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Introduction to solid state relays and the advantages and disadvantages

Solid State Relay (SSR), compared with electromechanical relays, and is not a mechanical movement, the relay does not contain moving parts, but it has the electromechanical relay is essentially the same functions. SSR is an all-solid-state electronic components of the non-contact switching elements, he points the use of electronic components, magnetic and optical properties to complete a reliable input and output isolation, using high-power transistor, power MOSFET, single controllable silicon and the triac switching characteristics of such devices, to achieve non-contact, non-charged spark to the circuit on and off.

Solid state relay has three parts: the input circuit, isolation (coupling) and the output circuit. An input voltage of the different categories, the input circuit can be divided into the DC input circuit, the AC input circuit and the AC and DC input circuit three. Some of the input control circuit also has a TTL / CMOS-compatible logic control and anti-equal positive and negative features. Solid state relay input and output circuits are isolated and the optical coupling and coupling transformer coupling two. Solid state relay output circuit can be divided into direct current output circuit, output circuit and the AC-DC AC output circuit in the form. AC output, usually with two thyristor or a triac, DC can use bipolar output devices or power FET.

Advantages and disadvantages of solid state relays

1, the advantages of solid-state relay

(1) high life, high reliability: SSR no mechanical parts, complete the contact device features a solid, no moving parts, so in high shock and vibration of the working environment, composed of solid state relays as components inherent characteristics, determine the solid-state relays, long life, high reliability.

(2) high sensitivity, small power control, electromagnetic compatibility: solid state relay wide input voltage range, low drive power, compatible with most logic integrated circuits or drive without additional buffers.

(3) fast conversion: Because the use of solid during solid-state relay, so the switching speed can be from a few milliseconds to a few subtle.

(4) Electromagnetic interference laugh: solid state relay does not enter "coil" and did not touch the light arc and rebound, thus reducing electromagnetic interference. Most AC output solid state relay is a zero-voltage switching at turn-on at zero voltage, zero current at turn-off, reducing the sudden interruption of current waveform, thus reducing the switching transients.

2, the shortcomings of solid-state relays

(1) After conducting tube pressure drop, phase control thyristor or silicon-forward step-down of up to 1 ~ 2V, high-power transistor saturation pressure slurry disaster 1 ~ 2V, and the average power FET The shoot-Zu are more mechanical contact of the contact resistance.

(2) The semiconductor device is shut off, can still be a few microamps to several milliamps of leakage current, it can not achieve the desired electrical isolation.

(3) The pipe pressure drop, power consumption and after the conduction of heat is also large, high-power solid-state relays with capacity far greater than the volume of the electromagnetic relay, and higher cost.

(4) the temperature characteristics of electronic components and electronic circuit of the poor anti-interference ability, resistance to radiation is also poor, if not take effective measures, the work of low reliability.

(5) solid-state overload relay for greater sensitivity, must be fast fuse or RC snubber circuit to protect them too. Solid state relay load and ambient temperature was significantly related to temperature, load capacity will decline rapidly.

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