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.
I have an application like this. The SCR is a triggering device, which does
not turn off after it turns on. This characteristic is useful in certain
situations. I have one where the SCR crowbars a coil which MUST stay off
after a certain time, no matter what. No matter whether the PIC hangs,
reboots, gets hit by a brownout, and so on.
In quantities a TO-92 SCR will cost you US$0.06 to 0.10, whereas a 2n2222
will cost you about a penny, so most people don't use an SCR unless it is
really needed.
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.
You can switch on an SCR. However, the SCR will not switch off until the
voltage across it terminals drops to near zero volts. This is good if you
are chopping a sine wave to generate an average voltage for light or motor
control. But not good if you want to switch DC loads on/off.
Also, switching an SCR on/off with a high dv/dt will cause noise, so don't
mix SCR switching with RF receivers....
<< 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.
> 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.
I guess technically, the SCR will turn off under specific conditions. #1 is
if the forward current in the SCR drops below a certain minimum value
(essentially zero for many practical circuits).
Careful here, an SCR will not turn off ntil the CURRENT drops below a
certain value. This can be very important when dealing with
inductive/capacitive loads. TTYL
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.
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.
> 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 removed}
> 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.
In low power electronics maybe, but at 100 to 1500 amperes and KV range
they don't have any substitute.
mitvma.mit.edu with SET PICList DIGEST in the body
