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WHAT IS A RELAY?

RELAY HISTORY

Samuel Morse and Joseph Henry are the inventors of the telegraph. With the telegraph, communication over long distances began. Due to this distance, there was a loss of power in the signals. While looking for a solution to the weakening of these signals, the Relay was found by Joseph Henry (1835).

WHAT IS A RELAY?

A relay is a passive circuit element. That is, it becomes active when current flows through it. In this way they work in an electromagnetic system. It is a kind of switch that can electrically control the circuit. The main reason why relays are used in electrical and electronic circuits is that low currents and high currents can be controlled. Structurally, they consist of three parts. These are coil, spring and contact.

When current flows through the coil, a magnetic field is created and this magnetic field changes the position of the moving contact in the relay. The voltage value should be selected according to the circuit in which the coil will be used. This voltage value is written on the box of the relay. Generally, voltage values such as 5V, 12V, 24V, 120V, 220V are used. These contacts have two positions as NC (Normally Close) and NC (Normally Open). These contacts are resistant to high currents. Since they have more than one contact structure, they can control the opening and closing of more than one element at the same time. These current values are also written on the relay. The biggest problem is that moving contacts stick to fixed contacts over time. This problem is overcome with Solid State Relays (SSR).

RELAY TYPES

  • Motor Protection Relays
  • General purpose Mechanical Relays
  • Thermal Relays
  • Time Relays
  • Residual Current Relays
  • Voltage Protection Relay
  • Frequency Protection Relay
  • Solid State Relay
  • Smart Relay
  • Reed Relay

SOLIDE STATE RELAY (SSR)

Unlike other mechanical relays, SSRs are completely electronic. Since they are electronic, they do not have a contact, spring and coil structure as in mechanical relays. These structures are provided by electronic elements. Since there is no mechanical situation, the problem of sticking of the moving contact to the fixed contact at high currents does not exist in these relays. Therefore, they can operate at very high current values. (100-2000 A level) The control of the SSR is provided by an external circuit element.

One of the main differences of SSR from relays and contactors is that the power circuit can only operate in certain lower and upper voltage ranges according to the type of SSR. In mechanical relays and contactors, the opening and closing state is the contact of the moving contact to the fixed contact or not. This mechanical event is independent of the voltage on the contacts.
For example, a contactor may be switching any DC or AC system between 0.1 V and 600V in the power circuit. In SSR it is not possible to switch from such a wide voltage range.

SOME SSR SWITCHING RANGE VALUES

  • 24-380VAC
  • 12-65VDC
  • 90-420VDC

Since transistor-like electronic elements need a threshold voltage to operate, it is not possible to switch a voltage such as 0.1, 1, 2 V in SSRs as in mechanical contacts. If a voltage such as 1 V is desired to be switched in the SSR, the SSR can control a value such as 10-12 V at this value. Therefore, SSRs are not produced at these values. In addition, if the operating range of the SSR is to be increased, this will increase the cost.

If a power circuit is desired to be switched at a voltage value below the lower voltage values of the SSR, the SSR will not be damaged, but the SSR will not perform the switching function. If switching is desired with a voltage value above the upper voltage values of the SSR, the upper voltage limit that electronic elements such as transistors can withstand is exceeded. Therefore, the device will be damaged and become unusable.