Solar Cells And Photothyristors

Solar cells are photo diodes with very large surface areas. The large surface area of makes the device more sensitive to incoming light, as well as more powerful (larger currents and voltages) than photo diodes. For example, a single silicon may be capable of producing a 0.5-V potential that can supply up to 0.1 A when exposed to bright light. They can be used to power small devices such as solar-powered calculators or can be added in series to recharge nickel cadmium batteries. Often solar cells are used as light-sensitive elements in detectors of visible and near-infrared light (e.g., light meters, light-sensitive triggering mechanism for relays).

Like photo diodes, they have a positive and negative lead that must be connected to the more positive and more negative voltage regions within a circuit. The typical response time for a solar cell is around 20 ms. Like batteries, cells can be combined in series or parallel configurations. Each solar cells produces an open-circuit voltage from around 0.45 to 0.5 V and may generate as much as 0.1 A in bright light. By adding cells in series, the output voltage becomes the sum of the individual cell voltages. When cells are placed in parallel, the output current increases.

Each cell provides 0.5 V, so the total voltage is 4.5 V minus a 0.6-V drop due to the diode). The diode is added to the circuit to prevent the NiCd cells from discharging through the solar cell during times of darkness. It is important not to exceed the safe charging rate of NiCd cells.To slow the charge rate, a resistor placed in series with the batteries can be added. Photothyristors are light-activated thyristors. Two common photothyristors include the light-activated SCR (LASCR) and the light-activated triac.

A LASCR acts like a switch that changes states whenever it is exposed to a pulse of light. Even when the light is removed, the LASCR remains on until the anode and cathode polarities are reversed or the power is removed. A light-active triac is similar to a LASCR but is designed to handle ac currents The equivalent circuit shown here helps explain how a LASCR works. Again, like other pn-junction optoelectronic device, a photon will collide with an electron in the p-semiconductor side, and an electron will be ejected across the pn junction into the n side.

When a number of photons liberate a number of electrons across the junction, a large enough current at the base is generated to turn the transistors on. Even when the photons are eliminated, the LASCR will remain on until the polarities of the anode and cathode are reversed or the power is cut. (This results from the fact that the transistors’ bases are continuously simulated by the main current flowing through the anode and cathode leads).