Hot Switching Relays

Hot switching is a term used to describe operations where a relay is either opened or closed while carrying a user signal. It is a parameter that can have a major impact on relay life, a relay used in a hot switching environment will experience contact erosion and heating that does not occur in a cold switching environment where the user signal is removed before changing relay state.

Relay vendors do provide information on ratings, but they have to provide these ratings under reproducible conditions which usually means they are specified with resistive source and loads. When relays are placed in switching systems other issues can arise, the following describes some of those issues.

Capacitive Hot Switching

It is not always obvious to users what level of hot switching is experienced by relays - even in relatively low current or voltage conditions. If a low impedance source is connected to a high impedance load (therefore implying a light loaded hot switch environment) but the cabling or load contain significant parasitic capacitance then on contact closure the relay can experience a high surge current as the source charges the capacitor. High surges can also be experienced if the relay connects a source to a capacitive load that is carrying a charge from a previous state. This might occur for example in a system where polarity reversal is required to be made by a relay system, or if a previous operation has left a charge on a high impedance load. For this reason some systems that test cable assemblies provide a mechanism for discharging any residual charge on the cable assemblies. 

Switching modules normally only define a maximum hot switch voltage/current/power which is characterized into a resistive load. So it should be noted that if long cables or other capacitive loads are attached the rating maybe affected. 

Switching systems are also typically naturally capacitive at their terminals, if a matrix has a large number of closures on a single Y axis to X connections the capacitance can create an extra load which effect relay life when hot switching. 

Resistance in the cable wiring and the PCB tracks can help reduce the surge currents involved and therefore increase life, however this effect declines rapidly with higher voltages. 

Power Supply Charge Exchange and Current Limiters

One potential misunderstanding on hot switch ratings is when a power supply is used to connect a power supply to a load with the power supply already generating voltage. The load will commonly have local decoupling capacitors and the sudden connection of the power supply can result in high inrush currents. This current is NOT limited by a current limit on the power supply since they usually arise from exchange of charge from the power supply output decoupling capacitor to the load decoupling capacitors. 

In addition current limiters on power supplies rely on a feedback mechanism to limit current, the current is sensed and the limiter starts to reduce the output voltage when the set threshold is reached. The time taken for the limiter to operate means that it cannot protect a hot switched relay from excessive current when it switches into an unexpected to load condition. Only a resistor will limit the current, and this is the test condition used when testing relays for hot switching capacity.  (see also: Life testing relays)

Some relays do have ratings for these high inrush events which are above the normal rating of the relay, these relays will be typically designed with contacts which have a large wiping action on closure and use high temperature materials in their contact construction. These types of relay exhibit higher levels of contact resistance with variation in current (resistance reducing with increasing current) than relays designed for small signal applications. Solid state relays are generally considered to be more robust for hot switching power supplies since their ability to handle inrush currents between capacitors is often more than an order of magnitude greater than their steady state current handling. (see also: Minimum switching capacity)

Hot Switch Failure on Closure Mechanism

The most common mechanical relay hot switching failure mechanism is either welded contacts or contacts having variable or intermittent contact resistance, particularly at low currents, because of erosion of the contact materials. Welded contacts are usually caused by high inrush currents as the contacts are closed creating molten or soft metal in the contact area. Failure to close can be caused by severe erosion of the contacts and the build up debris on the contacts. 

Hot switching can also induce resonant waveforms in switching systems, the sudden increase in current and change of voltage levels can excite any resonant circuits created in the traces and the external connections to the switching system since the closure event can be very abrupt. This is of very short duration compared to the operating time of the relay. 

Hot Switch Failure on Opening Mechanism

The most common cause is either misunderstanding the relays specification or switching an inductive load. 

When a relay opens in the presence of an inductive load the opening event is sudden, the sudden cessation of current creates a high voltage spike as the field in the inductor is discharged. On mechanical relays this can create multiple break events or the creation of a continuous arc.
Mechanical relays often have a rapidly declining current rating as voltage increases, in the presence of an inductive load this decline is significantly worse and the exact nature of the degradation can be difficult to quantify. 

On solid state relays the change in current is usually not as abrupt as on mechanical relays, but inductive loads still create spikes which might cause failures due to overvoltage breakdown exceeding the rating of the SSR transient power handling. Data sheets for SSR hot switching can include a maximum allowed switching energy, usually expressed in milliJoules (mJ). The energy stored in an inductive load can be calculated from 0.5*L*I*I, where L is the inductance and I is the current being carried. 

Hot Switch Failure on Opening Due to Arcing

In applications where high powers are being switched by a mechanical relay an arc (plasma) can be generated which increases the time during which contact erosion can occur. The arc is particularly damaging if the load or source contains a significant inductive component as it can create multiple arc's over a period of time as the contact opens. 

Operating a power relay frequently in a short space of time can cause additional problems as the arc generated leads to a general increase in the relay temperature, the more often the arc is generated in a given time the more erosion occurs and the more heat is generated. For this reason many high power mechanical relays are life tested at relatively low cycle rates, particularly as they are used towards their maximum rating. The more frequent an arc is generated the higher the temperatures that are reached leading to more rapid contact erosion and the damaging of mechanical parts in the relay, particular the plastic parts that hold the contacts in place. Some relays require the package to include a vent which is open to the environment when used to break a high current/voltage signal at regular intervals to help dissipate the plasma that forms in the relay case. 

Arcing is particularly a problem on DC signals because the arc can be sustained for a long time. The trace below was taken by loading a relay with 30V and an excess carry current, the relay has then broken the connection (a hot switch opening event). An arc is created as the relay contacts part and a gap is created (on very high current relays this creates a visual arc even thru the plastic casing). The arc creates a plasma which continues to conduct until it cools sufficiently. In this example (where the relay is being used beyond its rating) the arc endured for 1 second before cooling enough to be extinguished.

30V hot switch arc

While the arc was in progress an intermediate current flows, implying the relay package has to dissipate significant power for the arc duration.
This type of arcing event does not occur on solid state relays.

RF Hot Switching

A slightly different problem can occur when hot switching RF signals. If the source VSWR is high (so not a 50 Ohm source impedance) then a relay in the open position could have very high voltages appearing across an open contact before it closes or a high voltage is created as the relay is opened. The source VSWR being high allows reflected signals to build up a high voltage, insertion loss will reduce the maximum voltage by attenuating the reflected signals.

As the operating conditions have a large impact on the hot switching conditions for RF signals the hot switch power is normally quoted with a low source VSWR unless otherwise stated. If the relay is operated with poor source/load VSWR the hot switch performance will degrade, the degree to which it degrades being highly dependent on the conditions. The most common reason for high RF powers is that the source is a transmitter (or amplifier) and it is usual for these to have high output VSWR to prevent loss of power in the output source impedance.

Power Limit

The relay specification often contains a power limit number. In the case of DC signals it should be noted that this often is limited to a single voltage/current. As voltage increases beyond this the DC power the relay can handle may decrease rapidly.

Difference Between AC and DC

The hot switch rating of power relays is often different between DC and AC switching, the AC power ratings being higher. In DC switching metal transfer tends to be in one direction so erosion of the contacts accumulates more quickly. In AC applications the metal transfer occurs in both directions and arcs may be suppressed as the signal voltage falls towards zero with both effects extending the relay life.

When handling AC signals derived form the AC supply reactive components (inductors and capacitors) can be problematic and reduce the relay hot switch capacity. AC supply systems typically have many reactive components in them (filters, transformers) which can create issues. Inductive sources or loads can create prolonged arcing when breaking a signal, capacitive loads can create high inrush currents on path closure.

Minimum Capacity

It should also be noted that hot switching some types of EMR is required to achieve the best contact resistance stability where there is a minimum switching capacity stated. The minimum switch capacity can be quite low and for example residual cable charges and capacitance in a system can provide the required surge to clean contacts.

Relays intended primarily for RF switching can also exhibit problems after frequent DC hot switch operations. The operation can remove gold from the contacts and make low level RF switching more variable in loss and frequency dependency, especially at lower frequencies. Where possible it is best to avoid DC switching in RF applications.

Solid State Relays

Solid state relays can offer "infinite" lifetimes when used within their ratings, they are not subject to the arcs and contact problems that are found on mechanical relays. Frequent operation of these relays (above hundreds of Hz) can generate additional heat in the switching process. The faster the switching occurs the less heat that is generated. Solid state relays are a good solution for applications where long life is needed in a hot switch environment.

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