This characteristic of SCR's has interesting uses.  When I was a Senior in
High School, a buddy and I cooked up a squealing noisemaker box.  We locked
it inside a sturdy toolbox with a large battery, and put a bat handle switch
on the outside that operated and SCR.  Thus you could turn the annoying
squealer ON, but never turn it OFF until the battery died.  Chained to a
girder in the cieling above the school library, it made quite an interesting
problem for the hapless janitor trying to turn it off.  By that time, of
course, we were graduated and gone, guffawwing about our senoir week prank.

I suppose the statute of limitations is worn out, so I can 'fess up to this
now.

--Lawrence

----- Original Message -----
From: "Shawn Yates" <syates@CARETECH.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, February 19, 2002 1:08 PM
Subject: Re: [EE]: SCV vs Transistor


> In the security or safety call area where I work I can think of some neat
> uses because most things should not turn off till someone comes and pushes
a
> button.  So the SCR could be used to trigger something to turn on and stay
> on based on a transient event.  Then it would stay turned on till a user
got
> there to push a reset button.
>
> Thanks everyone, I got a lot of info from this.
>
> Shawn
>
> -----Original Message-----
> From: Olin Lathrop [mailto:olin_piclist@EMBEDINC.COM]
> Sent: Tuesday, February 19, 2002 11:43 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: SCV vs Transistor
>
>
> > I have always used a transistor driven by the pin of a PIC to turn on
and
> > off high current devices (relays, buzzers whatever).  I saw a design
where
> > someone was using an SCR.
> >
> > Is there any operational difference when the application is switching
not
> > amplifying?
> > Is there a signifigant cost difference?
> >
> > What I am getting at is why would one use and SCR instead of a
transistor.
> > They are even in the same TO package.
>
> A bipolar transistor or FET are on and off controlled by the instantaneous
> input signal (base current for bipolar, gate voltage for FET).  An SCR
goes
> on like a bipolar transistor, but stays on as long as current is flowing
> thru it.  You would typically pulse the gate of an SCR once, then let it
> turn itself off when the load current goes to zero.  SCRs are useless in
> applications where the load current would never go to zero, like driving a
> relay from a DC supply.
>
> A disadvantage of an SCR is that it will have a higher on voltage than a
> saturated bipolar or FET in most applications (depending on load current).
>
> Personally I find little use for SCRs nowadays.  They were used heavily in
> solid state lamp dimmers because they require few parts to drive from an
> analog circuit.
>
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, olin@embedinc.com, http://www.embedinc.com
>
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