Building a Bi-Directional SPDT Solid-State Relay Circuit with MOSFETs

  • Thread starter Jdo300
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In summary, Jason is trying to create a SSR circuit using some MOSFETs and is wondering if it is ok to reference the same floating ground to both drivers. There are multiple circuit designs that use FETs in an bidirectional SPDT configuration, but the datasheet for the FETs he is using does not show the topology.
  • #1
Jdo300
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Hello All,

I have been trying to think of a way to make a Single-pole double-throw solid-state relay circuit out of some MOSFETs that I have laying around. I determied that I wanted to this SSR to be bidirectional so that it will behave as much like a mechanical relay as possible. I first started with a SPST version using two Mosfets with their source pins tied together and used as a ground reference for the driver IC, which has it's own floating power supply (battery) referenced to the same point independent of the load circuit. I then took it a step further and added another pair of MOSFETs and a driver so that the setup could be driven as the compliment of the first pair. I drew a circuit diagram showing my idea but I'm wondering if it is ok that both drivers are referenced to the same floating ground. Could someone take a look and let me know if this is a good way to do this?

Thanks,
Jason O
 

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  • #2
From what I can see the two MOSFETs connected to B are shorted out of the circuit by the common ground.

Just a guess, but you may want to look into H-Bridge circuits.

IIRC they do make a variant of a triac that can be turned off by control signal.
 
  • #3
Yeah I thought there was something funny there. Good catch. The triac idea sounds interesting but i really want to stick with the use of Fets because of the fast switching times. As for the H-Bridge design, that's an interesting approach as well but then I'm not sure how to make it bidirectional, and I'm not sure that the general setup woud be good for a bidirectional switch type of setup.

Are there any circuits out there that use FETs in an bidirectional SPDT configuration?
 
  • #4
You would make such a switch out of an analog switch topology, with a mix of n-channel and p-channel FETs. Here is one example of a SPDT analog switch from Maxim:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2561

Unfortunately, the datasheet doesn't show you the FET topology...

This one has a better internal schematic (although drawn kind of funny):

http://www.fairchildsemi.com/ds/MM%2FMM74HC4066.pdf

And this is the basic analog switch schematic that I was looking for (but ignore the "amp" at the bottom):

http://www.freepatentsonline.com/6577263-0-large.jpg
 
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FAQ: Building a Bi-Directional SPDT Solid-State Relay Circuit with MOSFETs

1. What is a SPDT solid-state relay?

A SPDT solid-state relay (Single-Pole Double-Throw) is an electronic switching device that uses solid-state components, such as transistors and diodes, to control the flow of electricity. It is designed to switch a single input signal between two separate output signals.

2. How does a SPDT solid-state relay work?

A SPDT solid-state relay works by using a control signal to activate an LED, which then triggers a photosensitive element, such as a phototransistor, to switch on or off the output circuit. This process is controlled by the input voltage and allows for high-speed switching without any mechanical components.

3. What are the advantages of using a SPDT solid-state relay?

One of the main advantages of using a SPDT solid-state relay is its faster switching speed compared to traditional electromechanical relays. It also has a longer lifespan due to the absence of moving parts, and it can handle higher voltage and current ratings. Additionally, it produces less noise and generates less heat, making it more efficient and reliable.

4. What are the applications of a SPDT solid-state relay?

SPDT solid-state relays are commonly used in various industrial and commercial applications, such as motor controls, lighting systems, temperature controls, and power supplies. They are also used in electronic devices, such as robots, medical equipment, and automotive systems.

5. How do I choose the right SPDT solid-state relay for my project?

When choosing a SPDT solid-state relay, consider the input voltage and current requirements, the output voltage and current ratings, and the switching speed. You should also consider the load type (AC or DC) and the required isolation level. It is recommended to consult with a technical expert or refer to the manufacturer's specifications for guidance in selecting the appropriate relay for your project.

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