# Action of SCR & Triac?

Discussion in 'Electronic Basics' started by Louis Bybee, Dec 29, 2003.

1. ### Louis BybeeGuest

Could someone explain the reason why a SCR or Triac need a minimum load or
current level to conduct?

I understand (at least I think I do :-] ) the circuitry necessary to
utilize the component, but I'm having a little difficulty understanding why
there needs to be a minimum load in addition to a trigger circuit to cause
the device to conduct.

Thank you.

Louis--
*********************************************
Remove the two fish in address to respond

3. ### Keith R. WilliamsGuest

Think of an SCR as a relay with one set of contacts that continues to
energize the coil when the input is removed.

V
|
L
o
a
V V d
| +----+---+ | |
| | | | o / o /
| o | C| | / /
|=|> | C| | ./ ./
| o | C| | o o
| | | | | |
+--+ | +---+-----+
===
GND

Closer to the truth, an SCR is a pair of complementary bipolar
transistors:
Anode
|
|---+
/| |
| |
| |
| |
| |/
Gate o-------+---|
|>
|
Cathode

When a current is applied to the gate, the NPN (lower) transistor turns
on causing current to flow in the PNP (upper) transistor. The
collector current in the upper transistor continues to supply base
current to the NPN transistor (keeping it turned on) until there is no
current available in the upper transistor (no voltage across the Anode
to Cathode).

If there is no current in the anode then there isn't any current in the
PNP transistor to keep the NPN transistor turned on (thus keeping the
PNP turned on).

4. ### Louis BybeeGuest

It makes sense now.

Some of the electronic instructors that I've had wern't very good at
breaking concepts down for students!

Thanks to all who replied.

Louis--
*********************************************
Remove the two fish in address to respond

5. ### James WGuest

Keith, your drawing implies that ALL of the current that flows is thru
the base of either transistor. I suspect there must be more to this.

6. ### Keith R. WilliamsGuest

Yeah, well sorta! ;-) This is an "equivalent circuit" (I.e. *closer* to
the truth ;-) used to understand the SCR's operation. Indeed the
circuit will function as an SCR, as you note, perhaps sub-optimally).
In reality, the PNP's base and the NPNs collector are the same slab of
diffusion, as are the PNP's collector and the NPN's base. An SCR is a
three junction PNPN device.

Perhaps this will help some more: http://www.eteonline.com/scrs.html

--
Keith