Cold switching is a term used to describe the operation of a switch (relay) where there is no significant user signal when the relay contacts are open or closed. When the relay is carrying the user signal, the relay has a current flowing at the moment when the contact is closed. When the switch is open, the current will be interrupted.
The cold switching of mechanical relays is significantly reduced compared to thermal switching, which maximizes the service life of the relay. For solid state relays, there is usually no difference between the hot and cold switching ratings.
Cold switching can also have different (higher) voltage and current ratings than the thermal switching rating because it does not cause arcing, metal migration, and arc induced radio frequency interference (RFI) at the relay contacts. While thermal switching can be used to switch high power applications through mechanical relays, an arc (plasma) can be generated, which increases the time at which contact erosion can occur. If the load or power supply contains important inductive components, the arc will have a particularly severe effect, as more arcs may be generated over a period of time when the contacts are disconnected.
However, when the time interval from the application of the voltage to the measurement must be strictly controlled, thermal switching is required. Cold Switching In some designs, if the applied voltage has a high voltage rise rate coupled to the relay control system, there are large and rapid voltage changes that can interfere with the relay control system. For example, using a relay outside of a cold switching rated switching system can produce a very high rate of voltage change (1000 μW/μs) that propagates through the signal path and is coupled to the control system. When using cold switching ratings, it is recommended to manage the voltage rise time to avoid these voltage transients. Therefore, when digital logic is involved, since the state of the device may change even if the signal is interrupted instantaneously, hot switching is usually required.
For relatively large relays, thermal switching may also be necessary to ensure a good closure. If there is no current through the "wet" action of the contacts, the connection may be unreliable.
Cold switching can extend contact life and maximize contact life by 10 to 100 times. Cold switching avoids accidental break-before-break problems (short-circuits between devices). It also reduces transients when switching sensitivity loads (DUTs or instruments) as well as capacitive loads.
Mechanical relay
Cold switching is significantly less than hot switching, which maximizes the life of the relay. It does not cause arcing, metal migration, and radio frequency interference (RFI) caused by arcing at the relay contacts.
solid state relay
Provides an "infinite" life when used within its rating, independent of arcing and contact problems found on mechanical relays. Frequent operation of these relays (above hundreds of Hz) may generate additional heat during the switching process. The faster the heat switch, the less heat is generated. Solid state relays are a good solution for applications that require long life in a hot switching environment. At the same time, there is usually no difference in cold switching ratings. So for solid state relays, hot and cold switching is possible.
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