Vivitar 285 Multi flash setup

[Dan Quan]

PC cords are indispensable if their use affords you something you can't achieve any other way. Like in an RF controlled environment, or hardwiring to achieve an unusually fast shutter/flash sync speed, such as between 1/1000 sec and 1/8000 sec which can be done on a D70.

But if you don't need to have the cords laying around, the RF or optical triggers can be safer, more tidy and lead to fewer falling light stands and broken flash heads.

[MattKing]

I have some old Wein IR triggers that work well.

Recently though I was surprised when I found that the remote for our digital TV tuner/recorder would trigger the flashes!

[Steve Smith]

I was surprised when I found that the remote for our digital TV tuner/recorder would trigger the flashes!

A few years ago at work, we were making a circuit for a vending machine which used infra red to detect the position of a cup. I used an old remote control as part of the test equipment.


Steve.

[Soeren]

If within economical reach I'd go with radio or optical slave triggers. It probably will save you some strobes in the end and keep you from getting a wounded ego you know like in Bussy photog + Cables on floor =

[John Koehrer]

Recollections are kinda foggy but it seems that by putting two or three flash units through one PC contact that the voltages are added together, not parallel. If you have a multiple adapter it would be easy to check at the point where the single cable attaches.

[Steve Smith]

Recollections are kinda foggy but it seems that by putting two or three flash units through one PC contact that the voltages are added together, not parallel.

They won't add together but as the trigger voltage is derived from the HV capacitor via a high resistance, the voltage of the combined connection will be around that of the highest voltage flash.

However, some flashes (my 285s included) measure 5.6 volts which I think shows that there is a 5.6 volt zener diode limiting the voltage. In that case, it might pull the voltages of all the other flashes down to this level too.


Steve.

[John Koehrer]

The PC cord doesn't see trigger voltage, only open switch from the 6V batteries.

[Steve Smith]

The PC cord doesn't see trigger voltage, only open switch from the 6V batteries.

Within the flash there is a high value resistance (about 1M ohms) connected from the high voltage supply to a small capacitor (about 10nF) which is then connected to the primary of the trigger transformer (the other side being connected to ground).

As the main high voltage capacitor charges, so does the small trigger capacitor. The sync. cable is connected between ground and the junction of the resistor and capacitor so that when the shutter contacts close, the capacitor is connected across the trigger transformer primary. The energy in this small capacitor discharges into the trigger transformer which causes a high voltage pulse to appear on the transformer's output. This pulse ionises the gas in the flash tube and causes it to start conducting, taking the charge from the main high voltage capacitor. It will continue to conduct until the charge is gone, or in the case of thyristor flashes, until the thyrystor circuit stops it conducting.

So the voltage present on the sync. connection is likely to be the same as the internal high voltage supply unless it is limited by a zener diode or resistor network.


Steve.