On the picture below is the output circuit of AC/DC adapter. I understand how voltage level contol works, but I don't understand how exactly current limitation works and how should I calculate resistors to get output current 2A for example? Could someone clarify this?

Thank in advance.enter image description here

UPD. I find another example of almost the same circuit. It's from datasheet to AP4310A. I still have no clue how current limitation works... enter image description here

  • \$\begingroup\$ um, it's kind of obvious that your schematic has both sides of your optocoupler incorrectly connected. \$\endgroup\$ – Marcus Müller Dec 7 '19 at 10:43
  • \$\begingroup\$ The longer I look at this schematic, the more confusing it gets. What with the unlabeled pull-up resistor above the R3-R4 voltage divider? What's up with the polarized cap, isn't that going to undergo a really hard (and unnecessary) ripple current testing, being half a resonant circuit with the inductor? Where's this circuit from? \$\endgroup\$ – Marcus Müller Dec 7 '19 at 10:48
  • \$\begingroup\$ @MarcusMüller the first one is from datasheet to NCP4300A. Here is the link onsemi.com/pub/Collateral/NCP4300A-D.PDF \$\endgroup\$ – Victor Dec 7 '19 at 10:57
  • \$\begingroup\$ oh, the second schematic is much nicer in the way it's unrolled: for example, the engineer took a lot of effort to make sure that the "signal flow" is left-to-right where possible, making things much easier to understand! \$\endgroup\$ – Marcus Müller Dec 7 '19 at 11:08
  • \$\begingroup\$ It really looks like someone who had little grasp on schematic symbols "compressed" the second circuit into the first. \$\endgroup\$ – Marcus Müller Dec 7 '19 at 11:16

In your second schematic, opamp 2, is amplifying the voltage across the current sense resistor R2.

That voltage is simply the product of R2 and the current through the secondary side of the transformer.

Now, if that current is high, then the opamp has a very high output voltage. That means no current is sunk into the opamp output. If opamp 1's output also has high output voltage, then no current can be sunk there, either, turning off the LED in the optocoupler.

That Opamp1's output is also turned high when opamp2's output is high is ensured by the voltage divider R3R4-R5.


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