17-09-2012, 10:33 AM
SSR Technicalities
[attachment=33817]
What is a Solid State Relay?
An electronic switch made up of solid state
components.
No mechanical contacts or moving parts.
Basic Switching Element of the SSR
The SSR is powered by the AC
Line itself, by connecting the
2 gates of the output SCR’s
through a controlled switch.
When S1 is closed, current from the AC supply flows into
the gate of the forward biased SCR triggering it into
conduction.
As long as S1 is closed, this action continues, reversing
every half cycle of the AC supply.
When S1 is opened, the SCR presently in conduction will
continue to conduct until the zero current point is reached at
which time the SSR will be off.
Advantages of SSR’s over EMR’s
Unlimited Life when properly selected and applied
No moving contacts to burn, stick, arc, or bounce
Very low input current required
Very fast response time
Ability to switch at AC zero-cross point or randomly
Inherent characteristic of turn-off at zero current point
No audible noise
High surge current capability for severe inrush loads
Capability to >1500 Apk for 1 cycle.
Considerations for SSR Use
The Vf drop of the switching silicon will produce
internal heating that must be considered in the
system design.
Off-state leakage. In contrast to EMR’s, there is a
leakage current through the output in the off state of
typically 100 microamps to several milliamps
depending upon the specific model.
ZERO-CROSS SWITCHING
1. When the input signal is activated,
2. The internal zero-crossing detector
circuit triggers the output (Triac or
AC Switch) to turn on as the AC load
voltage crosses zero,
3. The load current is maintained by the
thyristors after the input signal is
deactivated,
4. The thyristor is turned off when the
load current crosses zero.
[attachment=33817]
What is a Solid State Relay?
An electronic switch made up of solid state
components.
No mechanical contacts or moving parts.
Basic Switching Element of the SSR
The SSR is powered by the AC
Line itself, by connecting the
2 gates of the output SCR’s
through a controlled switch.
When S1 is closed, current from the AC supply flows into
the gate of the forward biased SCR triggering it into
conduction.
As long as S1 is closed, this action continues, reversing
every half cycle of the AC supply.
When S1 is opened, the SCR presently in conduction will
continue to conduct until the zero current point is reached at
which time the SSR will be off.
Advantages of SSR’s over EMR’s
Unlimited Life when properly selected and applied
No moving contacts to burn, stick, arc, or bounce
Very low input current required
Very fast response time
Ability to switch at AC zero-cross point or randomly
Inherent characteristic of turn-off at zero current point
No audible noise
High surge current capability for severe inrush loads
Capability to >1500 Apk for 1 cycle.
Considerations for SSR Use
The Vf drop of the switching silicon will produce
internal heating that must be considered in the
system design.
Off-state leakage. In contrast to EMR’s, there is a
leakage current through the output in the off state of
typically 100 microamps to several milliamps
depending upon the specific model.
ZERO-CROSS SWITCHING
1. When the input signal is activated,
2. The internal zero-crossing detector
circuit triggers the output (Triac or
AC Switch) to turn on as the AC load
voltage crosses zero,
3. The load current is maintained by the
thyristors after the input signal is
deactivated,
4. The thyristor is turned off when the
load current crosses zero.