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

enter image description here

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?

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  • \$\begingroup\$ Supply voltage of 1.2 V ? \$\endgroup\$
    – Antonio51
    Commented Aug 22 at 17:21
  • \$\begingroup\$ @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. \$\endgroup\$
    – Thomas Anderson
    Commented Aug 22 at 17:42
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    \$\begingroup\$ 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. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Aug 22 at 17:51
  • \$\begingroup\$ VTM min should be ~1.4 V ... Difficult to use 1.2 V. But 3 V -> 4.2 V is ok. Tested. \$\endgroup\$
    – Antonio51
    Commented Aug 22 at 17:51
  • \$\begingroup\$ @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. \$\endgroup\$
    – Thomas Anderson
    Commented Aug 22 at 18:02

1 Answer 1

Reset to default
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Macro SCR.mac Made with microcap v12.

enter image description here

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

Passed parameters follow the format:
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)

enter image description here

Parameters (component's value) are more practical.

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Thomas Anderson
    Commented Aug 22 at 18:52
  • \$\begingroup\$ Yes, same problem ... but values are a bit "larger". Note that TIC126 is a "power" device. Use an SCR low power. \$\endgroup\$
    – Antonio51
    Commented Aug 23 at 6:33

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