The schematic is correct. Circle your grounds and you will figure it out. You will also find out the importance of the relationship of D10 and D13 and why replacing D13 with a power Schottky diode will prevent problems. Especially if you use two of them to make a split supply and do not disconnect them before power on or off.

Hi Mr Imsai Guy !, ... you've missed the most important point of this design: what is drawn as the "ground" is nothing but the Positive Regulated Output of the Power Supply and the +12V and +2.5V are over this Regulated Output. This "ground" is not the Negative (or 0V) of the PS. The connection of the Positive Regulated Output to "ground" is the only part of the schematics you have not "coloured" so far.

Great videos. The relay switching circuit uses the opamp inverting input connected to 12V from the top regulator and the non inverting is connected to chassis through a resistor which in this type of power supply is the positive output. Probably that confuses a lot since is less common to reference positve to chassis.

The configuration is a bit unusual. One way to get an overview is to draw the circuit with its main components Q3, R22, R27 and C6. The circuit is set to ground at the positive output. This ground is the reference point where the two op-amps are connected, the V op-amp with the negative input and the A op-amp with the negative input via R22. With a high current the minus input of the A op-amp gets higher than the set value. With a high output voltage (which is negative) the plus input of the V op-amp gets negative against ground. They both shut the transistor Q3.

I think the circuit diagram is correct. The positive output of the power supply in connected to circuit ground, which is unusual. (But as the whole circuit is floating, there is no connection between circuit ground and chassis ground.) Thus the output voltage is also connected to the comparator input, while the other comparator input is connected to the reference voltage.

These two videos are genius. Thank you.

Thanks for taking the time teach and share this very useful video really a blessing to and others BIG help

@3:45 That 2V zener blew my mind... @4:20 That cant be output V because that relay needs constant 12V+ at that point. You wouldn't have the V-OUT from that 7812 going to the regulated high current output ?? BUT MAYBE that other input to the op-amp should be connected to the main V-OUT and not that 12V rail ??

The negative output is not referenced to ground is floating from the reference. The dividers from 12 volts make sense then.

ImsaiGuy Where can I find the nice prototype board in the video. It is just what I am looking for. Thank for the great videos.

I watched a History channel show on toys where they said the Atari Pong game had no software code - it was all hard wired. I found a schematic but I went to KZfaq to see if anyone actually explained the Pong schematic when I found this. I would love to see a step through of that schematic - lots of 7400 series TTL chips.

Just wondering that if the documentation is incorrect and you send someone to do a field repair who is a mechanic how would you expect the repair to be done in a timely matter while the customer is watching and paying. Last time I checked companies that retain field service were charging upwards of $250/ hr. to industry. Fortunately most of us who were in that position had work arounds, but it could get interesting for fresh skin. Name of the game, documentation! Interesting post on your part.

I usually like IMSAI Guy's videos, but there are some glaring lapses in this two-part series. Errors in reading the schematic lead to incorrect analysis, which probably results in greater confusion for the students the videos purport to teach. The ICs are not adequately studied to see if they are actually op-amps or comparators, and if they ARE op-amps, are they being used as amps or as comparators? Nothing was done to look up what they are, and knowing that would have helped with the analysis. As others have commented, it is very clear from the schematic that what is 'marked in' as circuit ground, ISN'T. This skews much of the other conclusions, and contributes to more confusion. And while I like the methodology of starting with the easy parts and working up to the less obvious parts, more circuit study BEFORE making the video would have helped immensely. No teacher should ever be caught posing a question to students, and then not being able to explain it correctly to them. If this were my video, I would have A) studied it thoroughly before hand so as to not embarrass myself more than necessary, and B) if there was STILL some place that I could not figure out, I would have admitted that I "don't get it" instead of just concluding that the incomplete analysis presented is OK. Sometimes that happens when trying to reverse engineer a circuit, and there is no shame in admitting it. But to gloss over it as if it IS understood is just adding to the student's confusion.