A diode is an electronic component that is used to allow an electric current to flow in one direction but not in the other. This is useful for things like converting AC into DC, preventing a battery discharging back into a solar panel when the sun goes down.

Early diodes were made from light bulb shaped Thermionic Valves, but were soon replaced by modern silicon diodes because of their superior efficiency, smaller size and robustness.


Uses of diodes (also known as rectifiers) in electronic circuits

• Converting AC to DC Alternating Current (AC) is what you would get from the mains, and Direct Current (DC) is what you would get from a battery.
  
  In older-style power supplies, the mains current (Normally 110 ~ 230 Volts AC, and between 50~60 cycles per second) is stepped down by a transformer to a much lower voltage (Typically 5 ~ 12 volts AC). This is then passed through four diodes arranged as a bridge rectifier. The output is a pulsing DC voltage which is normally smoothed by a capacitor. A 12 volt AC current would produce approximately 17 volts DC because 12 volts is only the average voltage (Actually it's called RMS - Root Mean Square) and 17 volts is the peak (12 volts x 1.414 = 16.8 volts). In many cases the voltage wasn't smooth enough, so voltage regulators had to be used which wasted energy as heat.
  
  In newer switch-mode power supplies the raw mains is fed directly into a bridge rectifier and smoothed by a high voltage capacitor. On 230 volts AC mains this would yield 322 volts DC. This high voltage is "chopped" into 50 KHz slices by a transistor and passed to a small transformer. The resulting high frequency low voltage is passed through a bridge rectifier, smoothed by a capacitor and passed onto the device to be used.
  
  
• Preventing reverse power damage on low-voltage electronic devices who's power leads could get connected the wrong way. This is also useful when charging batteries from solar panels as they tend to leak a small amount when their voltage drops below the battery voltage - the down-side is that you lose about 0.6 volt from the incoming power when charging, but this is a small trade-off.