I'm trying to understand the function of the constant current regulator CCR and the zener diode 1N5338. I do have more questions but these two components is probably a good start.

I understand that a zener diode is voltage regulator but not sure why it is included. As for the CCR I have no idea what its purpose is in this circuit.

The link to the web page where I found this: https://www.electronicdesign.com/power-management/article/21801456/current-limiter-offers-circuit-protection-with-low-voltage-drop

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  • \$\begingroup\$ zener diode is voltage regulator but not sure why it is included ... think about this what is the value of Vin? \$\endgroup\$
    – jsotola
    Commented Jan 17, 2020 at 2:31
  • \$\begingroup\$ @jsotola 24V switch mode power supply \$\endgroup\$
    – Jacob
    Commented Jan 17, 2020 at 2:35
  • \$\begingroup\$ i was not asking a question ... i was asking you to ask the question of yourself and think about the answer .. it may help you to understand the reason for the use of D1 \$\endgroup\$
    – jsotola
    Commented Jan 17, 2020 at 2:44
  • \$\begingroup\$ btw, the answer to what is the value of Vin? is a voltage value, not the source of the voltage \$\endgroup\$
    – jsotola
    Commented Jan 17, 2020 at 2:48
  • \$\begingroup\$ @jsotola okay so D1 regulates whatever voltage is coming through CCR, so why is CCR there? \$\endgroup\$
    – Jacob
    Commented Jan 17, 2020 at 2:59

2 Answers 2


Reasons why I would reject this design by Sajjad Hajdar, technologist at UBC Apr 18, 2016

  1. There are no design specs for Input voltage range, Ripple , PSRR, Vout ripple heat loss, Temp rise,
  2. There is no apparent need for Opto-isolation.
  3. There is no need for a CCR to bias a Zener to power the LED current.
  4. There is no OTP
  5. There is no mention of Cap ESR limitations for phase margin stability.
  6. There are hundreds of MOSFET LDO's in stock everywhere. that do this already.

It might have been interesting 40 years ago as a Magazine Design Tip, but obsolete news today.

  • \$\begingroup\$ If my supply is rated 24V 25A how would I go about current limiting in your opinion? \$\endgroup\$
    – Jacob
    Commented Jan 17, 2020 at 3:09
  • \$\begingroup\$ That's a different question. I would modify Lab supply with schematic if avail. using SMPS with your design specs. for Step load range, V range, I range, Ripple max, Efficiency or loss max. Then Purpose. : Energy storage . CC limit by thermal of instant μs control? with/without foldback? nNEVER try to design a PSU without good specs. \$\endgroup\$ Commented Jan 17, 2020 at 3:14

As Tony mentioned it's not a good design;

It's a constant current source for the LED in the opto-coupler so that can't pull more than specified current from the circuit. the zener diode works as a voltage regulator for the CCR supply.

R7 is there just to avoid Q2 shorting the VCC to ground when the circuit goes to CC mode.

For example take look at this simple design to understand the concept of building a constant current source for the opto-coupler LED:

R1 limits the input current . Q1 is used as current sink in order to keep the current through the LED (the LED inside U1) at an almost stable value.

When the voltage across R2 reaches about 0.65V, Q1 begins to conduct, shunting current from the LED. The result is that, although the current of R1 rises as the input voltage rises, the current through the LED stops increasing at a specific value; The minimum LED current that will operate the TRIAC. This value is set by R2.

  • \$\begingroup\$ He has the 5V rail there why not use that for the opto-coupler? \$\endgroup\$
    – Jacob
    Commented Jan 18, 2020 at 3:19
  • \$\begingroup\$ @Jacob 5V or 24V, you have to maintain the current of the LED at an acceptable level. \$\endgroup\$ Commented Jan 18, 2020 at 7:33

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