OVERLOAD PROTECTION SWITCH Schematic Diagram: Functionalit: The electronic overload switch is a fast-acting circuit breaker. It trips faster fuses and mechanical circuit breakers. O!eration: The schematic diagram for an electronic overload switch is shown in Fig. 20. The curr ent at which this cir cuit inter rupts power to th e load is pres et by adjustment of potentiometer s . !hen the switch "# is closed$ the pulsating direct current from the bridge recti%er & # through & ' charges capacitor ( # through # . The voltage on ( # maintains a steady current through 2and &)$ which is su*cient to trigger the "(early in each input half cycle. +s a result$ the current ,ows through the load over almost #0 0 for each halfcycle. nce the "(is triggered all load current ,ows through /. If the current eceeds a certain value 1# +mp. for top setting of /the regenerative latch T #$ T2is trigger ed which prevents the %ring of "(from net half cycle and thus interrupt ing power to the load. The charge on ( #holds the latch in full conduction so that the "(cannot be re-triggered on succeeding input half cycles until the switch " #is opened long enough for (#to discharge completely and then closed again. 3ecaus e the load is connected on the dcside of the bridge recti%er$ this overload switch should be used only with ac/dcloads such as heaters and universal PART LIST C" )0uF 1#) voltsD"# D$ 34 #25 F Fuse #.)+ R" #0k 1#0!R% 220 hms #67 ! R&' R( '50 hms 8 ! R$ 0.) hms 12!R) )0 hms 1!.! potmeterSCR "9 #0' S" 96FF switch T" 3( #)5
Functionality:The electronic overload switch is a fast-acting
circuit breaker. It trips faster fuses and mechanical circuit
breakers.Operation: The schematic diagram for an electronic
overload switch is shown in Fig. 20. The current at which this
circuit interrupts power to the load is preset by adjustment of
potentiometer Rs. When the switch S1 is closed, the pulsating
direct current from the bridge rectifier D1 through D4 charges
capacitor C1 through R1. The voltage on C1 maintains a steady
current through R2 and D5, which is sufficient to trigger the SCR
early in each input half cycle. As a result, the current flows
through the load over almost 1800 for each half cycle.Once the SCR
is triggered all load current flows through R6. If the current
exceeds a certain value (1 Amp. for top setting of R6) the
regenerative latch T1, T2 is triggered which prevents the firing of
SCR from next half cycle and thus interrupting power to the load.
The charge on C1 holds the latch in full conduction so that the SCR
cannot be re-triggered on succeeding input half cycles until the
switch S1 is opened long enough for C1 to discharge completely and
then closed again.Because the load is connected on the dc side of
the bridge rectifier, this overload switch should be used only with
ac/dc loads such as heaters and universal motors. Incandescent laps
can be protected against overload provided that the potmeter R6 is
turned down first increasing the tripping current to allow for the
stating transients caused by the low cold resistance of lamp from
tripping the switch.
