Reverse conducting SCR?

Thread Starter

Hagen

Joined May 8, 2010
30
While testing an SCR module I came across a situation I can't figure out. I have simulated the situation in Spice. Simply stated, I am connecting the gate of the SCR to the Anode, and applying DC to the anode and cathode, with a current limiting resistor in series. The thing is, when I apply negative to the Anode, and positive to the Cathode, I was not expecting current flow.
Wrong..! What am I missing? Why does this "reverse biased" SCR conduct?
 

timrobbins

Joined Aug 29, 2009
318
It's a pnpn structure. With reverse voltage across anode/cathode but no bias on gate then acts like two reverse biased diodes (ie. only leakage current between anode and cathode). But if gate is biased then can act as transistor - you should be able to find a nice explanation showing the 'transistor' layout somewhere.
 

Ron H

Joined Apr 14, 2005
7,063
Connecting gate to anode is not a normal operating mode. I think you are seeing the gate-cathode breakdown voltage, analogous to Vbe breakdown voltage in an NPN.
In the datasheet of 2N5064 (the only datasheet I checked), this breakdown voltage is not even specified (at least, I couldn't find it).
The Teccor Spice model uses a value of 5V. See ZGK below.
Rich (BB code):
*SYM=SCR
*SRC=2N5064;2N5064;SCRs;TECCOR; 200V 0.8A 
.SUBCKT 2N5064   1 2 3
*    TERMINALS:  A G K
QP  6 4 1  POUT OFF
QN  4 6 5  NOUT OFF
RF  6 4    400K
RR  1 4    267K
RGK 6 5    80
RG  2 6    9.09
RK  3 5    0.112
DF  6 4    ZF
DR  1 4    ZR
DGK 6 5    ZGK
.MODEL ZF   D (IS=3.2E-16 IBV=100U BV=200)
.MODEL ZR   D (IS=3.2E-16 IBV=100U BV=200)
.MODEL ZGK  D (IS=3.2E-16 IBV=100U BV=5)
.MODEL POUT PNP (IS=320F BF=1 CJE=402P TF=102U)
.MODEL NOUT NPN (IS=320F BF=100 RC=0.45 CJE=2.01N CJC=402P TF=3.74U)
.ENDS
 

thatoneguy

Joined Feb 19, 2009
6,359
But I don't think it gives any insight as to how it would have transistor action if the gate was tied to the anode, and then the anode was reverse biased.
True, I was only providing the "equivalent transistor hookup diagram" mentioned in the post I quoted.

SCRs/DIACs/TRIACs are uncommon enough in most circuits that knowledge of their workings is a bit less than knowledge of inductors, usually less (e.g. not very good).
 
Top