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overview:
The Keep It Clean brand LED illuminated switches, series SW15, SW18 and SW19, manufactured by Miyama / Mountain Switch, are somewhat atypical illuminated switches: they have four terminals (illustration 1) instead of the typical three terminals in similar switches. The two LED terminals are isolated from the two SPST switch terminals, and the LED terminals only 'make' internal contact when the switch is mechanically closed.
 the equivalent circuit: LED terminals only 'make' when the switch is mechanically closed.
The switch is rated for AC applications; however, the LED is a DC device with an integrated current-limiting resistor for DC 12V automotive and similar applications. Installing the switch must accommodate power to the LED. For AC applications, the power terminals to the LED must be to a 12V DC source.
Per the distributor's documentation, the positive terminal of the LED (pin LED+ in illustration 1) can be connected in parallel to the input terminal (pin 2). This is suitable for MOST 12V automotive applications with a current draw of 4 Amps or less (schematic A). For 24V DC (such as nautical or industrial / commercial vehicle) applications, an appropriate current limiting resistor must be applied in series to pin LED+. A 750Ω 1/2 Watt resistor will be adequate.
The switch is rated for 10 Amps at 120V AC inductive / 4 Amps DC 'lamp' as per the manufacturer markings on the switch (and regardless of the vendor's diagram above, which lists a 15 Amp fuse in a DC circuit). It is recommended that the switch be used to control a relay for applications over 4 Amps (schematic B). Applications using a relay should have a transient voltage suppression diode across the coil to prevent power spikes from damaging the relay or erratically switching the devices controlled by the relay.
If the switch application involves 'logic level' switching or high power transistor switching, it is recommended that the application designer consult an electrical engineer with experience in automotive electronics.
 schematic A: typical low-current application.
 schematic B: typical high-power application with a relay and TVS diode.
connection:
The switch terminals are 'standard' 3/16" (0.187", 4.75mm) quick-connect. The LED terminals are solder lugs, but can be 'quick-connected' with TE Connectivity #42765-1 connectors (available from Digi-Key or LWE's eBay outlet). These connectors as manufactured by TE Conn are the right width but a bit too 'tall' for the LED terminals, and must be 'pre-crimped' to reduce the internal height. (These connectors, purchased through LWE with a series SW19 switch, will be pre-crimped to the appropriate height.)
The wiring for the switch contacts should be at least 18ga stranded copper for lengths of less than 6 feet (2m). Cables intended for passage through an engine compartment firewall MUST BE of a type with the appropriate SAE rating for high temperature applications.
The wiring for the LED terminals can be 22ga or 24ga, as these will be installed inside the cabin and will conduct no more than 20 mA.
 the TE Conn 42765-1 connector, for the LED terminals.
 a SW19 switch with quick-connects, crimped and wrapped.
installation:
The SW15 / ~18 / ~19 series switches are designed for panel mount, and can be mounted into panels up to 3/16" (4.75mm) thick. The panel cutout (illustration 2) is a 13/16" (20.64mm) round hole with 3/4" (19mm) anti-rotation detents. If cutting the anti-rotation detents is impractical, the switch can be installed with a size M18 toothed lock washer to prevent rotation. Note that this option reduces the workable panel thickness to 1/8" (3.17mm).
Ideally, this installation would be done with a template and the appropriate cutting and milling tools.
  important: Do note the orientation of the switch before installing. The switch lever is in the 'closed' position when it is in the position where the top of the switch is toward the LED terminals (illustration 3). Since the cutout is symmetrical with no 'keyed' detent, it is possible to install this switch UPSIDE DOWN.
 illustration 2: cutout and bezel dimensions for SW19.
 illustration 3: orientation of switch lever to LED terminals. Switch is in 'closed' position, LED is ON.
illustrations 1 and 3: Keep It Clean Wiring, www.keepitcleanwiring.com/
image of TE Conn 42765-1 connector: DigiKey, www.digikey.com
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