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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