Photothyristor
A photothyristor (also Optothyristor ) is a light-sensitive electronic semiconductor - component .
Structure and functionality
In terms of structure and functionality, the photothyristor is essentially the same as a normal thyristor . It consists of four semiconductor zones with alternating doping , for example npnp, and the anode and cathode connections . In contrast to normal thyristors, the photothyristor has a housing window with an optical connection or a built-in light-emitting diode instead of a gate electrode .
Accordingly, the photothyristor is not ignited as usual by a gate current, but by incident light on one of the middle pn junctions . In the blocking area, the middle pn junction is polarized in the reverse direction. The incident light energy acts on this and generates charge carriers ( electron-hole pairs ). If a certain illuminance is exceeded, the photothyristor ignites, i.e. sufficient charge carriers are generated that flood the space charge zone and make the pn junction conductive. The "ignition illuminance" (a few milliwatts) is influenced, among other things, by a bias voltage on the gate. After ignition, the photothyristor behaves like a normal thyristor. It switches off as soon as the current drops below the holding current.
application
The particular advantage of photothyristors is that ignition takes place without a galvanic connection to the load circuit. This allows the construction of mains-separated control circuits without a transformer . Any high voltage differences can be overcome with fiber optic cables. For this reason, photothyristors are used in high-voltage direct current (HVDC) transmissions .
Photothyristors allow the construction of optocouplers without a downstream switching amplifier. The thyristor load current can also switch larger relays or mains-fed loads directly . Most of these optocouplers are designed with triacs and can therefore switch the mains AC voltage directly.