[TriEmbed] MOSFET or Relay (Brian)

Martin Brooke martin.brooke at gmail.com
Tue Dec 8 10:04:55 CST 2015


couple of additional details

Relays click when they work.  Drove docs in operating room crazy in one
system I built.

Vds can be made as low as you want.  You just need a "big" enough MOSFET.
You may have to use PMOS or NMOS or both (CMOS) devices if you want to
switch close to the power supply.

MOS FET devices all have some small leakage current when off check the data
sheets.  You get what you pay for with this.  Generally relays are very low
leakage unless they get dirty.

Cheers,

---

Martin Brooke
Associate Professor,
and Philip Baugh Scholar
Department of Electrical and Computer Engineering
Box 90291
Duke University
Durham, NC 27708-0291

Phone: (919) 660-5504
Secretary:  (919)-660-5252
Fax:   (919)-660-5293
E-Mail: mbrooke at ee.duke.edu
On Dec 8, 2015 7:00 AM, <triembed-request at triembed.org> wrote:
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>    1. Re: MOSFET or Relay (Brian)
>    2. Re: MOSFET or Relay (Brian)
>
>
> ---------- Forwarded message ----------
> From: Brian <triembed at undecidedgames.net>
> To: triembed at triembed.org
> Cc:
> Date: Mon, 7 Dec 2015 10:44:54 -0500
> Subject: Re: [TriEmbed] MOSFET or Relay
> Adam's responses are good.  One more considration:  Voltage drop.  Even
in saturation ("on"), a semiconductor will have a current-independent
voltage drop (known as Vsd) across it that will likely be a good bit*
larger than the drop across the relay's contacts.
>
> To summarize:
>
> MOSFET -
>  - Fast switching (nanoseconds)
>  - Very low drive current (gate current)
>  - Very small "on" resistance (Rds - 10s of mOhms)
>  - Fairly large "on" voltage drop (Vsd - ~1.5 V)
>
> RELAY -
>  - Slow switching (micro- to milliseconds)
>  - Much larger drive current (coil current)
>  - Larger "on" resistance (contact resistance - 100s of mOhms)
>  - Negligible "on" voltage drop
>
>
> Personally I'd rather have seen amazing strides on the Philadelphia
Experiment!
>
> -B
>
> On 12/7/2015 8:41 AM, Grawburg via TriEmbed wrote:
>>
>> Just for my future information. Presuming that current draw is not an
issue, given the choice between using a MOSFET or a suitably opto-isolated
relay to switch 12V, is there any significant advantage of one over the
other? The MOSFET seems more 'elegant', but that hardly seems like a good
answer.
>>
>> BTW, there should be some kind of law against so many people helping
guys like me. :-)  Imagine if the Manhattan Project had a group like
TriEmbed.
>>
>>
>> Thanks,
>> Brian Grawburg
>>
>>
>>
>>
>>
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>
> ---------- Forwarded message ----------
> From: Brian <triembed at undecidedgames.net>
> To: triembed at triembed.org
> Cc:
> Date: Mon, 7 Dec 2015 11:05:37 -0500
> Subject: Re: [TriEmbed] MOSFET or Relay
> One important note on SSRs: If you're switching DC, make sure you get a
DC-rated SSR.  Many AC-rated SSRs have zero-cross circuits to minimize
switching noise; this means that once the drive signal is received, the
relay waits until the voltage across its "contacts" crosses zero before it
actually turns on.  Obviously with a DC supply, the voltage never crosses
zero, so the relay would never turn on.
>
> As a side note, zero-cross circuits also have the effect of making most
AC SSRs useless for triac-style dimming.  The relay won't turn on until the
next zero-cross, and the silicon won't turn off until another zero-cross.
>
> On 12/7/2015 8:50 AM, Adam Haile via TriEmbed wrote:
>>
>> A FET is going to be WAY faster at switching than a relay. Multiple
>> orders of magnitude faster. Relays will often carry far more current for
>> less cost. Then there is the weird in between of the two... the Solid
>> State Relay. Also, not that a mechanical relay is true on/off. Whereas a
>> FET or SSR (which is usually just an opto isolated FET in a nice
>> package) are /generally/ on/off, but there's fancy things you can do to
>> actually have the output voltage vary... or if you supply way to much
>> voltage in it will just allow it through and short out. I don't really
>> understand that part... I just ensure I read the datasheet and stay
>> within the specs.
>>
>> here's a good comparison of mechanical relays vs SSRs:
>>
http://electronicdesign.com/components/electromechanical-relays-versus-solid-state-each-has-its-place
>>
>> On Mon, Dec 7, 2015 at 8:41 AM, Grawburg via TriEmbed
>> <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
>>
>>     Just for my future information. Presuming that current draw is not
>>     an issue, given the choice between using a MOSFET or a suitably
>>     opto-isolated relay to switch 12V, is there any significant
>>     advantage of one over the other? The MOSFET seems more 'elegant',
>>     but that hardly seems like a good answer.
>>
>>     BTW, there should be some kind of law against so many people helping
>>     guys like me. :-)  Imagine if the Manhattan Project had a group like
>>     TriEmbed.
>>
>>
>>     Thanks,
>>     Brian Grawburg
>>
>>
>>
>>
>>
>>     _______________________________________________
>>     Triangle, NC Embedded Computing mailing list
>>     TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>>     http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>     TriEmbed web site: http://TriEmbed.org
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>>
>>
>>
>> _______________________________________________
>> Triangle, NC Embedded Computing mailing list
>> TriEmbed at triembed.org
>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
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---

Martin Brooke
Associate Professor,
and Philip Baugh Scholar
Department of Electrical and Computer Engineering
Box 90291
Duke University
Durham, NC 27708-0291

Phone: (919) 660-5504
Secretary:  (919)-660-5252
Fax:   (919)-660-5293
E-Mail: mbrooke at ee.duke.edu
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