Reed Relay Protection

The life expectancy of reed relay (like those found on the NI PXI-2530 and NI SCXI-1130 multiplexers or the NI PXI-2532 matrix) can be extremely long compared to an armature relay. Life expectancy, however, can be greatly affected by the nature of the load. This document discusses reed relay behavior in different situations and ways to protect them.

Reed Relay Overview

A reed relay consists of a coil wrapped around a reed switch. The switch is composed of two overlapping ferromagnetic blades (called reeds) that are hermetically sealed within an inert-gas-filled glass capsule. When current flows through the coil, a magnetic field is produced that pulls the two reeds together. This completes a signal path through the relay. When the coil is de-energized, the spring force in the reeds pulls the contacts apart.



Reed relays have several advantages over traditional armature-based electromechanical relays. Reeds are generally smaller, allowing for greater density on a board. The lower mass of the reeds compared to larger armatures means they can actuate faster than electromechanical relays. Reed relay cycle speeds are as much as 10 times faster than electromechanical relays. Also, there is less actuation stress on a reed switch so that mechanical lifetimes of up to 1 billion cycles are possible. All of these advantages could also be achieved with solid-state relays (SSR). Reeds, however, have the added advantage of low contact resistance, allowing them to carry larger currents than SSRs. Reeds also have better linearity, bandwidth, and isolation than solid state relays.

Bounce

Armature and reed relay contacts “bounce” when they close. When closing, the contacts touch momentarily, making and breaking continuity, until finally remaining in the closed position. Figures 3 and 4 demonstrate relay bounce.Every switching system will have some inductance. When a relay opens a circuit with inductance, an arc occurs across the relay contacts, sometimes causing significant damage. The same physical characteristics of reed relays that provide the benefits mentioned above also result in some significant disadvantages. The small mass of the reed switch makes it easier to damage during arcing.

During the bounce phase, the first momentary closure starts current flow through the relay. As the contacts open, an arc forms that can melt part of the contact surface. If the molten contacts solidify in the closed position, a micro-weld may form, sticking the relay closed. The spring force of the reeds may not be sufficient to break this weld when the current stops flowing through the coil. Figure 5 shows a sequence of a relay bouncing and welding shut.
Another way to weld contacts is to send a large current through relays that are already closed. The non-zero contact resistance can heat up and cause the same welding phenomenon described above.


Obviously, such contact welding constitutes end-of-life for the relay.

Capacitance

Electrical lifetime specifications for reed relays almost always assume a resistive load. There is, however, always capacitance due to such things as board traces and external cables. When a relay closes, this capacitance acts as a transient short circuit until it is re-charged. These short circuits generate high inrush currents that are usually much larger than the steady state current. This high inrush current can damage the relay.

How much capacitance is too much? This is a very difficult question, and the simple answer is: “It depends on the application.” That is not a very satisfying answer, but relay vendors can not be more specific because many factors external to the relay affect inrush current.

A purely resistive system is shown in Figure 8. When the relay closes, current flows that is proportional to the source voltage V1 and the load resistance Rload. The current though the relay is shown in Figure 9.

A more realistic system is shown in Figure 10. The relay contact has a non-zero resistance, Rcontact and an output load capacitance, C. When the relay closes, an inrush current limited only by Rcontact, flows through the contacts to charge the capacitor. The inrush current may be much higher than the intended steady state current and will decrease to steady state according to the time constant defined by Rcontact and C. See Figure 11. Note that the peak inrush current is much higher than in the purely resistive example.
As current flows through the relay, power is dissipated in the contacts: Rarc is the resistance of the arc that exists between the relay contacts during bounce. This power will dissipate as heat, raising the temperature of the contacts. Inrush currents expose the contacts to very high momentary power levels. The energy associated with inrush current may be sufficient to melt the contact surfaces. After bouncing, when the contacts come back together they can weld shut. Inrush currents usually decrease rapidly to the steady state levels. Contact bounce, however, can subject the relay to multiple inrushes per activation, causing further damage.

Figure 12 shows a relay within a switch module connected to a voltage source and a load. Cint1, Cint2 and Rcontact will always produce an inrush current proportional to the input voltage. The duration of the inrush current increases (as will contact heating) with the addition of external capacitance at the load. Inrush current flows while load capacitance Cint2 and Cext are being charged.


Inrush Current Protection
To limit inrush current an impedance (e.g. a resistor) can be placed in series between Cint2 and Cexternal. This resistor acts to isolate the unwanted effects of the load capacitance and limit damage to the relay contacts. A protection resistance, Rp should be selected such that:
Figures 13 and 14 illustrate the use of the protection resistor Rp.



The energy associated with the inrush currents in these two circuits can be expressed as:

and:

Note that while the peak currents are the same, the associated energy for the protected circuit is 21 times less.

Board Layout and Topology

The open contact capacitance of a single reed relay is often very low (< 1 pF), and cannot store enough energy to damage the relay when switching at the rated voltage. The amount of capacitance across an open relay is directly related to the topology of the switch, the layout of the PCBs, and the loads connected to the module.

• Topologies with long traces and many connected relays, such as matrices, will have higher capacitances than simpler topologies.

• Most instrumentation switches have arrays of relays on a circuit board and a connector on one side. All of the traces must route to the connector. Dense, parallel routes on a PCB will increase capacitance between the traces.

• Connecting large capacitive loads directly to the switch terminals or connecting multiple terminals together can substantially increase the load capacitance.

Figure 22 illustrates the test system with a capacitance of 200 pF added to simulate cable capacitance. Also, the external load has been identified as the input to a high-impedance DMM. Note that even the input to a DMM presents a substantial capacitive load. The addition of the cable to connect the switch to the load increases this load further. The total load capacitance is 300 pF. Table 1 shows lifetime test results for a circuit with no protection and for a circuit with 200 ohms of protection resistance.

This test shows nearly a 10 to 1 increase in lifetime when a protective series impedance is used to isolate the switch from the capacitive load.

Conclusion

Reed relays are vulnerable to contact welding from high currents. Parasitic capacitance will unavoidably create high inrush currents. To minimize inrush current, care must be taken to decrease or isolate external capacitance. This can be accomplished by:

1. Using lower switching voltages.
2. Avoiding large capacitive loads to the switch.
3. Isolating external capacitance with series impedance Rp such that:

Reader Comments | Submit a comment »

This is a thorough and well done coverage of an occasionally-perplexing subject.
- Jan 5, 2004