Which Switch Is Which?

DN Staff

January 2, 2014

2 Min Read
Which Switch Is Which?

My girlfriend's travel trailer has several 12V interior light fixtures of a style widely used in the industry. Replacement fixtures are widely available, and I found out why.

These fixtures are designed to use type 1141 lamps, although most folks replace with 1156 lamps for a little extra brightness -- 27W vs. 18W. That is, the lamps draw 2.1A or 1.4A. The fixtures incorporate a standard slide switch, which fails regularly. In searching the Internet for switches, I learned that they are made in a variety of current ratings. The highest DC rating is 0.5A DC. No wonder they fail frequently and replacement fixtures are so widely available, most folks replace the fixture instead of the switches.

This is a clear misapplication of the switch. Not only is it underrated by 300 or 400%, but a slide switch is the worst possible choice for the application. A snap-action switch would be much better.

The resistance of Tungsten filament lamps varies with filament temperature. A cold filament has nearly zero resistance. As the filament heats, the resistance increases until an equilibrium is reached. The initial current is several times the final 1.4A or 2.1A. As the user is sliding a slide-type switch it continually arcs across the entire contact surface. A snap-action switch is preferred for this type of application, even considering the effects of contact bounce.

The second design flaw in this lamp is the plastic diffuser lens. They scorch and melt from the heat of the lamp. Apparently this is a frequent issue as these are also readily available from many sources.

I resolved these problems by replacing the lamps with LEDs made with the same 15 mm bayonet base. The models I got were at least as bright and only draw 0.4A within the rating of the switch. Furthermore, the resistance surely does not vary with temperature, eliminating the damage caused by large inrush currents.

Tell us your experiences with Monkey-designed products. Send stories to Lauren Muskett for Made by Monkeys.

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