Relays are conceptually very simple devices. Energize a relay and two pieces of metal come together to complete a circuit. When the two pieces of metal come together, however, the environment is far from simple. The particulars of this metal to metal interface and the load level can determine whether the relay will work as intended or not. It is intuitive that switching higher than rated currents will damage relay contacts. It is not so obvious that switching very low current can have a detrimental effect. This paper discusses the affects of low current switching and how this affects relay performance.
Load current and voltage are important parameters when considering which relay to use in a particular application. Most relay vendors publish minimum current and voltage levels required for optimum switching performance. This is still not a guarantee for success, however. Switching very low currents and voltages can still, over time, cause increased contact resistance. This can become a big problem for a test system that is relying on relays to keep their normally low resistance values.
Inside of sealed relay housings there exist gases that are emitted from the plastics, adhesives, and etc. that go into making the relay. This material is organic and is adsorbed onto the contacts where it can then polymerize under these low voltage and current switching conditions. Low level contact loads do not provide enough energy to form an arc sufficient to clean off the contacts. Instead polymerization occurs which leaves behind a powdery substance on the contacts causing the resistance to rise. Switching voltage/current, and contact material also contributes to the severity of this problem. Higher voltages and currents ensure that switching arcs are of sufficient energy to clean deposits from the contact surfaces. Also, some contact materials inhibit the formation of organic deposits and some materials promote it.
Problematic Use Case Example (2-Wire, Low Value Resistance Measurement using a DMM)
It is not always apparent when minimum voltage or current specifications are being violated. This is the case for 2-wire resistance measurements. Different DMMs use different current source values to perform this measurement. The PXI-407x series of DMM uses a 1mA source when measuring low resistance values. This is in a current range that can cause the issue discussed above.
Higher resistances are less of a problem even though the measurement current will be lower. This is because the increased contact resistance is still much lower as a percentage of the resistance to be measured.
The best way to avoid creating high resistance contacts is to read the relay module specifications carefully. Most relays (all armature relays) should have minimum voltage and current specifications. Make sure that your application will be above these values.
Use reed relays if possible for low current and voltage applications. Because of the inert atmosphere surrounding the contacts, they typically don’t have this type of increased contact resistance behavior.
2. Use a Different Type of Measurement Device
To increase the current used when making 2-wire resistance measurements a precision power supply like the PXI-4110 can be used. It is possible to source a higher current (e.g. 10mA) and then read the power supply output voltage. Combining these two values will yield the resistance. This method will help ensure that the minimum voltage or current is met or exceeded. In the event of high contact resistance the current can be increased until the relay contacts are clean again. This current should not exceed 100mA.
3. Use 4-Wire Resistance Measurement
Performing 4-wire resistance measurements eliminate the effects of contact resistance. This is the preferred method for doing low resistance measurements.
If switching very low currents with armature relays is unavoidable, there are still things that can be done.
1. Increase the Switching Current
As stated above, increasing the switching current can clean the adsorbed/polymerized material from the contacts. The amount of current will vary depending of relay size. For small signal relays this could be as little as 10mA. There is no one correct answer for this value. The lowest value that can clean the contacts should be used. A current that is too high can cause other problems that also degrade the contact resistance.
2. "Buzz” the Relay
Another technique for cleaning contacts is to “buzz” them. This method actuates the problem relay as fast as possible for about one second. The act of rapidly cycling the relay knocks deposits from the relay contacts. This method has the advantage that no fixtures or other measurement devices are needed.
Neither of these two methods is a permanent solution and may need to be performed periodically. Every application is different and it is not possible to say in advance how many switch cycles can be performed before maintenance is required. When this is determined, the relay operations counter function found on most National Instruments switch products can be used to predict future cleaning cycles.
Increased contact resistance of armature relays is a well known phenomenon. By choosing the right relay for the application can eliminate of minimize these effects. Even if the problem can’t be totally avoided there are ways to work around the issue.