Doing some reverse engineering to a PCB, I drew this part of the circuit. Looking into the datasheets, I found out the comparator (LMC6772) is able to drive the SCR (S6S2) by itself. But the designer used a PNP transistor in between. I see that the comparator is fed with single power supply, so I assume the PNP is to do some kind of shift as the SCR is referenced to -13V. Why would the designer do this? It's like the current from the coil is sinking to -13V instead of the ground, but why? If you need more details about the circuit I can expand the drawing, but that's the basic concept of my question.

Below there's a link which is a follow up of this thread. Some notes:

  • TR1 in the old thread is U5 in this thread.

  • V+ in this thread is the point between R2 and R3 in the old thread.

  • L1 in this thread is the coil in parallel with D9 in the old thread.

  • R20 in this thread is (R2 + Rds (FET)) in the old thread.

  • In this thread, I just drew the circuit for simplicity, since in the old thread the path from R3 is bypassed when the SCR is fired.

Understanding this power supply design

enter image description here

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    \$\begingroup\$ Where is V+ connected to? \$\endgroup\$ – Huisman Sep 16 '19 at 13:37
  • \$\begingroup\$ I have a thread on the power supply of the circuit. Look at Dave's reponse, V+ is the point between R2 and R3. electronics.stackexchange.com/questions/444253/…. \$\endgroup\$ – user115094 Sep 16 '19 at 13:56
  • \$\begingroup\$ Please don't answer in comments, but rather edit your original question with this information. I assume U5 in this schematic =/= TR1 in the other question? \$\endgroup\$ – Huisman Sep 16 '19 at 14:04
  • \$\begingroup\$ Yes, TR1 is U5 in this case. I'll edit the question. Thanks. \$\endgroup\$ – user115094 Sep 16 '19 at 14:06
  • \$\begingroup\$ But L1 and R20 are missing in the other question!?! \$\endgroup\$ – Huisman Sep 16 '19 at 14:08

Looking into the datasheets, I found out the comparator (LMC6772) is able to drive the SCR (S6S2) by itself.

You are mistaken about that. The LMC6772 has open-drain outputs, which means that they can only sink current, not source it. However, the SCR requires current to be driven into its gate terminal for triggering. Therefore, the PNP is required to convert a current sink into a current source.

It really doesn't matter to the transistor whether the current is returned to ground or to the negative rail. Either way, the loop is completed through one or both power supplies.


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