# Circuit with dual power supply

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.

• 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 • Where is V+ connected to? – Huisman Sep 16 '19 at 13:37
• 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/…. – Blue_Electronx Sep 16 '19 at 13:56
• 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? – Huisman Sep 16 '19 at 14:04
• Yes, TR1 is U5 in this case. I'll edit the question. Thanks. – Blue_Electronx Sep 16 '19 at 14:06
• But L1 and R20 are missing in the other question!?! – Huisman Sep 16 '19 at 14:08