# SCR relaxation oscillator does not oscillate when it should

I am studying this SCR relaxation oscillator and I cannot predict when it oscillates and when it does not.

The idea seems simple: as soon as the cathode voltage drops below SCR's gate trigger voltage, SCR turns on and the capacitor should discharge through the SCR, and it would rise the cathode voltage, and it would decrease the SCR a-k current. As soon as the current drops below SCR's holding current, the SCR should turn off, and the cycle should repeat.

But when I simulate this circuit it oscillates only with a certain very narrow set of parameters. For instance, it stops oscillating if I change R3 to 5 or 3 Ohm, if I change C1 to 1uF, if I change R1 to 8 or 10 Ohm, if I change R2 to 41 or 48 Ohm.

How can it be explained that this circuit stops oscillating when its parameters are changed so little? How can I predict without a simulation if the circuit oscillates or not?

• Supply voltage of 1.2 V ? Commented Aug 22 at 17:21
• @Antonio51 That's correct. I was wondering if it can be powered from a single NiMH battery. But the same problem happens when I use a different voltage: it just stops oscillating without any apparent reason, and I have to tweak the parameters by trial and error to get it oscillating. Commented Aug 22 at 17:42
• The holding current has no minimum value specified in the datasheet. Nor is there a typical value. Have you examined or tested the model to see what they are using? This seems like a very poorly controlled circuit using a loosely specified (for this kind of application) obsolete part. Commented Aug 22 at 17:51
• VTM min should be ~1.4 V ... Difficult to use 1.2 V. But 3 V -> 4.2 V is ok. Tested. Commented Aug 22 at 17:51
• @Spehro'speff'Pefhany They seem to be using this: .macro TIC126A SCR(40m,5m,.8u,1,100,100MEG,11u,6,8) Don't know what it all means. Commented Aug 22 at 18:02

Macro SCR.mac Made with microcap v12.

Macro thyristor used ...
International Rectifer Power Modules
Texas Instruments Power Products Data Book 1990
Motorola Thyristor Device Data

SCR (IH, IGT, TON, VTMIN, VDRM, DVDT, TQ, K1, K2)

.macro TIC126A SCR(40m, 5m, .8u, 1, 100, 100MEG, 11u, 6, 8)

SCR - refers to the macro circuit SCR.CIR
IH - DC Holding Current
IGT - Gate Trigger Current
TON - Turn on time
VTMIN - Minimum anode to cathode on state voltage
VDRM - Maximum repetitive peak off-state voltage
DVDT - Critical rate of rise of off-state voltage
TQ - Turn off time
K1 - DVDT adjustment (set to 1 unless otherwise specified)
K2 - TQ adjustment (set to 1 unless otherwise specified)


Parameters (component's value) are more practical.

• Thank you for the valuable info. It does not quite answer my question, though, but it is educational. Did you find your parameters by trial? Say, your circuit will not oscillate if you change R4 to 80 or 15 Ohm. The same problem. Commented Aug 22 at 18:52
• Yes, same problem ... but values are a bit "larger". Note that TIC126 is a "power" device. Use an SCR low power. Commented Aug 23 at 6:33