861SSRA208-DC-2 Magnecraft / Schneider Electric, 861SSRA208-DC-2 Datasheet - Page 26

• Manufacturer Part Number: 861SSRA208-DC-2
• Description: Modular Style
• Manufacturer: Magnecraft / Schneider Electric

Specifications of 861SSRA208-DC-2

• Control Voltage Range: 3 VDC to 32 VDC
• Load Voltage Rating: 24 VAC to 280 VAC
• Load Current Rating: 8 A
• Contact Form: SPST - NO
• Mounting Style: DIN Rail
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Data
26
(continued)
Magnecraft
Thermal Considerations
One of the major considerations when using a SSR is properly managing the heat
that is generated when switching currents are higher than 5 A� In this scenario
mount the base plate of the SSR on a good heat conductor, such as aluminum, and
use a good thermal transfer medium, such as thermal grease or a 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.
Thermal Calculations
To understand the thermal relationship between the output semiconductor junction (T
and the surrounding ambient temperature (T
drop of temperature, from junction to ambient (T
thermal resistances multiplied by the junction power dissipation�
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 example:
1) Determine the maximum allowable ambient temperature, for a 1 ˚C/W heat sink
and a 10 A load (12 watts) with a maximum allowable junction temperature (T
of 100 ˚C, and assume a thermal resistance from junction to case (R
2) Determine the required heat sink thermal resistance, for 71.2 ˚C maximum
ambient temperature and a 10 amp load (12 watts):
3) Determine maximum load current, for 1 ˚C/W heat sink and 71.2 ˚C ambient temperature:
Where: T
T
R
P
J
өSA
- T
A
= P (R
= 12 (1.3 + 0.1 + 1.0) hence:
= 28.8
= T
= 100 - 71.2 - (1.3 + 0.1)
= 1 °C/W
=
=
= 12 watts
T
P
R
R
R
A
J
(R
өJC
өCS
өSA
J
1.3 + 0.1 + 1.0
өJC
P
T
- T
= Junction Temperature, °C
= Ambient Temperature, °C
= Power Dissipation (I
= Thermal Resistance, junction to case, °C/W
= Thermal Resistance, case to sink, °C/W
= Thermal Resistance, sink to ambient, °C/W
12
100 - 71.2
J
өJC
- T
+ R
®
A
T
J
- (R
A
+ R
Solid State Relays
- T
өCS
= P (R
өJC
A
өCS
+ R
+ R
+ R
өSA
өJC
өCS
өSA
)
A
+ R
) measure the temperature gradient, or
)
)
hence:
J
- T
өCS
A
); which equals the sum of the
+ R
LOAD
өSA
I
T
3 E
LOAD
A
)
= T
= 100–28.8
= 71.2 °C
DROP
=
= 10 amperes
J
E
1.2
) watts
–28.8
12
өJC
DROP
P
) of 1�3:
J
)
J
)