Solid state switch or MOSFET: Which is the better relay replacement?

In summary, the conversation discussed the use of solid state switches/relays as replacements for traditional relays on boards. The difference between a solid state relay and something like a MOSFET or transistor was explained, with the main advantage of a solid state relay being its ability to pass AC and offer good isolation between the control signal and the switch. The suggestion of using a triac with a photodiode on the gate was also brought up, but it was noted that this would require multiple components and may not be as efficient as a true solid state relay.
  • #1
Jdo300
554
5
Hi, I've been hearing people talk about using solid state switches/relays as replacements for relays on boards and I was wondering if someone could explain to me the difference between a solid state relay and something like a MOSFET or transistor. Why not just use one of those?

Thanks,
Jason O
 
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  • #2
A true solid state relay is able to pass AC and offers very good isolation between the control signal and the actual switch. A single MOSFET or BJT cannot offer this.
 
  • #3
but two back to back can.
 
  • #4
light_bulb said:
but two back to back can.

Reread my post. This will not accomplish isolation.
 
  • #5
what about a triac with a photodiode on the gate.
 
  • #6
Yeah I suppose, I've never looked into it. The original argument was:

Hi, I've been hearing people talk about using solid state switches/relays as replacements for relays on boards and I was wondering if someone could explain to me the difference between a solid state relay and something like a MOSFET or transistor. Why not just use one of those?

So I'm going to stick with that. A triac with a photodiode on the gate sounds a lot like a solid state relay which the OP was questioning the replacement of with a single transistor. A triac is basically 2 SCRs back to back. An SCR is basically 2 transistors arranged in a fashion so they latch on until current is interupted. To provide the isolation needed it takes an opto-coupler which is usually 2 devices, but usually the output is 2 transistors in darlington configuration. So, let's count up the devices. One triac consists of 4 transistors, 2 more transisors for the photo darlington pair, and an LED. That's 7 individual components. Quite a stretch from the OPs idea of a single part.
 
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  • #7
Thanks for the clarification on that guys.
 

1. What is the difference between a solid state switch and a MOSFET?

A solid state switch is a semiconductor device that is used to control the flow of current in a circuit, whereas a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of solid state switch that is used for amplifying or switching electronic signals.

2. Which one is better for high power applications?

In general, MOSFETs are better suited for high power applications due to their low on-resistance and high current handling capabilities. However, the choice ultimately depends on the specific requirements and design of the circuit.

3. Which one is more efficient?

MOSFETs tend to have higher efficiency compared to other types of solid state switches. This is because they have a low on-resistance and can operate at high frequencies, resulting in less power loss and improved efficiency.

4. Are there any limitations of using a solid state switch or MOSFET?

One limitation of using a solid state switch is that it can only handle a limited amount of current and voltage. Similarly, MOSFETs have a maximum voltage and current rating that they can operate within. It is important to choose the right type and size of switch for the specific application to avoid exceeding these limitations.

5. Can a MOSFET be used as a solid state switch?

Yes, a MOSFET can be used as a solid state switch. In fact, MOSFETs are commonly used in solid state switches due to their fast switching speeds, low on-resistance, and high efficiency.

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