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

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Discussion Overview

The discussion revolves around the comparison between solid state relays and MOSFETs or transistors as potential replacements for traditional relays in electronic circuits. Participants explore the differences in functionality, isolation capabilities, and component requirements.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Jason O questions the differences between solid state relays and MOSFETs or transistors, seeking clarification on why one might be preferred over the other.
  • One participant asserts that true solid state relays can handle AC and provide good isolation between the control signal and the switch, which a single MOSFET or BJT cannot achieve.
  • Another participant suggests that using two back-to-back MOSFETs could provide isolation, but this claim is challenged by a subsequent reply stating that this configuration does not accomplish isolation.
  • A participant proposes using a triac with a photodiode on the gate as an alternative, noting that it resembles a solid state relay.
  • Further elaboration indicates that a triac consists of two SCRs and requires additional components like opto-couplers for isolation, leading to a total of seven components, which contrasts with the original idea of a single part replacement.

Areas of Agreement / Disagreement

Participants express differing views on the capabilities of MOSFETs and triacs in providing isolation compared to solid state relays. The discussion remains unresolved regarding the best approach for replacing relays.

Contextual Notes

There are limitations regarding the assumptions made about isolation capabilities and the specific configurations of components discussed. The complexity of the proposed alternatives is also noted, which may affect their practicality as replacements.

Jdo300
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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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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.
 
but two back to back can.
 
light_bulb said:
but two back to back can.

Reread my post. This will not accomplish isolation.
 
what about a triac with a photodiode on the gate.
 
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.
 
Last edited:
Thanks for the clarification on that guys.
 

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