Amplifier Relay DC to Low Frequency High Gain Signal and Power

Ever since the discovery of electricity and applying it in communications, the need for amplifying useful signals became apparent. As the length of the telegraph lines increased the signal progressively became weaker. The telegraph companies developed methods that ranged from simply increasing the battery voltage or to use repeater stations to allow the signals ever-longer reach.

Figure 1. Power and Signal Relays

Perhaps the simplest and most often used, amplifier device is a special electromagnet outfitted with one or more contacts and is named a relay Figure 1. The relay is one of the oldest electromechanical devices invented about the year 1835 to be used as a repeater in the then new Morse telegraph. As its name implies, the useful signal gets relayed, and passed on amplified through the device. The weak incoming signal coming from a telegraph wire is connected to a cylindrical coil of fine copper wire that is wound around a soft iron core Figure 2.

A coil like that is often referred to as a solenoid and called a field coil. As the voltage appears across the two terminals of the solenoid, the energized field coil becomes an electromagnet. When a magnetic field builds inside the electromagnet its iron core attracts a moveable iron piece named armature that is nearby and is attached to one or more sets of contacts. The movement of the armature closes a pair of contacts that are connected to an outgoing telegraph wire that now carries a much stronger copy of the incoming signal faithfully.

Figure 2. A signal amplifier relay in a long transmission line

A relay with many turns on its solenoid field coil is very sensitive and its contacts can be made to easily switch a lot of current. A relay can pull in with as little as a mA [milli-Ampere - a thousandth of an Ampere] of current while the contact currents can be many hundreds of amperes. In any amplifier the gain of the device is the ratio of the output current to the input current. If a relay pulls in with 1 Amperes current and its contacts switch 100 Amperes than its gain is 100. If the same relay is sensitive enough to pull in with 1 mA, than its gain are a thousand times more or 100,000! Relays used to start automobile engines and other heavy machinery switches 4-500 Amperes reliably for a long time.

Relays come in a large number of different sizes both physically and electrically Figure 2. They are made for a large number of different voltages and activating and output switching currents. Telephone company central offices and their field equipment used many special versions of relays. Some of them have a magnetic bias, others have mercury-coated contacts to increase the switching current and the life of the contacts, again others are super sensitive to minute amounts of currents.

Figure 3. 120 VAC Relay with Dual Set 10 A Contacts

The sensitivity of any relay is mostly dependent on the number of turns its coil. The more turn a coil has the smaller current it needs for activation. Unfortunately, the more turns a coil has the longer and finer the wire it uses and the larger is the wire's resistance. Larger coil resistance requires higher voltage to pass the same current trough the device so there is only so far that turn numbers and sensitivity can be increased.

Relays are terrific DC [Direct Current] amplifiers. A problem with relays as amplifiers is that being electromechanical devices they can only follow slow signals. Past a few hundred Cycles-per-Seconds [cps or Hertz] other types of amplifiers are needed. Far the best devices were for awhile the electron or vacuum tubes. Some specially built vacuum tubes useful for amplifications for very high frequencies. Newer solid state transistor amplifiers are even faster but they don't have the power handling capabilities of the tubes so where power is an issue, electron tubes still rule. Many different amplifier devices were designed in the past century. Some are quite exotic and little used. Others like magnetic amplifiers are simple and still often used. For DC and very low frequency pulse amplification relays are still often used for their simplicity and reliability.