Relays

Relays

There are a variety of devices which are classed as output devices and are therefore commonly called Actuators. Actuators convert an electrical signal into a corresponding physical quantity such as movement, force, sound etc. Actuators can also be considered as either Binary or Continuous devices based upon the number of stable states their output has. For example, A relay is a Binary Actuator as it has two stable states, latched and unlatched while a motor is a Continuous Actuator. The most common types of actuators or output devices are Relays, Lights, Motors and Loudspeakers and in this tutorial we will look at a Electromechanical Relays and Solid State Relays.

The Electromechanical Relay.

The term Relay generally refers to a device that provides an electrical connection between two or more points in response to the application of a control signal. The most common and widely used type of relay is the Electromechanical Relay or EMR. Relays are basically electrically operated switches that come in many shapes, sizes and power ratings suitable for all types of applications but in this section we are just concerned with the fundamental operating principles of "light duty" electromechanical relays. Such relays are used in general electrical and electronic control or switching circuits either mounted directly onto PCB boards or connected free standing and in which the load currents are normally fractions of an Ampere up to 20+ Amperes.

As their name implies, Electromechanical Relays are Electro-Magnetic devices that convert a magnetic flux generated by the application of an electrical control signal either AC or DC current, into a pulling mechanical force which operates the electrical contacts within the relay. The most common form of electromechanical relay consist of an energizing coil called the "Primary Circuit" wound around a permeable iron core. It has both a fixed portion called the Yoke, and a moveable spring loaded part called the Armature, that completes the magnetic field circuit by closing the air gap between the fixed electrical coil and the moveable armature. This armature is hinged or pivoted and is free to move within the generated magnetic field closing the electrical contacts that are attached to it. Connected between the yoke and armature is normally a spring (or springs) for the return stroke to "Reset" the contacts back to their initial rest position when the relay coil is in the "de-energized" condition, ie. turned "OFF".

Example of a simple low power electromechanical relay.

In our simple relay above, we have two sets of electrically conductive contacts. One pair which are classed as Normally Open, (NO) or make contacts and another set which are classed as Normally Closed, (NC) or break contacts. These terms "Normally Open, Normally Closed" or "Make and Break Contacts" refer to the state of the electrical contacts when the relay coil is "de-energized", i.e, no supply voltage connected to the coil. An example of this arrangement is given below.

The relays contacts are electrically conductive pieces of metal which touch together completing a circuit and allows the circuit current to flow, just like a switch. When the contacts are open the resistance between the contacts is very high in the Mega-Ohms, producing an open circuit and no circuit current flows. When the contacts are closed the contact resistance should be zero a short circuit, but this is not the case. All relay contacts have a certain amount of "contact resistance" when they are closed and this is called the "On-Resistance". With a new relay and contacts this on-resistance will be very small, generally less than 0.2Ω's because the tips are new and clean.

For example. If the contacts are passing a load current of say 10A, then the voltage drop across the contacts using Ohms Law is 0.2 x 10 = 2 volts. As the contact tips begin to wear, and if they are not properly protected from high inductive or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the contacts open. This arcing or sparking will cause the contact resistance of the tips to increase as the contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the point were they are physically closed but do not pass any or very little current.

If this arcing damage becomes to severe the contacts will eventually "weld" together producing a short circuit condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due to arcing to say 1Ω's the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty relay will have to be replaced.

To reduce the effects of contact arcing and high "On-resistances", modern contact tips are made off, or coated with, a variety of Silver based alloys to extend their life as given in the following table.

Relay manufacturers data sheets give maximum contact ratings for resistive d.c. loads only and this rating is greatly reduced for either AC loads or highly inductive or capacitive loads. In order to achieve long life and high reliability when switching AC currents with inductive or capacitive loads some form of arc suppression or filtering is required across the relay contacts. This is achieved by connecting a RC Snubber network in parallel with the contacts. The voltage peak, which occurs at the instant the contacts open, will be safely short circuited by the RC network, thus suppressing any arc generated at the contact tips. For example.

Relay Snubber Circuit

Relay Contact Types.

As well as the standard descriptions of Normally Open, (NO) and Normally Closed, (NC) used to describe how the relays contacts are connected, relay contact arrangements can also be classed by their actions. Electromechanical relays are made up of one or more individual switches with each "switch" being referred to as a Pole. Each one of these switches or poles can be connected or "thrown" together by energizing the relays coil and this gives rise to the description of the contact types as:

• SPST - Single Pole Single Throw
• SPDT - Single Pole Double Throw
• DPST - Double Pole Single Throw
• DPDT - Double Pole Double Throw

with the action of the contacts being described as "Make" (M) or "Break" (B). Then a simple relay with one set of contacts as shown above can have a contact description of:

      "Single Pole Double Throw - (Break before Make)", or SPDT - (B-M).

Examples of just some of the more common contact types for relays in circuit or schematic diagrams is given below but there are many more possible configurations.

Relay Contact Configurations

One final point to remember, it is not advisable to connect relay contacts in parallel to handle higher load currents. For example, never attempt to supply a 10A load with two relays in parallel that have 5A contact ratings each as the relay contacts never close or open at exactly the same instant of time, so one relay contact is always overloaded. While relays can be used to allow low power or computer type circuits to switch a relatively high currents or voltages both "ON" or "OFF". Never mix different load voltages through adjacent contacts within the same relay such as for example, high voltage AC (240v) and low voltage DC (12v), always use sperate relays.

One of the more important parts of any relay is the coil. This converts electrical current into an electromagnetic flux which is used to operate the relays contacts. The main problem with relay coils is that they are "highly inductive loads" as they are made from coils of wire. Any coil of wire has an impedance value made up of Resistance R and Inductance L in series (AC Circuit Theory). As the current flows through the coil a self induced magnetic field is generated around it. When the current in the coil is turned "OFF", a large back EMF (Electromotive Force) voltage is produced as the magnetic flux collapses within the coil (Transformer Theory). This induced reverse voltage value may be very high in comparison to the switching voltage, and may damage any semiconductor device such as a transistor, FET or microcontroller connected to the coil and used to control the relay.

One way of preventing damage to the transistor is to connect a reverse biased diode across the relay coil. When the current flowing through the coil is switched "OFF", an induced back EMF is generated as the magnetic flux collapses in the coil. This reverse voltage forward biases the diode which conducts and dissipates the stored energy preventing any damage to the semiconductor transistor. When used in this type of application the diode is generally known as a "Flywheel Diode". Other types of inductive loads which require a flywheel diode for protection are solenoids and motors.

As well as using Flywheel Diodes for protection of semiconductor components, other devices used for protection include RC Snubber Networks, Metal Oxide Varistors or MOV and Zener Diodes.

The Solid State Relay.

One of the main disadvantages of an Electromechanical Relay (EMR) is that it is a "mechanical device", that is it has moving parts. Over a period of time these parts will wear out and fail, or that the contact resistance through the constant arcing and erosion may make the relay unusable and it will therefore need to be replaced. Also, they are electrically noisy with the contacts suffering from contact bounce which may affect any electronic circuits to which they are connected. There is another type of relay called a Solid State Relay or (SSR) for short which is a solid state contactless, pure electronic relay. It has no moving parts with the contacts being replaced by transistors, thyristors or triacs. The electrical separation between the input control signal and the output load voltage is accomplished with the aid of an opto-coupler type Light Sensor.

The Solid State Relay provides a high degree of reliability, long life and reduced electromagnetic interference (EMI), (no arcing contacts or magnetic fields), together with a much faster response, as compared to the conventional electromechanical relay. Also the input control power requirements of the solid state relay are generally low enough to make them compatible with most IC logic families without the need for additional buffers, drivers or amplifiers. However, being a semiconductor device they must be mounted onto suitable heatsinks to prevent the output switching semiconductor device from over heating.

Example of a Solid State Relay.

The AC type Solid State Relay turns "ON" at the zero crossing point of the AC sinusoidal waveform, prevents high inrush currents when switching inductive or capacitive loads while the inherent turn "OFF" feature of thyristors and triacs provides an improvement over the arcing contacts of the electromechanical relays. Like EMR's an RC (Resistor-Capacitor) snubber network is generally required across the output terminals of the SSR to protect the semiconductor output switching device from noise and voltage transient spikes when used to switch highly inductive or capacitive loads and in most modern SSR's this RC snubber network is built as standard into the relay itself. Non-zero detection switching (instant "ON") type SSR's are also available for phase controlled applications such as the dimming or fading of lights at concerts, shows, disco lighting etc, or for motor speed control type applications.

As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching applications, or a Triac/Thyristor combination for AC switching), the voltage drop across the output terminals of an SSR when "ON" is much higher than that of the electromechanical relay, typically 1.5 - 2.0 volts. If switching large currents for long periods of time an additional heat sink will be required.

Input/Output Interface Modules.

Input/Output Interface Modules, (I/O Modules) are another type of solid state relay designed specifically to interface computers, microcontrollers or PIC's to "real world" loads and switches. There are four basic types of I/O modules available, AC or DC Input voltage to TTL or CMOS logic level output, and TTL or CMOS logic input to an AC or DC Output voltage with each module containing all the necessary circuitry to provide a complete interface and isolation within one small device. They are available as individual solid state modules or integrated into 4, 8 or 16 channel devices.

Example of a Modular Input/Output Interface System.

The main disadvantages of solid state relays (SSR's) compared to that of an electromechanical relay (EMR) are higher costs, only Single Pole Single Throw (SPST) types available, "OFF"-State leakage currents flow through the switching device, high "ON"-State voltage drop and power dissipation resulting in additional heatsinking requirements. Also they can not switch very small load currents or high frequency signals such as audio or video signals although Solid State Switches are available for this.