Application Tables and Guideline Data for Use of Electrical Contact Design
Contents
6.1 Application Ranges for Switching Contacts
6.1.1 Low and Medium Electrical Loads
Switching processes at low and medium electrical loads are experienced for example in relays and switches for the measuring technology, telecommunications, automotive usage, and appliances. The switching voltage ranges from μV to 400V with currents between μA and about 100A.
Main Article: Low and Medium Electrical Loads
6.1.2 High Electrical Loads
At high electric loads that usually occur in power engineering devices the switching phenomena are mostly related to arc formation. For most applications the management of the switching arc is the key problem. Depending on the device type different require-ments are dominant which influence the selection of the contact material. Similar to those in communications engineering, issues related to the switching characteristics and current path have to be considered.
Main Article: High Electrical Loads
6.2 Contact Materials and Design of Contact Components
The highest reliability and electrical life of electromechanical components and switching devices can only be achieved if both, the material selection and the design of the actual contact parts, are optimized. Economic considerations must of course also be applied when selecting the most suitable contact material and its way of application as an electrical contact. In the following Table 6.1 recommendations are made for selected application examples for contact materials and contact shape or configuration.
Table 6.1: Material Selection and Contact Component Design
| Type of Contacts or Devices | Characteristic Requirements for Contacts | Contact Material | Design Form of Contacts |
|---|---|---|---|
| Contacts for dry circuits | Reliable contacting at very low currents and voltages and mostly at also low contact forces |
AuAg alloys, (AuPt), Au | Contact rivets, welded miniature profiles (tapes), electroplated Au, sputtered Au layers |
| Switching contacts in measuring devices | Reliable switching at low voltages and currents at low contact forces | Au and Pt alloys, (AgPd alloys) | Contact rivets, welded tips, clad parts |
| Keyboard contacts | Defined contacting, close to bounce-free make, high reliability at low switching loads | Au alloys, (AgPd), Au on Ni substrate | Au plated snap discs, Au clad wires and stamped parts, hard gold electroplated contact spots on printed circuit boards |
| Rotary switches on printed circuit boards | Good frictional wear resistance, low contact résistance | Sliding track: hard gold on Ni substrate Slider: AgPd alloy, (Hard silver) | Electroplated coatings on slide tracks; clad, welded, or riveted stamping parts |
| Slip rings with high reliability | Low and consistent contact resistance at low contact forces | Brushes: Au alloys, AgPd, AgPdCu; Slip rings: Au alloys, Ag alloys (Rh); For higher currents: Ag/C brushes against Ag slip rings | Brush wires, stamped brushes; solid, clad, or electroplated slip rings, Ag/C formed parts |
| Sliding contacts in miniature motors | Very high frictional wear resistance, sure contacting even at very low contact forces | Ag and Au alloys, Pd alloys, Au multi component alloys | Brushes from flat rolled wire or stamped; collector hard gold electroplated or clad, or made from miniature profile segments |
| Centrifugal controllers for small motors | Little shape changes, defined contacting at very low contact forces and high frequency of operation | Pd alloys | Contact rivets, contact screws, welded parts |
| Connectors | Low contact resistance, corrosion resistance, sufficient frictional wear resistance, good sliding capabilities | Ag and Au alloys, Pd, PdNi; For automotive and consumer electronic at low operation numbers: Sn and Sn alloys | Electroplated layers or clad, often Au flash plated, mostly with Ni substrate layer, stamped parts from hot tin dipped strip |
| Telecommunication relays | Reliable contacting even at high operational frequency | Ag, AgPd, Au alloys, PdRu | Rivets, welded profile segments |
| Reed relay contacts | High reliability at low currents independent of atmospheric environment | Au, (Rh) | Switch paddles FeNi with partially diffused Au, (electroplated Rh) |
| Relays in electronic circuits | High reliability at low switching loads and compact device design | Au alloys, AgPd, Ag alloys | Stamped springs from seam-welded profiles, welded miniature profile (tape) segments, contact rivets |
| GP relais (Elementary relays) | Low arc erosion, high weld resistance, low and consistent contact resistance | Ag/Ni, Ag/SnO2, (Ag/CdO), Ag/ZnO,AgNi0.15, (Ag) | Solid and composite contact rivets, welded miniature profile (tape) segments |
| Automotive relays | Low material transfer, low contact resistance, high weld resistance | AgNi0.15, Ag/SnO2, Ag/Ni | Contact rivets, welded miniature profile (tape) segments |
| Flasher relays (automotive, > 3 Mio operations) | Low material transfer, high arc erosion resistance, low contact resistance | PdCu15 and 40 (Anode) vs. AgNi0.15, AgCu3 (Cathode), Ag/ZnO, Ag/SnO2 | Contact rivets, welded miniature profile (tape) and strip segments |
| Breaker points (automotive ignition) | Very high arc erosion resistance, high switching frequency | W | Tips or discs welded to formed parts or Fe supports |
| Automotive horn contacts | High arc erosion resistance at extremely high number of switching operations | W, Ag/SnO2 | Contact rivets, W weld buttons, springs or formed parts with brazed or welded tips |
| Appliance switches | Low contact resistance, reasonable arc erosion and weld resistance | AgNi0.15, Ag/Ni, Ag/SnO2, (Ag/CdO) | Contact rivets, welded contact parts |
| Temperature controllers (Thermostats) | Defined contacting point even at slow motion make, high operating temperatures | AgNi0.15, Ag/Ni, Ag/SnO2, (Ag/CdO) | Contact rivets, welded contact parts, weld buttons |
| Wiring devices (Light switches) | Low contact resistance, reasonable arc erosion and weld resistance | AgNi0.15, AgCu, Ag/Ni, with make peaks also Ag/ZnO, (Ag/CdO) | Contact rivets, welded contact parts |
Table 6.1: Material Selection and Contact Component Design (Fortsetzung - 2 Teile!)
| Type of Contacts or Devices | Characteristic Requirements for Contacts | Contact Material | Design Form of Contacts |
|---|---|---|---|
| Contacts for dry circuits | Reliable contacting at very low currents and voltages and mostly at also low contact forces |
AuAg alloys, (AuPt), Au | Contact rivets, welded miniature profiles (tapes), electroplated Au, sputtered Au layers |
| Switching contacts in measuring devices | Reliable switching at low voltages and currents at low contact forces | Au and Pt alloys, (AgPd alloys) | Contact rivets, welded tips, clad parts |
| Keyboard contacts | Defined contacting, close to bounce-free make, high reliability at low switching loads | Au alloys, (AgPd), Au on Ni substrate | Au plated snap discs, Au clad wires and stamped parts, hard gold electroplated contact spots on printed circuit boards |
| Rotary switches on printed circuit boards | Good frictional wear resistance, low contact résistance | Sliding track: hard gold on Ni substrate Slider: AgPd alloy, (Hard silver) | Electroplated coatings on slide tracks; clad, welded, or riveted stamping parts |
| Slip rings with high reliability | Low and consistent contact resistance at low contact forces | Brushes: Au alloys, AgPd, AgPdCu; Slip rings: Au alloys, Ag alloys (Rh); For higher currents: Ag/C brushes against Ag slip rings | Brush wires, stamped brushes; solid, clad, or electroplated slip rings, Ag/C formed parts |
| Sliding contacts in miniature motors | Very high frictional wear resistance, sure contacting even at very low contact forces | Ag and Au alloys, Pd alloys, Au multi component alloys | Brushes from flat rolled wire or stamped; collector hard gold electroplated or clad, or made from miniature profile segments |
| Centrifugal controllers for small motors | Little shape changes, defined contacting at very low contact forces and high frequency of operation | Pd alloys | Contact rivets, contact screws, welded parts |
| Connectors | Low contact resistance, corrosion resistance, sufficient frictional wear resistance, good sliding capabilities | Ag and Au alloys, Pd, PdNi; For automotive and consumer electronic at low operation numbers: Sn and Sn alloys | Electroplated layers or clad, often Au flash plated, mostly with Ni substrate layer, stamped parts from hot tin dipped strip |
| Telecommunication relays | Reliable contacting even at high operational frequency | Ag, AgPd, Au alloys, PdRu | Rivets, welded profile segments |
| Reed relay contacts | High reliability at low currents independent of atmospheric environment | Au, (Rh) | Switch paddles FeNi with partially diffused Au, (electroplated Rh) |
| Relays in electronic circuits | High reliability at low switching loads and compact device design | Au alloys, AgPd, Ag alloys | Stamped springs from seam-welded profiles, welded miniature profile (tape) segments, contact rivets |
| GP relais (Elementary relays) | Low arc erosion, high weld resistance, low and consistent contact resistance | Ag/Ni, Ag/SnO2, (Ag/CdO), Ag/ZnO,AgNi0.15, (Ag) | Solid and composite contact rivets, welded miniature profile (tape) segments |
| Automotive relays | Low material transfer, low contact resistance, high weld resistance | AgNi0.15, Ag/SnO2, Ag/Ni | Contact rivets, welded miniature profile (tape) segments |
| Flasher relays (automotive, > 3 Mio operations) | Low material transfer, high arc erosion resistance, low contact resistance | PdCu15 and 40 (Anode) vs. AgNi0.15, AgCu3 (Cathode), Ag/ZnO, Ag/SnO2 | Contact rivets, welded miniature profile (tape) and strip segments |
| Breaker points (automotive ignition) | Very high arc erosion resistance, high switching frequency | W | Tips or discs welded to formed parts or Fe supports |
| Automotive horn contacts | High arc erosion resistance at extremely high number of switching operations | W, Ag/SnO2 | Contact rivets, W weld buttons, springs or formed parts with brazed or welded tips |
| Appliance switches | Low contact resistance, reasonable arc erosion and weld resistance | AgNi0.15, Ag/Ni, Ag/SnO2, (Ag/CdO) | Contact rivets, welded contact parts |
| Temperature controllers (Thermostats) | Defined contacting point even at slow motion make, high operating temperatures | AgNi0.15, Ag/Ni, Ag/SnO2, (Ag/CdO) | Contact rivets, welded contact parts, weld buttons |
| Wiring devices (Light switches) | Low contact resistance, reasonable arc erosion and weld resistance | AgNi0.15, AgCu, Ag/Ni, with make peaks also Ag/ZnO, (Ag/CdO) | Contact rivets, welded contact parts |
Notes: Table 6.1 is meant to give suggestions for the use of contact materials for the specified devices. For most of the contact materials we deliberately did not indicate the exact composition and, as for Ag/SnO2 and AgZnO, did also not include specific additives. The final material composition depends on specific design parameters of the electrical device. Advise on the special properties of specific contact materials can be found in chapter 2.
6.3 Design Technologies for Contacts
A multitude of technologies is available and used for the actual manufacturing of contact components (see chapter 3). The desired contact shape however requires specific material properties like for example formability and weldability which cannot be fulfilled by all materials in the same way. In addition the design of the contact part must be compatible with the stresses and requirements of each switching device. The following table 6.2 combines contact design, contact material, and specific applications.
Table 6.2: Design Technologies for Contacts (2 Teile!)
Table 6.2: Design Technologies for Contacts (Fortsetzung) - (2 Teile!)
6.4 Formulas and Design Rules
6.4.1 Definition of Terms and Symbols
Note: The symbols for electrical contact specific terms (i.e. contact area, contact resistance, etc. have been retained from the german version of the Data Book. In related English literature some of them may vary using subscript symbols related to the language used – for example “contact resistance”: as used here from german Rk, in english mostly Rc.
Main Article: Definition of Terms and Symbols
6.4.2 Contact Physics – Formulas
Main Article: Contact Physics – Formulas
6.4.3 Closed Contacts
Fig. 6.5: Rough flat surface. a) before and b) during making contact with an ideally smooth flat surface; c) Schematic of the apparent, load bearing and effective contact areas (not to scale; dashed lines are elevation lines)
Fig. 6.6: Contact resistance of crossed rods as a function of the contact force for gold, silver and silver-palladium alloys
Table 6.3: Thermo-electrical Voltage of Contact Materials (against Copper)
6.4.4 Switching Contacts
Main Article: Switching Contacts
6.4.5 Physical Effects in Sliding and Connector Contacts
Main Article: Physical Effects in Sliding and Connector Contacts
6.4.6 General Rules for Dimensioning of Contacts
Main Article: General Rules for Dimensioning of Contacts
6.4.7 Contact Spring Calculations
Main Article: Contact Spring Calculations
References
Vinaricky, E. (Hrsg): Elektrische Kontakte-Werkstoffe und Anwendungen. Springer-Verlag, Berlin, Heidelberg 2002
Schröder, K.-H.: Grundlagen der Werkstoffauswahl für elektrische Kontakte. Buchreihe „Kontakt & Studium“, Band 366:zit. in „Werkstoffe für elektrische Kontakte und ihre Anwendungen“, Expert Verlag, Renningen, Bd. 366, (1997) 1-30
Horn, J.: „Steckverbinder“. zit. in Vinaricky, E. (Hrsg): „Elektrische Kontakte- Werkstoffe und Anwendungen“, Springer-Verlag, Berlin, Heidelberg 2002, 401- 419
Holm, R.: Electric Contacts, Springer-Verlag, Berlin, Heidelberg, New York 1967
Sauer, H. (Hrsg): Relais-Lexikon. 2. Aufl. Hüthig-Verlag, Heidelberg 1985
Greenwood J.A.: Constriction Resistance and the Area of Contact, Brit.J.Appl.Phys. 17 (1966) 1621
Biefer, H.: Elektrische Kontakte, Technische Rundschau (Bern) (1954/10) 17
Thielecke, K.: Anwendung von Kontakten in Schwachstromschaltern, in “Kontaktwerkstoffe in der Elektrotechnik”, Akademie-Verlag Berlin 1962, 107
Kirchdorfer, J.: Schalter für elektrische Steuerkreise, Blaue TR-Reihe, Heft 91, Verlag Hallwag, Bern und Stuttgart 1969
