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Mos relays
Electro-mechanical relays are electrically operated switches. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Electro-mechanical relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first electro-mechanical relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Electro-mechanical relays were used extensively in telephone exchanges and early computers to perform logical operations.

Reed relays are reed switches enclosed in a solenoid. The switch has a set of contacts inside an evacuated or inert gas-filled glass tube which protects the contacts against atmospheric corrosion; the contacts are made of magnetic material that makes them move under the influence of the field of the enclosing solenoid. Reed relays can switch faster than larger relays, require very little power from the control circuit. However Reed relays have relatively low switching current and voltage ratings. Though rare, the reeds can become magnetized over time, which makes them stick 'on' even when no current is present; changing the orientation of the reeds with respect to the solenoid's magnetic field can resolve this problem.

Solid state relays  are solid state electronic component that provides a similar function to an electro-mechanical relay but does not have any moving components, increasing long-term reliability. Solid state relays have a small voltage drop across it. This voltage drop limits the amount of current a given SSR can handle. The minimum voltage drop for such a relay is a function of the material used to make the device. Solid state relays rated to handle as much as 1,200 amperes have become commercially available. Compared to electromagnetic relays, they may be falsely triggered by transients.