direct sunlight at fast shutter speeds. However, its main purpose is to be a mobile two-light studio for use in interesting locations with no electricity, and I have yet to try it out fully in this role.

It remains to determine how many flashes that are possible from one charging of the battery. The available battery capacity is reduced at high currents, and it is known that more flashes could be produced if the recycling speed was reduced. However, fast recycling was given priority over battery capacity in this project.

By the nature of the energy selection and voltage regulation techniques used, the colour and exposure stability of the generator is similar to that of my other, outlet-powered unit.

Warning! Flash circuits are very, very dangerous! Never attempt a project similar to this one if you do not have the right experience and equipment. If you just want to get a flash unit at low cost, there are plenty on the second-hand market.

(c) Marcus Gunnarsson 2005-2006

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