Thyristor SCR testing circuit

[HDaniel]

Hello,

In attachment a simple testing circuit for a thyristor (SCR).

Normally this circuit should work for a TIC 106D but it doesn't. (Even tried with "real" components. Lamp starts burning without pressing on switch 2. The lamp we used was a halogen light bulb 230V/10W).

I replaced the TIC106D with a BT150-500R in Multisim but same result/problem.

Could someone tell me what's wrong and what to do to get it right?

Kind regards,

Daniel

[ThiCop]

Hello HDaniel,

You should connect a Ground.

If there is no ground, then it doesn't work. (just like in real life)

[HDaniel]

Hello,

Thanks for your reply.

But the problem would be solved by grounding.

(I send the cicuit in attachement, if you add a ground it still won't work).

Do you have a better advise?

Kind regards,

[ThiCop]

Hello,

The ground should be added like in attachment.

Can you test this?

Is there a specific reason why you're using version 10 of Multisim?

Do you have a license for version 12?

If yes, then this would allow me to share my design that works correctly (in version 12) with you.

[HDaniel]

Hello,

The problem should be comming from the multisim 10 program, i guess. I adjusted it like you mentioned it, but the SCR is still conducting when closing J1 (when J2 is open).

Reason why whe are still using multisim 10 is because an upgrade is to expensive for our school and we need only the basic things.

Seems to me that this topic can be closed without a solution (for multisim 10).

Kind regards,

Daniel

[digitalguy]

Just out of curiosity, I also tried the modified circuit and I got the same results as you did Daniel. I have multisim 12.

[HDaniel]

OK. Thanks anyway for trying in multisim 12. But the problem stays.

[HDaniel]

Hello,

I did some adjustments myself and the circuit is now working.

The only problem (i think) is that the gate current is too high (203 µA while datasheets says that Ig max is 200 µA for this component). When i change the resistor R1 to a higher level (up to 53 k) the SCR won't start conducting. Strange.

Try it out next week "in real life".

Working circuit added.

[ThiCop · Accepted solution]

Hello Daniel,

It is indeed correct that the lamp keeps burning if you just open J1 and leave J2 closed.

The reason that I didn't pick this up, was because you use the same keyboard shortcut (space) for both switches.

In general this is done when both switches are controlled by the same inputs.

I see that there have been several changes in the schematic. (eg. placement of the thyristor also changed)

Should the lamp be 10W or 5W (forum post says 10W, schematic says 5W)?

If you want to have the J1 switch to actually turn off the lamp when you turn off the power supply, then there's one thing that you missed.

Any voltage source (no matter if it can be turned off or not) on a schematic should always have some discharge capacity.

This discharge capacity should provide a path from the soruce to the ground.

If you don't include this, then the voltage source will (depending on the schematic) not correctly turn off.

In attachment there's an example of how you could do this.

In practice this might not always be needed, but it is always a good practice with custom designs.

In simulation you should always check that there's a discharge path when you encounter these kinds of issues.