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| JEL |
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| Operating principle of SSR |
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 1.SSR for AC Loads (Zero-cross
type) |
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The Zero-crossing SSR uses
a phototransistor or photo trial coupler to isolate
the input from the output (see Fig1-1). When the
input signal is activated,the internal Zero-crossing
detector circuit triggers the trial to turn on
as the Ac load voltage crosses Zero. The load
current is maintained by the trial's Latching
effect after the input signal is Deactivated,
until the trial is turned off When the load voltage
crosses Zero. |
1-1)Resistive
loads
Since resistive loads cause no phase shift between
the voltage and current, the trial turns on when
the AC load voltage crosses Zero after the input
signal is activated. The SSR turns off when the
AC load voltage crosses Zero and the load current
is turned off after the input signal is sub sequently
deactivated. |
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 1-2)Inductive loads
The SSR turns on when the load voltage crosses
Zero after the input signal activated. It turns
off when the loads current subsequently cresses
Zero after the input signal is deactivated. A
phase difference between the voltage and current
may supply a transient spike to the SSR when it
is turned off. While the snuffer circuit allsorts
this spike, an excessively large spike may result
in a dv/dt error in the SSR's internal triad. |
 1-3)Capacitive loads
The SSR turn. On the load voltage crosses Zero
after the input signal activated. It turns off
when the load current subsequently cresses Zero
after the input signal is deactivated. A capacitive
loads draws on in rush current to charge the load
capacitor when the SSR turns en. Choose an SSR
so the peak of in rush current does not exceed
50% of the SSR surge-on current. SSR should have
a high marginal voltage ratings since a voltage
twice as high as the load supply voltage develops
across the SSR in the turn off process. |
| 2.SSR for DC Loads |
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The
SSR for DC loads uses a phototransistor Coupler
to isolate the input from the output. The output
immediately responds to the input. Since the phototransistor
coupler directly turns the output transistor ON
or Off.
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