I'm trying to simulate the following circuit to test how it works:

enter image description here

The control circuit uses a PIC 18F4550.

The isolation circuit, and main circuit are these:

enter image description here

Unfortunately when I try to simulate this circuit on proteus, i get the following signals as result

enter image description here

When I connect this circuit with a resistor and with ground connected to the base of the circuit like this: enter image description here

I get the expected result:

enter image description here

But if I add the inductor, the program doesn't show anything on the oscilloscope

enter image description here

What am I doing wrong?, What's the right way to connect the ground on this circuit? Are there some additional considerations i should keep in mind?

  • \$\begingroup\$ Try reducing the value of the inductance. You also have grounded the common cathode for the SCRs in the first picture. \$\endgroup\$ Jun 22, 2021 at 21:37

1 Answer 1


You need to check your connections. The first picture you're showing doesn't have the best drawn conections, but you can infer that the commanding pulse is applied relative to the common point of the two SCRs.

Then, in your 1st schematic, you have grounded the common point, while in the 2nd you haven't. Both are not driven correctly, but it shows carelesness.

And the load you're using has a 400 mH inductor, which is a lot in terms of the time constant involved (40 seconds), so you may want to reduce it at least by a factor of 10.

With these in mind, here's the reworked schematic:


The driving is done relative to the common point of the two SCRs while the load has the same 100 Ω resistor but with a .stepped value for the inductance, whose current is plotted: 1 mH (black), 10 mH (blue), 100 mH (red), and 1 H (green).

The SCR used is a simple approximation with two BJTs, but it works pretty decent and it's at the basis of many commercial models for SCRs (Littlefuse, for example). The diodes are randomly chosen.


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