An internal camera circuit connects the center contact and shoe mount to trigger the flash. The magnitude and polarity of the voltage between the contacts on the flash in the open-circuit condition has varied between different flash units; this is of no consequence for a simple electromechanical contact on the camera so long as the energy is not so high as to damage the contacts. However, with more recent cameras with electronic triggering, excessive or insufficient voltage, or incorrect polarity can cause failure to fire, or damage the camera. The
ISO 10330 specification allows for a trigger voltage of 24 volts. Some manufacturers, particularly Canon, ask for no more than 6 volts. Flash units designed for modern cameras use voltages which are safe and effective, but some older flashes have much higher voltages, up to hundreds of volts, which damage electronic triggering circuits. Some use negative DC polarity, or AC.
iISO hotshoe contacts are only protected up to ca. 5 volts in some cameras. Minolta documented all their cameras' electronically controlled
PC terminals and ISO hot shoes to be protected up to 400 volts. It is possible to connect an older high-voltage triggering flash to a camera which can only tolerate 5 or 6 volts through an adaptor containing the necessary voltage protection circuitry, typically using a high power
TRIAC. Such adapters drain power from the flash's trigger voltage and therefore often do not need a power supply of their own. In order to avoid dangerous loops when connecting equipment in complex studio setups, better adapters offer voltage protection and
galvanic isolation of the units. Such adapters will ensure that there is no electrical connection of any kind between both sides of the adapter, including ground. They use either
transformers or
opto-couplers to transfer a safe trigger impulse from the camera to the flash. They are powered by batteries, as their electronics cannot be powered from the flash. As an example, Minolta offered the PC terminal adapter
PCT-100 (
8825-691) for this purpose, which worked as a galvanic isolator and could withstand 400 volts DC or AC. The similar Sony
flash sync terminal and ISO hotshoe adapters
FA-ST1AM and
FA-HS1AM also offer galvanic isolation as well, but only up to 60 volts DC or AC. Flash servos and radio triggers, e.g.
PocketWizard, can also provide electrical isolation, as trigger and receiver unit are physically separate. The camera is only presented with the low voltage used by the local trigger unit, and the remote receiver unit is designed to tolerate up to 200 volts from its flash port. Slave flash, where the flash from a safe flash unit connected to the camera triggers an unconnected flashgun which, if connected, would present a dangerous voltage, is another way to use a flashgun which cannot be connected to a hot shoe; indeed, it can be used for a camera with built-in flash and no hot shoe. The trigger voltages provided by some modern flashes or radio triggers can also be too low to reliably trigger electronic circuits in the camera or receiver. Trigger circuit voltages below ca. 2 to 2.5 volts may exhibit this problem, unless the triggering circuit is specifically designed to work with such low voltages. Older cameras equipped with an electro-mechanical trigger contact may exhibit yet another problem. If they provide both an ISO hotshoe and a PC terminal, both are typically wired to the same trigger contact in the camera rather than triggered independently as in cameras with electronic triggering circuits. When only the PC terminal is used and nothing is connected to the hotshoe, a flash with high trigger circuit voltages connected to the PC terminal delivers this voltage on the normally unprotected middle contact of the camera's ISO hotshoe. If the photographer's eyebrows accidentally make contact with the middle contact, the
electrical shock can cause pain or even injuries. One way camera manufacturers have used to avoid this problem was to use two independent trigger contacts, which, however, could cause flash synchronization problems. Another, as utilized by Minolta in all such cameras supporting a PC terminal, was to add a small switch on the side of the ISO hotshoe which disabled the middle contact until something was inserted into the hotshoe. == Modern cold shoes and other devices ==