MICRO SWITCH General Technical Bulletin No. 14 APPLYING PRECISION SWITCHES General Technical Bulletin #14 - Applying Precision Switches TABLE OF CONTENTS INTRODUCTION......................................................................................................................................................................1 BRIEF HISTORY OF THE PRECISION SWITCH ..............................................................................................................1 I. THE MECHANICAL CHARACTERISTICS OF A PRECISION SWITCH...................................................................5 A. MECHANICAL ACTION AND TERMINOLOGY..........................................................................................................................5 B. THE RELATION BETWEEN PLUNGER FORCE AND PLUNGER POSITION....................................................................................6 C. THE RELATION BETWEEN CONTACT FORCE AND PLUNGER POSITION....................................................................................8 D. CONTACT BOUNCE AND TRANSIT TIME................................................................................................................................9 E. POLES, THROWS AND BREAKS............................................................................................................................................10 II. THE ELECTRICAL CHARACTERISTICS OF A PRECISION SWITCH................................................................11 A. THE RESISTANCE OF AN OPEN SWITCH...............................................................................................................................11 B. CAPACITANCE OF AN OPEN SWITCH...................................................................................................................................13 C. SWITCHES AND RADIO FREQUENCY INTERFERENCE (RFI)..................................................................................................13 D. THE RESISTANCE OF A CLOSED SWITCH.............................................................................................................................14 III. SWITCH RESISTANCE ..................................................................................................................................................15 A. SOURCES OF SWITCH RESISTANCE .....................................................................................................................................16 B. TYPICAL PATTERN AND MAGNITUDE OF SWITCH RESISTANCE............................................................................................16 C. WHY SWITCH RESISTANCE BEHAVES AS IT DOES................................................................................................................18 D. THE ROLE OF CONTAMINATION IN SWITCH RESISTANCE.....................................................................................................19 Solder Flux......................................................................................................................................................................................20 Crushed Solids ................................................................................................................................................................................20 Carbon.............................................................................................................................................................................................20 Polymer ...........................................................................................................................................................................................21 Chemical Compounds of Contact Material and Foreign Material ...................................................................................................21 Condensed Metal Vapor..................................................................................................................................................................21 Effect of Silicone on Electrical Contacts.........................................................................................................................................21 Migration of Silicones .....................................................................................................................................................................22 Wear Particles .................................................................................................................................................................................22 E. CHOICES OF CONTACT MATERIAL.......................................................................................................................................22 F. HOW SWITCH DESIGN AFFECTS RESISTANCE.......................................................................................................................24 G. HOW TO MEASURE SWITCH RESISTANCE............................................................................................................................25 Voltage ............................................................................................................................................................................................25 Current ............................................................................................................................................................................................25 The Measuring Circuit.....................................................................................................................................................................26 Switch Plunger Position ..................................................................................................................................................................26 Sequence of Operations in Measuring Switch Resistance ...............................................................................................................27 H. WHEN TO GIVE SWITCH RESISTANCE SPECIAL ATTENTION.................................................................................................27 I. HOW TO BE CONSTRUCTIVELY CAUTIOUS ABOUT SWITCH RESISTANCE...............................................................................27 IV. STABILITY OF MECHANICAL OPERATING CHARACTERISTICS...................................................................29 A. NORMAL VARIATION OF OPERATING AND RELEASE POINT..................................................................................................29 B. HOW STABILITY IS AFFECTED BY FACTORS IN THE APPLICATION........................................................................................29 Selection of the Switch....................................................................................................................................................................29 Mounting the Switch .......................................................................................................................................................................29 Soldering Lead Wires to the Terminals ...........................................................................................................................................30 Actuation.........................................................................................................................................................................................30 Electrical Load ................................................................................................................................................................................30 Summary of Facts About Operating and Release Point Versus Electrical Load..............................................................................32 Acceleration, Vibration and Shock..................................................................................................................................................32 Temperature and Humidity..............................................................................................................................................................32 Contamination .................................................................................................................................................................................32 C. HOW TO STABILIZE OPERATING AND RELEASE POINT IN END USE.......................................................................................32 For application help: call 1-800-537-6945 Honeywell • MICRO SWITCH Sensing and Control i General Technical Bulletin #14 - Applying Precision Switches V. SWITCH LIFE ................................................................................................................................................................... 34 A. THE NATURE OF SWITCH LIFE............................................................................................................................................ 34 Contact Welding .............................................................................................................................................................................34 Contact Material Migration.............................................................................................................................................................35 Contact Erosion...............................................................................................................................................................................35