# Simple DIAC circuit which I do not understand - LTspice

Here is the circuit :

Here is the model and the parameters of the DIAC :

.SUBCKT DIAC DIAC_IN DIAC_OUT PARAMS:
+Tr=0.342
+Vbo=13.6V
+Delta_V=19V
+Ibo=15uA
*
*Tr:      Rise time (in µs)
*Vbo:             Break over voltage
*Delta_V: Dynamic breakover voltage
*Ibo:     Breakover current
C_C1         N02098 DIAC_OUT 1u IC=0
V_IDIAC1     DIAC_IN N01041 DC 0Vdc AC 0Vac
R_R1         N02098 TRG  {1.462*Tr}
D_D1         N06161 N01041 DZ19V
E_ABM1       TRG DIAC_OUT VALUE { IF(ABS(I(V_IDIAC1))>{Ibo},1,0)}
D_D2         N06161 N01060 DZ19V
D_D3         N10655 N01060 DZ14V
D_D4         N10655 DIAC_OUT DZ14V
S_S1         N01041 N01060 N02098 DIAC_OUT _S1
RS_S1        N02098 DIAC_OUT 1G
C_C2         DIAC_IN DIAC_OUT 10p
****************
* Switch Model *
****************
.MODEL        _S1 VSWITCH Roff=1e7 Ron=2.2 Voff=0.1V Von=0.99V
****************
* Diodes Model *
****************
.model DZ14V    D(Is=3.142f Rs=0.1 Ikf=0 N=1 Xti=3 Eg=1.11 M=.3282
+                               Vj=.75 Fc=.5 Isr=1.973n Nr=2 Bv={Vbo} Ibv=.14467 Nbv=1.093
+                               Ibvl=.1m Nbvl=1.2722 Tbv1=001433.3u)
*
.model DZ19V D(Is=6.994f Rs=5.612 Ikf=0 N=1 Eg=1.11 M=.2906
+                    Vj=.75 Fc=.5 Isr=2.088n Nr=2 Bv={Delta_V} Ibv=.17098 Nbv=1.2072
+                    Ibvl=2.002m Nbvl=1.1457 Tbv1=888.89u)
*               Vz = 18 @ 14mA, Zz = 37 @ 1mA, Zz = 11 @ 5mA, Zz = 7.9 @ 20mA
.ends
*
*********************************************************************
*                                                               Standard DIACs                                      *
*********************************************************************

*\$
.subckt myDIAC DIAC_IN DIAC_OUT
X1 DIAC_IN DIAC_OUT DIAC params:
+Tr=0.342
+Vbo=13.6V
+Delta_V=19V
+Ibo=16uA
* 2008 / ST / Rev 1
.ends


where :

* This DIAC model simulates:
*Tr:      Rise time (in µs)
*Vbo:             Break over voltage
*Delta_V: Dynamic breakover voltage
*Ibo:     Breakover current
*
*All these parameters are constant, and don't vary neither
*with temperature nor other parameters.


What I think : When Vc reaches the breakover voltage (32V), the DIAC begins to conducts, until the current is lower than the breakover current, ie 16uA. The current of the source voltage is able to provide 15uA due to the limiting resistor 60 Meg (this is a simulation). So it should conduct until the capacitor is able to provide 1 uA. And then wait again that the voltage across the capacitor rise to 32V. The voltage capacitor should "oscillate" over the time. Nevertheless, here what the simulation shows :

What is weird is that the capacitor voltage stays @ 32V whereas if the DIAC was not conducting, it should continue to grow until the voltage source. Nevertheless when conducting the DIAC have a "forward" voltage lower than 32V. In the model the forward voltage is equal to 32V - 19V = 13V.

Thank you very much and have a nice day ! :D

• Are you sure you want those values? 900V, 60Meg+10n, 1m (1e-3)? Commented May 2, 2020 at 10:56
• No it was just for understanding ;)
– Jess
Commented May 2, 2020 at 12:28
• Thank you :) (to take care about my circuits :) )
– Jess
Commented May 2, 2020 at 12:52