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What is Reed Switch (Reed Switch) Dynamic Contact Resistance (DCR)?

- May 22, 2018 -

The dynamic contact resistance (DCR) measures the actual dynamic state of the switch. As previously described, the contact resistance is mostly composed of the resistance of the dry reed itself or the lead resistance. Measuring the resistance between the terminals of the reed switch can only roughly show whether the switch function is normal or not. To provide a more detailed description of the operating function of the switch, it is necessary to observe the operation of the reed switch in a dynamic situation, so as to be more accurate. Operating the switch in the frequency range of 50Hz to 200Hz can exhibit more detailed information. Switching the voltage of 0.5V and the current of about 50mA can detect potential problems. This kind of test can be realized by an oscilloscope, or digitized through some automatic test methods. Special attention should be paid to avoiding the test voltage exceeding 0.5 volt to avoid the phenomenon of “through”.

If a reed switch is not clean during the manufacturing process, several etched thicknesses of non-conducting thin layers will remain when switching very low signals or near no current (after any voltage or current flow through the reed Before the switch, let the reed switch close first. This thin layer is like an open circuit. When using a high voltage test, you may miss the opportunity to discover this potential problem.

When applied to a coil at a frequency of 50Hz to 200Hz, the switch will operate at approximately 0.5ms. This switch will oscillate for approximately 100μs and will experience a period of dynamic noise at 0.5ms. This dynamic noise is caused by the switch oscillating but not turning on As a result, the contact resistance changes correspondingly with the pressure experienced by the switch dry reed. During about 0.5 ms or less, the reed oscillating spring force suddenly decays.


A typical dynamic contact resistance waveform showing initial closure, oscillation, dynamic noise, and wobble swing

Once this dynamic noise disappears, the switch will experience a "wave period." At this point the switch is still closed but keeps oscillating for 1 ms or longer. When the switch oscillates in a magnetic field, it will generate current between the reeds. Once the oscillation is stopped, the switch will be in a static state.

The variety of wave types generated by dynamic tests can show the quality level of the reed switch. Usually, when the coil voltage is applied, the dynamic state of the switch should be stabilized within 1.5 ms. If the switch continues to oscillate for more than 250 μs, the closing force will weaken, resulting in a shorter switch life, especially if there is no load.

If the dynamic noise or oscillation duration is extremely long, this may indicate poor reed switch sealing or excessive stress. This condition may cause the glass tube to crack or break. If the oscillation amplitude is too large, it means that the glass tube may be subjected to excessive stress and may result in cracks. In this situation, external air and moisture may penetrate the glass tube, causing pollution on the switch.

At the same time, when the switch is continuously closed and the contact resistance is accompanied by small changes in chaos, this condition may be caused by contamination, cracks in the seal, loose plating or peeling, and may shorten the life of the reed switch.

Changing the frequency added to the coil can sometimes detect more resonance problems. An increase in the amplitude of the switching noise or ripple or a prolonged time will also show similar problems.

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