Perfectly efficient diodes appear to be open circuits with a negative voltage and they look like short circuits otherwise. But because diodes admit to some inefficiency, their current-to-voltage relationship is nonlinear. As such, you’ll want to consult a diode’s datasheet to see a graph of the curve of any given diode’s forward voltage relative to its forward current, so that you pick the right diode for your particular project.
Applications of Diodes
Despite being just simple two-pin semiconductor devices, diodes are vital in modern electronics. Some of the typical applications of diodes include:
Rectifying a voltage, such as turning AC into DC voltagesIsolating signals from a supplyControlling the size of a signalMixing signals
Power Conversion
One significant application of diodes is to convert AC power to DC power. A single diode or four diodes transform 110V household power to DC by forming a halfway (single diode) or a full-wave (four diodes) rectifier. The diode allows only half of the AC waveform to travel through it. When this voltage pulse charges a capacitor, the output voltage appears to be a steady DC voltage with a small voltage ripple. Using a full-wave rectifier makes this process even more efficient by routing the AC pulses so that both the positive and negative halves of the input sine wave are seen as only positive pulses, effectively doubling the frequency of the input pulses to the capacitor, which helps keep it charged and deliver a more stable voltage. Diodes and capacitors create different voltage multipliers to take a small AC voltage and multiply it to create very high voltage outputs. Both AC and DC outputs are possible using the right configuration of capacitors and diodes.
Demodulation of Signals
The most common use of diodes is to remove the negative component of an AC signal. Because the negative portion of an AC waveform is usually identical to the positive half, very little information is effectively lost in this process of stripping it away, leading to more efficient signal processing. Signal demodulation is commonly used in radios as part of the filtering system to help extract the radio signal from the carrier wave.
Over-Voltage Protections
Diodes also function well as protection devices for sensitive electronic components. When used as voltage protection devices, the diodes are nonconducting under normal operating conditions but immediately short any high-voltage spike to ground where it cannot harm an integrated circuit. Specialized diodes called transient voltage suppressors are designed specifically for over-voltage protection and can handle very large power spikes for short time periods, typical characteristics of a voltage spike or electric shock, which would normally damage components and shorten the life of an electronic product. Similarly, a diode can regulate voltage by serving as a clipper or a limiter — a specialized purpose that caps the voltage that can pass through it at a certain point.
Current Steering
The basic application of diodes is to steer current and make sure it only flows in the proper direction. One area where the current steering capability of diodes is used to good effect is in switching from the power coming from a power supply to power running from a battery. When a device is plugged in and charging — for example, a cell phone or uninterruptible power supply — the device should be drawing power only from the external power supply and not the battery, and while the device is plugged in the battery should be drawing power and recharging. As soon as the power source is removed, the battery should power the device so that no interruption is noticed by the user. A good example of current steering is through reverse current protection. Consider, for example, your car. When your battery dies and a friendly passerby offers to help with jumper cables, if you mix the order of the red and black cables you won’t fry your car’s electrical system because diodes adjacent to the battery block the wrong-direction current.
Logic Gates
Computers operate in binary — an endless sea of zeroes and ones. The binary decision trees in computing are based on logic gates enabled by diodes that control whether a switch is on (“1”) or off (“0”). Although hundreds of millions of diodes appear in modern processors, they’re functionally the same as the diodes you buy at the electronics store — just much smaller.
Diodes and Light
An LED flashlight is just a flashlight whose illumination is sourced from a light-emitting diode. In the presence of positive voltage, LEDs glow. A photodiode, by contrast, accepts light through a collector (like a mini solar panel) and converts that light into a small amount of current.
