2 . T Y P E S OF SWITCHES
2 . 1 Classification by USES
T h e y can be classified into three; power, selector and dig
ital switches.
a) P o w e r Switch
P o w e r switch connects or disconnects an A C power line
t o the primary winding of a power transformer. This switch
has been so designed to stand high heat to deal with a large
current which generates arcs and heat when switched.
T h e contact pieces should, therefore, be m a d e of a mate
rial which has a high melting point and should move quick
ly t o minimize sparks, and the gap between the contact
points should be wide enough to prevent the pieces from
melting a n d sticking. If the gap is narrow or the opera
t i o n is slow, the arc will continue for a long period. Fig.
3 shows a typical power switch. T h e contact points are
m a d e of silver cadmium oxide (AgCdO) and are operated
quickly with the help of a spring. All Pioneer products em
ploy power switches which have preliminary been approved
by the authorities of safety standards in each country.
M a j o r safety standards are U L (USA), C S A (Canada),
S E M K O (Sweden), D E M K O ( D e m m a r k ) , N E M K O (Nor
way), BS (GB), VDE (West Germany), EI (Finland) and
SEV (Switzerland).
Bs (Ag-plated)
Bs or Cu
AgCdO
Bs or Cu
- Bs (Ag-plated)
Terminals
Fig. 3 Contact Point of Power Switch
b) Selector Switch
Selector switch is a multiposition switch t h a t permits one
or m o r e conductors to be connected to one of other con
ductors. This is used in the following circuits to the secon
dary winding of a power transformer. Most switches are
for selecting a signal and some for switching a DC cur
rent. H e r e we will discuss the switches for selecting signal
channels. As to the switches for opening and closing D C
circuits and for sensing various conditions, refer to the item
" 2 . 2 Switches Classified by F u n c t i o n s " .
Switches for selecting signals usually employ the sliding
mechanism which scrubs and clean the contact surface by
itself because the signals to be selected are very low in volt
age, small in current and are highly affected by the resis
tive dust film. Figure 4 shows t h e mechanism of a linear
sliding signal selector.
The clip or moving piece is m a d e of silver-plated phos
p h o r o u s bronze and the fixed piece is silver-plated brass.
Low tenacity grease is applied o n the contact surfaces to
protect silver from being affected by the sulphur in the air.
The sulphur affects silver, forms a resistive film on the sil
ver surface and causes p o o r contact.
Terminals
Fig. 4 LINEAR Slider Type Signal Selector
There are shorting (make-before-break) and nonshorting
(break-before-make) types in this switch. Fig.5 shows the
difference. Type (a) once connects and type (b) once opens
all terminals of A , B and C when switching. Generally,
shorting type is used in muters, attenuators, mode-
selectors, etc. and nonshorting type is used in function
selectors, t a p e m o n i t o r s , etc.
Fig. 5 Shorting and Nonshorting Types of Contacts
c) Digital Switch
The switch exclusively used in a digital circuit is called
digital switch. This is also a kind of signal selectors.
Generally they are used in key input and d a t a circuits.
3
Clip
Conductive piece
