From: Sean Breheny <shb7@cornell.edu>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sender: "piclist-bounces@mit.edu" <piclist-bounces@mit.edu>
Date: Sun, 8 Aug 2021 02:21:03 -0700
Subject: Re: [EE] Latching relay ratings
Thread-Topic: [EE] Latching relay ratings
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Vacuum tubes came in a great variety of types. The main ones were triode,
tetrode, and pentode.

Triode behavior is pretty close to MOSFET. In fact, one of the regions of
MOSFET operation is sometimes called the "triode" region. This is the same
as the ohmic region. However, the equivalent "Rds ON" is rather large for
vacuum tubes. The "linear" region for both triodes and MOSFETs obeys a
quadratic relationship (i.e., drain or plate current is proportional to
(Vgs-Vth)^2). Grid leakage is considerably higher than gate leakage, though=
..

Note that the term "saturation" is used differently in the BJT vs MOSFET.
In a BJT, saturation is the region where the BJT is fully ON and dissipates
the least heat. In a MOSFET, the equivalent is the ohmic region. The MOSFET
"saturation" region is where increasing Vds doesn't increase current any
further - where it acts like a constant current sink/source. This is the
highest dissipation region.

Pentode tubes are similar to BJTs (except that the control grid draws much
less current than a transistor base). The ohmic region is only a small Vce
or Vplate-cathode range wide, and the linear region is very flat - a more
ideal current source.

Tetrode is in-between.

Sean


On Sun, Aug 8, 2021 at 4:01 AM Justin Richards <justin.richards@gmail.com>
wrote:

> Re Glass FET's, I have heard them referred to as a FET with a pilot light=
..
> I know very little about these and never considered that they had a ohmic
> region, does that  imply they also have a saturated region where they
> dissipate less heat?
>
> On Sat, Aug 7, 2021 at 3:45 AM David Van Horn <
> david.vanhorn@backcountryaccess.com> wrote:
>
> > Amusing analogy:  I've heard tubes called glass FETs, and you could thi=
nk
> > of relays as Mech-FETs.
> > You don't want to spend time in the ohmic region.
> >
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