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Connecting a Solid State Relay for DC Switching
Introduction:
In a MOSFET based relay, both AC and DC signals can be switched. The output utilizes two n-channel type MOSFETs. When controlling an AC load, one of the MOSFETs is used to switch the positive phase of the AC cycle while the other is used to switch the negative phase. When controlling a DC load, only one MOSFET is needed, and the relay can be configured to enhance switching characteristics.
Output Configurations:
Figures 1 through 3 show typical wiring diagrams for a 6 pin, 1 Form A relay, with an On Resistance of 35 Ohms, continuous rated Load Current of 120mA, and a package power dissipation of 500mW.
Figure 1 shows the load connected across pins 4 and 6. This option allows for both AC and DC signal switching. Whether switching AC or DC signals, the On Resistance and Continuous Load Current of the relay will be 35 Ohms (Maximum) and 120mA respectively.
Figure 1: AC/DC Configuration
Figure 2 shows the load connected across pins 4 and 5 (or pins 6 and 5). By wiring the relay in this configuration the user bypasses the second MOSFET, and will find that the On Resistance is between 20 and 25 Ohms instead of 35 Ohms.
Figure 2: Single MOSFET DC Configuration
It is possible to actually connect two loads to the relay utilizing this configuration as long as the loads share a common ground. If the relay is used in this manner (2 Form A), two items must be taken into consideration. First, both loads must be only DC and share a common ground. Second, the total power dissipated to the relay from both loads must not exceed the Package Power Dissipation of 500mW. For example, if the relay is used to switch two 50mA DC loads at room temperature (25°C), the total power dissipated through the package would be:
P = [(0.050A)2 X 25W] x 2 = 125mW
This value is well below the maximum of 500mW allowable, and the relay would function normally. For loads greater than 100mA, the relay would be dissipating over 500mW and normal performance is no longer guaranteed.
Figure 3 shows a third option for DC signal switching. By shorting pins 6 and 4, then connecting the load across pins 6 and 5, the On Resistance is reduced significantly.
Figure 3: Dual MOSFET DC Configuration
In this configuration, the On Resistance drops down to below 15 Ohms. The advantage of the lower On Resistance means that a higher load current may be dissipated by the package.
Conclusion
Any MOSFET based relay can switch a DC load. Through various configurations, the DC switching characteristics of the relay can be enhanced, even to the point of creating a 2 Form A relay.
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