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TM-499 Page 28
PART NAME CIRCUIT SYMBOLPICTUREFUNCTION
SCR A Silicon Controlled Rectifier (SCR) is an
electrical device with three connections,
anode, cathode, and gate. It will allow current
to flow in only one direction and will only do
so after receiving a signal on the gate lead.
SCR’s are used to change AC to DC and to
control the output to a load such as a welding
arc.
K
A
G
SHUNT A precision low-value resistor typically used
to supply a small voltage to drive an
ammeter.
SHUNT
SNUBBER A device used to absorb voltage spikes,
sometimes found connected across the
points of a relay.
SN1
SPARK GAP The arrangement of two electrodes between
which a discharge of electricity will occur.
Typically used to produce “ high frequency”
which can be used for arc starting when TIG
welding.
G
STABILIZER/
REACTOR
When placed in a DC circuit, the inductor or
stabilizer as it is usually called, will oppose
any change in existing current. It is therefore
widely used to “stabilize” the welding arc.
When the inductor is placed in an AC circuit it
is referred to as a reactor where it now acts
to restrict the flow of current. A “tapped
reactor” is used to create current ranges for
welding.
Z1
L1
SWITCH
A mechanical device that completes or
breaks the path of the current or sends it over
a different path.
SPDTSPST
A multi-pole switch will use dashed lines to
connect poles.
SPST = S
ingle Pole Single Throw
SPDT = S
ingle Pole Double Throw
DPST = D
ouble Pole Single Throw
DPDT = D
ouble Pole Double Throw
DPDTDPST
MOMENTARY
SWITCH
A switch that “springs back” to its original
position.
S1
LIMIT SWITCH A switch mounted in the path of a moving
object and actuated by its passage.
NRM OPEN NRM
CLOSED
S1
S1
PUSH BUTTON
SWITCH
A switch in which a button must be
depressed each time the contacts are to be
opened or closed.
NRM
OPEN
NRM
CLOSED
PB1 PB1
ROTARY SWITCH A switch in which its shaft must be rotated to
actuate the contacts.
S1 S1