Simply put: For switching small loads, you can't beat the cost of small
switching transistors. For switching large loads, you can't beat the cost of
SCR's.

SCR's are great for switching high-current AC with few additional
components. I suppose that could be done with IGBT's, but the cost with
SCR's is still comparatively low W for W.
An SCR is basically a switch. Gate it when the anode is positive and it will
conduct. No biasing is required to keep it in a switching region. The gate
needs to be protected from reverse current, and forward gate current needs
to to be limited. Other than that you only need to keep from exceeding dv/dt
and reverse current through the anode/gate. There's less effort required to
figure heat dissipation also- as long as it's gated properly, W loss can be
figured on almost a straight line.

Chris

 Both have their place as is usually the case - depends upon your
application specification.

Chris



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Shawn Yates
Sent: Tuesday, February 19, 2002 6:47 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: SCV vs Transistor


I know this is a basic question, but I need to ask it.

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.

Any input appreciated.

Shawn

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