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MEI DN series

DN series is a type of compact metal contact keyboard switch from Mechanical Enterprises. The switches were offered in two basic configurations: open chassis (DN500) and fully enclosed (DN600). DN500 switches still had fully enclosed switch contacts, but the slider and return spring were not enclosed in a shell; these switches were PCB-mount only. The D600 enclosed type supported plate mounting.

Thus far, DN series has yet to be encountered. However, DN500 has NSN 5930-01-268-6004.

Design

The 1983 advertisement in EEM notes:

From top of PC board to top of double shot keytop, DN switches measure less than 15mm — easily meeting the new standards for low profile terminal keyboards.

Although DIN norms are not specifically mentioned, it is fairly clear that this is a DIN-compliant design.

The tall version (D600) uses a “keytop coupling”, which presumably extends the slider height, as the switch in its bare form is very low profile. Where Cherry M8 had separate sliders for the uncovered and covered versions, it appears that D600 is simply an adapted D500.

Operation

The switch operation is covered in European patent EP0084331A1 “Controlled floating contactor switch”, filed on 1983-01-07 with a priority date of 1982-01-18, and granted on 1985-08-14. The patent only describes the contact assembly itself and does not depict a complete switch. The 1983 advertisement contains a clear illustration of the switch contact operation, but the scan of the page (or some prior document reproduction) has lost most of the detail; even so, it can be seen that DN series (at least, at that point in time) opted for the version depicted in figure 6 with dual ball contacts.

In brief, one or two metal balls (two in the catalogue depiction) are held in a silicone rubber tube, surrounded by metal rods, also within the same tube. Pressing against the centre of the rubber tube brings the two balls into contact with the ends of the metal rods, bridging the two rods electrically. When releasing the switch, the tube returns to its unstretched, undeformed state and moves the balls away from the the rods. (Concave ends to the rods maintain a gap between the rods and the balls.) The elasticity and compressibility of the rubber tube accommodates ovetravel and avoids excessive contact pressure. The choice of two balls prevents the rubber tube from getting pinched in between the contact surfaces by the actuator, as the pressure is applied at the tube in between the balls.

The balls are silver plated for good electrical characteristics; they are prevented from oxidising by the rubber tube that contains them.

Specifications

The DN series specifications are shown below alongside the specifications for the later T-15S (sealed contact) switches. In most cases, DN series is equal or better, but T-15S has increased travel and 33% greater longevity (20 million instead of 15 million cycles).

Characteristic DN series T-15S series (momentary)
Contact rating, DC resistive 3 W max 1 W max
Voltage 24 V max
Initial contact resistance 20 mΩ max 100 mΩ max
Plunger travel 0.135″ (3.43 mm) 3.8 mm
Pretravel 0.70±0.25″ (1.78 mm) 1.9±0.6 mm
Operating force to make 60±15 gf 65±20 gf
Temperature range, operating −20°C to 75°C −20°C to 65°C
Temperature range, storage −40°C to 85°C
Contact bounce 3 ms max 5 ms max
UL flammability rating 94-HB
Life, actuations 15×10⁶ 20×10⁶

Literature