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Making Solid State Relays — Parallax Forums

Making Solid State Relays

william chanwilliam chan Posts: 1,326
edited 2008-09-01 17:00 in General Discussion
Hey,

Anybody knows the best way to make a solid state relay from 2 mosfets?

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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-09-01 02:31
    Is it for AC or DC? Isolated? If DC and not isolated, high-side or low-side switching?

    -Phil
  • william chanwilliam chan Posts: 1,326
    edited 2008-09-01 02:33
    All solid state relays can handle AC right?

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    www.fd.com.my
    www.mercedes.com.my
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2008-09-01 03:13
    William,

    No, there are types made specifically for AC or DC. You definitely want to use the right one. Now, on the other hand, the AC units will typically run at voltages lower than what they state. DC units usually specify a range you want to stay in.

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    Chris Savage
    Parallax Engineering
  • william chanwilliam chan Posts: 1,326
    edited 2008-09-01 03:25
    This is where I don't understand....
    If you need only DC, why buy DC rated SSR? Why not just use a mosfet or any other transistor?

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    www.fd.com.my
    www.mercedes.com.my
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2008-09-01 03:50
    A MOSFET alone does not offer any isolation. Higher current SSRs often have thermal considerations in the design, whereas a MOSFET alone does not. A transistor will require drive current relative to what it is driving.

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    Chris Savage
    Parallax Engineering
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-09-01 03:54
    So-called "solid state relays" usually provide galvanic isolation. MOSFETs alone do not and can be configured for either low-side or high-side switcing. Switching AC nearly always entails isolation. SSRs that can switch AC usually use triacs instead of MOSFETs, but there are some that employ back-to-back MOSFETs in conjunction with a photovoltaic isolation stage. So ... if we knew more about what you're trying to accomplish, we might better be able to help.

    -Phil

    Addendum: One additional note regarding AC SSRs: These usually include a zero-crossing detector, which prevents the load from being switched mid-cycle and causing a current surge. Normally, such a capability is provided by triac-type SSRs and not those that employ back-to-back MOSFETs.

    Post Edited (Phil Pilgrim (PhiPi)) : 9/1/2008 4:40:03 AM GMT
  • william chanwilliam chan Posts: 1,326
    edited 2008-09-01 07:17
    What is low-side high-side switching?

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    www.fd.com.my
    www.mercedes.com.my
  • LeonLeon Posts: 7,620
    edited 2008-09-01 12:10
    Google?

    Leon

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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-01 17:00
    Phil makes an excellent point.... without Zero-Cross detection you subject your components to more wear and tear because of the inrush currents potentially involved.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
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