Signal Diodes

Signal diodes are a two-terminal semiconductor circuit element that transmits the current when polarized forward and prevents the flow of current when it is inverted.

Semiconductor signal diodes are a small, nonlinear semiconductor device, usually used in electronic circuits, involving small currents or high frequencies such as radio, television and digital logic circuits.

Glass-enclosed signal diodes, called Point Contact Diodes or glass diodes, are physically very small compared to their larger Power Diode cousins.

signal diodes
Zener Diode and Silicon Diode

In general, the PN connection of a small signal diode is encapsulated in glass to maintain the PN connection, and usually there is a red or black band at one end of their body to help determine which end is the cathode terminal.The most common of all glass encapsulated signal diodes are 1N4148 and equivalent type 1N914.

Small signal and switching diodes have much lower power and current values of around 150mA, maximum 500mW compared to rectifier diodes, but they can work better in high frequency applications or trimming and switching applications that deal with short-term pulse waveforms.

The properties of a signal diode are different for both germanium and silicon types and are given as follows:

  • 1. Germanyum Signal Diodes – These have a lower forward voltage drop throughout the connection, a low resistance value that typically provides only about 0.2 to 0.3v, but has a higher forward resistance value due to small connection areas.
  • 2. Silicon Signal Diodes – These have a very high reverse resistance value and provide a forward voltage drop of approximately 0.6 to 0.7v throughout the connection.They have very low forward resistance values and give them high forward current and reverse voltage peak values.

The electronic symbol given for any type of diode is an arrow with a bar or line at the end, which is shown below, along with the Stable State VI Characteristic Curve.

signal diodes
Silicon Diode IV Characteristic Curve

The arrow always indicates the direction of the current passing through the diode, which means that the diode will transmit only if a positive feed is connected to terminal Anoda , ( a ) and a negative feed is connected to the Cathode ( k ) terminal. it acts more like a one-way electric valve, allowing it to flow through the current in only one direction.

However, we know from the previous tutorial that if we connect the external energy source in the other direction, it will block any current passing through the diode and instead act as an open switch (being inverted polarity), as shown below.

Forward and Reverse Polar Diode

signal diodes

Then we can say that an ideal small signal diode transmits the current in one direction (forward forward) and blocks the current in the other direction (reverse blocking).Signal Diodes are used in a wide range of applications such as a switch in rectifiers, current limiters, voltage breakers or waveforming circuits.

Signal Diodes and Parameters

Signal Diodes areproduced at various voltage and current degrees and care should be taken when choosing a diode for a specific application.There are a surprising set of static features associated with the signal diode, but we share with you the more important ones:

Maximum Forward Current

Maximum forward Current (IF(max)is, as the name suggests, the maximum forward current is the maximum forward current that the diode allows to flow into or even the circuit to which it is connected.When the diode is conductive in the case of forward pre-voltage, it has a very small "ON" resistance throughout the PN connection, and therefore the power is distributed throughout this connection (Ohm's Law) in the form of heat.

Then exceeding the value (IF(max)will cause more heat to be generated throughout the connection, and the diode will fail, often with devastating consequences, due to thermal overload.When operating diodes around maximum current values, it is always best to provide additional cooling to dissipate the heat generated by the diode.

For example, our small 1N4148 signal diode has a maximum current rating of about 150mA with a power loss of 500mW at 25o C. Then a resistance should be used in series with diode to limit forward current.

Inverted Peak Voltage

Inverted peak voltage (PIV) or reverse voltage (VR(max)is the operating voltage that can be applied throughout the maximum permissible diode.Therefore, this rating is usually lower than the level of "avalanche breakage" in the characteristic curve of the reverse pre-reclaent.Typical V R(max) values vary from a few volts to thousands of volts and should be taken into account when replacing a diode.

Inverse peak voltage is an important parameter and is mainly used to correct diodes in AC rectifier circuits, referencing the amplitude of the voltage, in which the sinusoidal waveform changes from positive to negative in each cycle.

Total Power Consumption

Signal diodes have a Total Power Consumption (P D(max) rating.This rating is the maximum possible loss of power in the case where the diode is forward (conductor).When the current passes through the signal diode, the deviation of the PN connection is not perfect and shows some resistance to the flow of the current, causing the power to dissipate (disappear) in the diode in the form of heat.

Since small signal diodes are nonlinear devices, the resistance of the PN connection is not constant, it is a dynamic feature, then we cannot use the Ohm Law to define as much power as we can for resistors in current and resistance or voltage and resistance.Then, in order to find the power to be expended by the diode, we must multiply the voltage drop on it by the current passing over it: P D = V*I

Maximum Operating Temperature

The maximum operating temperature is actually associated with the junction temperature (T J diode) and is related to maximum power consumption.The structure of the diode is the maximum temperature allowed before it deteriorates and is expressed in degrees Celsius per watt, (o C/W).

This value is closely linked to the maximum forward current of the device, so that the temperature of the connection at this value is not exceeded.However, the maximum forward current will also depend on the ambient temperature at which the device operates, so the maximum forward current is usually quoted for two or more ambient temperature values, such as 25 o C or 70 o C.

Then there are three main parameters to consider when choosing or changing a signal diode, and these are:

  • Reverse Voltage Rating
  • Forward Current Rating
  • Advanced Power Consumption Rating

Signal Diode Arrays

Diode arrays can be very useful when space is limited or pairs of matching switching signal diode are required.They usually consist of low-capacitance high-speed silicon diodes such as 1N4148, which are connected in multiple diode packages called a series for use in switching and clamping in digital circuits.They are stored in single-row packages (SIP) containing 4 or more internally connected diodes to give a separate isolated array, common cathode (CC), or common anode (CA) configuration, as shown.

Signal Diode Arrays

signal diodes

Signal diode arrays can also be used in digital and computer circuits to protect high-speed data lines or other input/output parallel ports from electrostatic discharge, (ESD) and voltage transitions.

By serially connecting the two diodes along the supply rails with the data line connected to their connection, unwanted passes are quickly distributed and they can protect eight data lines in a single package because the signal diodes are present in 8-storey arrays.

signal diodes

Signal diode arrays can also be used to connect diodes in series or parallel combinations to create voltage regulator or voltage-reducing type circuits or even produce a known fixed reference voltage.

We know that the forward voltage drop along a silicon diode is about 0.7v, and by connecting several diodes in series, the total voltage drop will be the sum of the separate voltage drops of each diode.

However, when the signal diodes are connected to each other in series, the maximum forward current should not be exceeded as the current will be the same for each diode.

Serially Connecting Signal Diodes

Another application for small signal diode is to create an regulated voltage source.Diodes are serially connected to ensure a constant DC voltage throughout the diode combination.The output voltage on the diodes remains constant despite changes in the load current drawn from the series combination or changes in the DC power supply voltage that feeds them.

Serial Signal Diodes

signal diodes

Since the forward voltage drop along a silicon diode is almost constant at about 0.7v, it can make a simple voltage regulator circuit with an advanced signal diode, while the current passing through it varies in relatively large quantities.Separate voltage drops on each diode are removed from the feed voltage to leave a certain voltage potential throughout the load resistance, and in our simple example above this is given as 10v – (3*0.7V) = 7.9V.

This is because each diode has a connection resistance related to the small signal current flowing through it, and the three signal diodes in the series, together with the load resistance R, have three times this resistance , creating a voltage divider throughout the feed.

A larger voltage drop will occur by adding more diodes in series.In addition, serially connected diodes can be placed in parallel with load resistance to serve as voltage regulator circuits.The voltage applied to the load resistance here will be 3*0.7v = 2.1V.Of course, we can produce the same constant voltage source using a single Zener Diode.Resistance is used to prevent overcurrent flowing through the diodes when the RD load is removed.

Free Rotary Diodes

Signal diodes can also be used in various clamping, protection and waveforming circuits; The most common docking diode circuit is one that uses a coil or a diode that connects in parallel with the inductive load to prevent damage to the sensitive switching circuit. voltage spikes and/or transient events that occur when the load is suddenly "OFF" are intended to be suppressed.This type of diode is often known as "Free Rotating Diode", "Flywheel Diode", or, more commonly, free rotary diode.

The most conceivable use case is diodes used to prevent reverse jumps when trading with relays.

signal diodes
Free Rotary Diode Use

Modern fast switching requires fast switching diodes such as free rotary diodes to protect against inductive loads such as semiconductor power devices, engine coils or relay windings.Each time the above switching device is placed in the "ON" position, the free wheel diode switches from a conductive state to a blocking state, as it becomes inverted.

However, when the device quickly moves to the "OFF" position, the diode is polarized forward, and the collapse of the energy stored in the coil causes a current to flow through the free rotary diode.Without the protection of the free diode, high di/dt currents occur, which causes high voltage rise or temporary current to flow around the circuit and possibly damages the switching device.

Previously, the operating speed of the transistor, MOSFET, IGBT or digital semiconductor switching device was impaired by the addition of free rotary diode along the inductive load using Schottky and Zener diodes instead in some applications.But in the last few years, free rotary diodes have regained importance, mainly due to their advanced reverse-recovery properties and the use of superfast semiconductor materials that can operate at high switching frequencies.

Other special types of diodes not included here are Photo-Diodes, PIN Diodes, Tunnel Diodes and Schottky Barrier Diodes.Other types of semiconductor devices can be made by adding more PN connections to the basic two-layer diode structure.

For example, a three-layer semiconductor device is actually a transistor, a four-layer semiconductor device becomes a thyristor or Silicon Controlled Rectifier, and five-layer devices known as Triage are also available.

In the next tutorial on diodes, we will look at the large signal diode called The Power Diode.Power diodes are silicon diodes designed for use in high voltage, high current main straightening circuits.