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Solid State Relays Application Data
SECTION 4
Magnecraft Solution Guide 105A
Magnecraft Solution Guide 105A
Magnecraft Solution Guide 105A
Definition:
A SSR (solid state relay) can perform many tasks that an EMR (electromechanical relay) can perform. The
SSR differs in that it has no moving mechanical parts within it. It is essentially an electronic device that relies on the
electrical, magnetic and optical properties of semiconductors, and electrical components to achieve its isolation and
relay switching function.
Principle of Operation:
Solid State Relays are similar to electromechanical relays, in that both use a control circuit
and a separate circuit for switching the load. When voltage is applied to the input of the SSR, the relay is energized
by a light emitting diode. The light from the diode is beamed into a light sensitive semiconductor which, in the case
of zero voltage crossover relays, conditions the control circuit to turn on the output solid state switch at the next zero
voltage crossover. In the case of nonzero voltage crossover relays, the output solid state switch is turned on at the
precise voltage occurring at the time. Removal of the input power disables the control circuit and the solid state
switch is turned off when the load current passes through the zero point of its cycle.
Applications:
Since its introduction the SSR, as a technology, has gained acceptance in many areas, which had
previously been the sole domain of the EMR or the Contactor. The major growth areas have come from Industrial
Process Control applications; particularly heat/cool temperature control, motors, lamps, solenoids, valves, and
transformers. The list of applications for the SSR is almost limitless.
The following are typical examples of SSR applications: industrial automation, electronic appliances, industrial
appliances, packaging machines, tooling machines, manufacturing equipment, food equipment, security systems,
industrial lighting, fire and security systems, dispensing machines, production equipment, on-board power control,
traffic control, instrumentation systems, vending machines, test systems, office machines, medical equipment, display
lighting, elevator control, metrology equipment, and entertainment lighting.
Advantages:
When used correctly in the intended application, the SSR provides many of the characteristics that are
often difficult to find in the EMR; a high degree of reliability, long service life, significantly reduced electromagnetic
interference, fast response and high vibration resistance are significant benefits of the SSR. The SSR has no moving
parts to wear out or arcing contacts to deteriorate, which are often the primary cause of failure with an EMR.
Thermal Considerations:
One of the major considerations when using a SSR is properly managing the heat that is
generated when switching currents higher than about 5 amps. In this scenario one must mount the base plate of the
SSR onto a good heat conductor, typically aluminum; along with utilizing a good thermal transfer medium such as
thermal grease or heat transfer pad. Using this technique, the SSR case to heat sink thermal resistance is reduced to
a negligible value of 0.1 C/W.
INDUSTRIAL
AUTOMATION
ALARM
SYSTEMS
ELECTRONIC
APPLIANCES
PACKING
MACHINES
MEDICAL
EQUIPMENT
INDUSTRIAL
APPLIANCES
TOOLING
MACHINES
Long life (reliability) > 10
9
operations
Zero voltage turn on, low EMI / RFI
Shock and Vibration resistant
Random turn-on, proportional control
No contact bounce
Arc-less switching
No acoustical noise
Microprocessor compatible
Fast response
No moving parts
Solid State Relays Application Data
4/2 Magnecraft Solution Guide 105A
Magnecraft Solution Guide 105A
Magnecraft Solution Guide 105A
Thermal Calculations:
To understand the thermal relationship between the output semiconductor junction (T
J
) and
the surrounding ambient temperature (T
A
) one has to look at the temperature gradient or drop of temperature from
junction to ambient (TJ - TA); which simply equals the sum of the thermal resistances multiplied by the junction
power dissipation.




T
J
- T
A
= P
(R
JC
+ R
CS
+ R
SA
)
Where

T
J

= Junction Temperature, C
T
A

= Ambient Temperature, C
P
= Power Dissipation (I
LOAD
X E
DROP
) watts
R
JC

= Thermal resistance, junction to case, C/W
R
CS
= Thermal resistance, case to sink, C/W
R
SA
= Thermal resistance, sink to ambient, C/W
To use the equation, the maximum junction temperature of the semiconductor must be known, typically 125 C, along
with the actual power dissipation. When these two parameters are known, the third can be found as shown in the
following examples:

1
.) Determine the maximum allowable ambient temperature, for 1 C/W heat sink and 10 amp load (12

watts) with a maximum allowable junction temperature (T
J
) of 100 C and assume thermal resistance from

junction to case (R
JC
) of 1.3:




T
J
- T
A
= P
(R
JC
+ R
CS
+ R
SA
)



T
A

= T
J
28.8




= 12 (1.3 + 0.1 + 1.0)
hence,

= 100 28.8





= 28.8





= 71.2 C

2
.) Determine required heat sink thermal resistance, for 71.2 C maximum ambient temperature and a

10 amp load (12 watts):




R
SA
= T
J
- T
A





P





= 100 - 71.2





12





= 1 C/W

3
.) Determine maximum load current, for 1 C/W heat sink and 71.2 C ambient temperature:




P
= T
J
- T
A




I
LOAD
= P












E
DROP





= 100 71.2

hence,

12





1.3 + 0.1 + 1.0



1.2





= 12 watts





= 10 amperes
Load Considerations:
The major cause of application problems with SSRs is improper heat sinking. Following
that, are problems which result from operating conditions which specific loads impose upon an SSR. The surge
characteristics of the load should be carefully considered when designing in an SSR as a switching solution.
Resistive Loads:
Loads of constant value of resistance are the simplest application of SSRs. Proper thermal
consideration along with attention to the steady state current ratings will result in trouble free operation.
(R
JC
+ R
CS
)
(1.3 + 0.1)
(R
JC
+ R
CS
+ R
SA
)
(R
JC
+ R
CS
+ R
SA
)
(R
JC
+ R
CS
+ R
SA
)
SECTION 4
4/3 SECTION 4
Magnecraft Solution Guide 105A
Magnecraft Solution Guide 105A
Magnecraft Solution Guide 105A
DC Loads:
This type of load should be considered inductive and a diode should be placed across the load to
absorb any surges during turn off.
Lamp loads:
Incandescent lamp loads, though basically resistive, present some special problems. Because the
resistance of the cold filament is about 5 to 10 percent of the heated value, a large inrush current can occur. It
is essential to verify that this inrush current is within the surge specifications of the SSR. One must also check that
the lamp rating of the SSR is not exceeded. This is a UL rating based on the inrush of a typical lamp. Due to the
unusually low filament resistance at the time of turn-on, a zero voltage turn on characteristic is particularly desirable
with incandescent lamps.
Capacitive Loads:
These types of loads can also prove to be problematic because of their initial appearance as
short circuits. High surge currents can occur while charging, limited only by circuit resistance. Caution must be
used with low impedance capacitive loads to verify that the di/dt capabilities are not exceeded. Zero voltage turn
on is a particularly valuable means of limiting di/dt with capacitive loads.
Motors and Solenoids:
Motor and solenoid loads can create special problems for reliable SSR functionality.
Solenoids have high initial surge currents because their stationary impedance is very low. Motors also frequently
have severe inrush currents during starting and can impose unusually high voltages during turn off. As a motors
rotor rotates, it creates a back EMF that reduces the flow of current. This back EMF can add to the applied
line voltage and create an over voltage condition during turn off. Likewise, the inrush currents associated with
mechanical loads having high starting torque or inertia, such as fans and flywheels, should be carefully considered
to verify that they are within the surge capabilities of the SSR. A current shunt and oscilloscope should be used to
examine the duration of the inrush current.
Transformers:
In controlling transformers, the characteristics of the secondary load should be considered because
they reflect the effective load on the SSR. Voltage transients from secondary loads circuits, similarly, are frequently
transformer and can be imposed on the SSR. Transformers present a special problem in that, depending on the state
of the transformer flux at the time of turn off, the transformer may saturate during the first half-cycle of subsequently
applied voltage. This saturation can impose a very large current (10 to 100 times rated typical) on the SSR which
far exceeds its half cycle surge rating. SSRs having random turn on may have a better chance of survival than a
zero cross turn on device for they commonly require the transformer to support only a portion